Discussion of Novel Organic Materials with Potential Conductivity.

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Potential Conductivity. (Under the direction of Dr. Christopher B. Gorman.)

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By

Andrew Thomas Hermann

A dissertation submitted to the Graduate Faculty of North Carolina State University

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Chemistry

Raleigh, North Carolina 2016

APPROVED BY:

_______________________________ _______________________________ Dr. Christopher B. Gorman Dr. David Shultz

Committee Chair

_______________________________ _______________________________

Dr. Maria Oliver-Hoyo Dr. Walter Weare

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Dedication

First, and most of all, I dedicate this to my loving wife and best friend, Allison P. Hermann. Her dedication to my happiness and success has gotten me where I am today, not only as a chemist, but also as a man. I would also like to thank my daughters, Jade and Ella, for keeping me going when life has gotten hectic, and reminding me to always make time for fun. I love you both more than you will ever understand. Lastly, I dedicate this to two of my best friends, Deanna and Scott, whose advice, motivation, and even jeering has been

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Biography

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Acknowledgments

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Table of Contents

List of Tables...viii

List of Figures...xi

List of Schemes...xii

1. Chapter 1: Introduction...1

A. Organic Conducting Materials...2

1. Rational for Moving to Organic Conductors...2

2. Aromatic Ladder Polymers...5

3. Direct-writing...6

B. Outline of Dissertation...7

1. Development of Precursor Polymer...7

2. Conversation to Ladder Polymer...9

C. References...11

2. Chapter 2: Monomer Development ...13

A. Introduction...14

1. Basic Structure of Monomer...14

2. AA: BB Versus AB: AB monomers...15

3. Variable Portions of the Monomer...17

B. Results and Discussion...18

1. Alkyl, Bromine Monomer...18

2. Pyridine Monomer...23

3. Fluorine Monomer...25

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D. Experimental...31

E. References...38

3. Chapter 3: Polymerization of Di-Cyano Containing Monomers...39

1. Types of Polymerization...40

2. Palladium Cross-coupling Chemistry ...43

3. Nucleophilic Aromatic Substitution Reactions...45

1. Palladium Cross-Coupling Polymerizations of Alkyl Derivatives...47

a. Microwave Conditions...47

b. Thermal Conditions...51

c. Effect of Initiation...56

2. Palladium Cross-Coupling Polymerizations of Pyridine Derivative...60

3. Nucleophilic Aromatic Polymerization of Fluorine Derivative...63

C. Conclusions...67

D. Experimental...69

E. References...74

4. Chapter 4: Preparation of Thin-Films and Cascade Cyclization...76

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2. Post-formation modification...80

3. Characterization of Thin-Films...81

1. Preparation of Precursor Polymer Thin-Film...82

2. Cascade Cyclization Via Acidic Conditions...84

3. Attempts to Prepare Thin-Film with a Uniform Thickness...88

C. Conclusions...94

D. References...94

5. Appendix: Alkyne-Capped Au Nano-particles...97

1. Nano-particle Overview...98

2. Synthesis of Au NPs...101

3. Alkynes as Capping Ligands...103

1. Updated Synthesis of Alkyne Capped Au NPs...105

2. Characterization of Alkyne Capped Au NPs...108

3. Alkyne capped Au NPs synthesized from a Known Alkyne Au Complex Instead of a Salt ...112

C. Conclusions...122

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List of Tables

Table 2- 1: Reaction conditions for the hydrolysis /

decarboxylation of MeM4 to prepare MeM5 ...22

Table 2- 2: Reaction conditions to prepare FM4 ...29

Table 3- 1: Microwave polymerizations of MeM5 ...51

Table 3- 2: Thermal polymerizations of MeM5 ...53

Table 3- 3: Thermal polymerization of MeM5 w/o initiator ...58

Table 3- 4: MS data comparing thermal and microwave reactions ...59

Table 3- 5: Microwave polymerization of PyM5 ...61

Table 3- 6: Thermal polymerization of FM5 ...67

Table 4- 1: Drying conditions used in the attempt to prepare a uniform thin-film surface via drop casting...89

Table 4- 2: Initial parameters used in the attempt to prepare a uniform thin-film via spin coating. ...90

Table 4- 3: Parameters used in the attempt to prepare a uniform thin-film via spin coating on surfaces pre-treat with a hydrophobic material ...91

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List of Figures

Figure 1- 1: Example of Electromigration in copper wire...3 Figure 1- 2: Example structures of conducting polymers, which have been

synthesized...4 Figure 1- 3: Theoretical calculated ionization potentials (IP) in eV,

and band gap energies (Eg) in eV for ladder polymers...5 Figure 1- 4: Graphical representation of direct-writing, as a way to

convert an insulator to a conductor...7 Figure 2- 1: Example reactions that aided in the development of the

selected monomers...15 Figure 2- 2: Graphical representation of A/B polymerizations versus

A/A B/B polymerizations...16 Figure 2- 3: Structures of proposed monomers, A/B, B/B, and A/A ...17 Figure 2- 4: Sites on the proposed monomer that can be varied, X= Br,

or F, R= H, Methyl, or Hexyl...18 Figure 2- 5: Mass spec of the observed products of the NAS performed

between difluorobenzonitrile and ethyl cyanoacetate...26 Figure 2- 6: 1H-NMR of the observed products of the NAS performed

between difluorobenzonitrile and ethyl cyanoacetate ...27 Figure 2- 7: 13C-NMR of the observed products of the NAS preformed

between difluorobenzonitrile and ethyl cyanoacetate...28 Figure 3- 1: Graphical representation of chain growth polymerization

vs. step growth polymerization. The chain growth system shown here is for the case where initiation is slow relative to propagation, thus the time for each chain to ‘start’ and ‘stop’ is fast compared to the rate

at which new chains ‘start’...41 Figure 3- 2: Example palladium cross coupled reactions...44 Figure 3- 3: A schematic representation of the change to the set up

for the polymerization reactions. A.) set up from previous group members

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Figure 3- 4: Example 1H NMR of the desired product from the

polymerization of MeM5...55

Figure 3- 5: IRs of MeM5 monomer (blue) and polymer (red); Arrows show effect of the polymerization on the cyano stretches...56

Figure 3- 6: Graphical depiction of the percentage of initiator added and its effect on the molecular weight of the polymer obtained...57

Figure 3- 7: NMRs of PyM5 and its polymer products, A.) starting material; B.) Product heated for 5 minutes; C.) Product heated for 10 minutes...62

Figure 3- 8: GPC of PyM5 polymerization product, dashed lines are weights from polystyrene standards with Mn = 3,370 and 162...63

Figure 3- 9: Example NMR of the 1.) FMe5 monomer and 2.) polymerization product...65

Figure 3- 10: MS of FM5 polymerization prepared with two equivalents of t-BuOK at 70°...66

Figure 4- 1: Graphical representation of the three primary techniques for preparing thin films. A.) Drop casting B.) Blade coating C.) Spin coating...79

Figure 4- 2: Graphical representation on masking, a postproduction modification technique...80

Figure 4- 3: Graphical depiction of a multi-layer deposited thin film...81

Figure 4- 4: Image taken of initial thin film prepared by drop casting...83

Figure 4- 5: Schematic of gaseous HBr treatment of thin-film...85

Figure 4- 6: UV-Vis of thin-film, prepared by spin coating, on glass slide before (Blue) and after (Red) acid treatment...86

Figure 4- 7: IR of thin-film, prepared by drop casting, on KBr plate before the acid treatment is shown in blue while the red shows the film after the acid treatment...87

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Figure 4- 9: Image of thin-film, ~ 400 µm in thickness, before

(A) and after (B) acid treatment...93 Figure Ax- 1: Plot of number of ligands vs. number of gold atoms

for known, discrete gold clusters...99 Figure Ax- 2: ORTEP diagram of Au38SR18 (R = hexane thiol) showing

the staple motif with both SR-Au1+-SR and SR-Au1+-SR-Au1+-SR staples. The blue atoms are Au0, red atoms are Au1+, and the

yellow atoms are S...100 Figure Ax- 3: Three potential binding modes of the alkynes to the gold

surface. (a) an alkylidene end on interaction, (b) a side on interaction,

and (c). an end on acetylide anion interaction...104 Figure Ax- 4: Representative NMR spectrum...109 Figure Ax- 5: Cleanest NMR of alkyne capped Au NP collected...110 Figure Ax- 6: Representative IR spectrum of dodecyne capped Au NP

(black) vs. free dodecyne (blue)...111 Figure Ax- 7: IR of (AuC2C13H9O)10 ...113 Figure Ax- 8: UV-Vis of (AuC2C13H9O)10 ...114 Figure Ax- 9: ESI/MS of (AuC2C13H9O)10 calculated molecular

weight 4039.47...115 Figure Ax- 10: A.) Full NMR B.) is zoomed in on aromatic region

for (AuC2C13H9O)10. Peaks at 7.2 and shoulder at 7.6 are from

excess ligand...116 Figure Ax- 11: UV-Vis of the A.) Au10 complex on top for comparison,

and of NP products from the reductions with B.) NaBH4 and C.) TBAB...117 Figure Ax- 12: TEM of particles from NaBH4 reductions 3 days

after reaction...119 Figure Ax- 13: IR of the A.) Au10 complex on top for comparison,

and of NP products from the reductions with B.) NaBH4

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List of Schemes

Scheme 1- 1: Proposed five-step synthesis of precursor polymer...8

Scheme 1- 2: Proposed synthesis of conversion to ladder polymer...10

Scheme 2- 1: Four-step synthesis of alkyl monomer ...19

Scheme 2- 2: Two-step synthesis of pyridene monomer...23

Scheme 2- 3: Two-step synthesis of fluorine monomer ...25

Scheme 3- 1: Example of chain growth polymerization ...42

Scheme 3- 2: Example Pd cross-coupling reaction w/o organometallic reagent ...45

Scheme 3- 3: Example nucleophilic aromatic substitution polymerization ...46

Scheme 3- 4: Initial polymerization optimized by previous group members ...47

Scheme 3- 5: Polymerization of pyridine derivative ...60

Scheme 3- 6: NAS polymerization of fluorine monomer ...64

Scheme 4- 1: Cascade cyclization to form ladder polymer ...84

Scheme Ax- 1: Brust method ...101

Scheme Ax- 2: Modified Brust method ...102

Scheme Ax- 3: Purposed synthesis with Au1+ source ...105

Scheme Ax- 4: Synthesis of Au(THT)Cl ...112

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A. Organic conducting materials

1. Rational for studying organic conductors

In the ever-growing world, the desire for smaller and faster electronic devices grows rapidly.1 As these devices decrease in size so must the components that facilitate their operation. One of the primary components in electronics is the material used to move charge

from one site to another through a conductive medium: the wires or interconnects.2 The

typical wires that have been used for this kind of function are made of copper or gold. However, as these materials are made smaller and smaller, problems arise as discussed below. As a result, there has been a push among the scientific community to develop new conductive materials to replace the current precious metals.3

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Figure 1- 1: Example of electromigration in copper wire5

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Figure 1- 2: Example structures of electrically conducting polymers, which display

semiconducting to metallic conductivity upon oxidation or reduction.6

A variety of electrically conducting polymers7 have been synthesized to date. Each of

them is fully congregated and displays a large increase in electrical conductivity upon oxidation or reduction. Figure 1-2 shows some of the have conducting or semi-conducting

polymers that have been prepared.8 These polymers provide a template that can be used for

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opening up the possibility of flexible devices, which are more difficult to construct with the more rigid, metal interconnects.9

2. Aromatic Ladder Polymers

Aromatic ladder polymers are a subtype of conducting polymers. They are composed

of fused-ring aromatic monomers.10 While only a few examples of this type of polymer have

been synthesized to date, there has been extensive theoretical / computational examination of them, at least in their idealized form. Examples of these structures and calculations can be seen in Figure 1-3. In each case chosen, the polymer is predicted to have a zero or very small band gap, opening up the possibility that it could be an intrinsic conductor (e.g. electrically

conductive without oxidation reduction).11,12

Figure 1- 3: Theoretical calculated ionization potentials (IP) in eV, and band gap energies (Eg) in eV for ladder polymers

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Ladder polymers, like many materials of interest, also have their drawbacks along side of their benefits.13 The primary drawback of these materials is their limited solubility in traditional solvents used when processing polymers. This drawback counters one of the major reasons for using organic materials as conductors. Processability is perceived to be one of the major advantages of organic, conducting materials.14

3. Direct-writing

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Figure 1- 4: Graphical representation of direct-writing, as a way to convert an insulator to a conductor.

B. Outline of this Dissertation

1. Development of Precursor Polymer

This dissertation will describe the development of multiple precursor polymers, which will be prepared as thin-films and then converted from insulating polymers to ladder polymers that are hypothesized to be low band-gap, electrical conductors. The first step of this project was to prepare monomers via standard, small-molecule, organic synthetic routes. Each of the monomers prepared followed the general synthetic route shown in Scheme 1-1.

Former group members had prepared some of these monomers previously,17,18 but issues

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Scheme 1- 1: Proposed five-step synthesis of precursor polymer

The polymerization of the monomers was then explored. Previous work indicates that, while the polymerization was originally designed as a step growth system, perhaps it could be converted to a chain growth polymerization with the introduction of an initiator. The process for doing this switch and the validity of this idea will be probed here.

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2. Conversion to Ladder Polymer

The final aspect of this project was the preparation of thin-films of the synthesized polymers followed by exposure of the films to acidic conditions in an attempt to convert them to ladder polymers. The acidic conversion was attractive as there are a variety of commercially available, photo acid generators (PAGs) that could be used to facilitate a

cascade19 cyclization/tautomerization sequence (Scheme 1-2) to form an all aromatic ladder

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Scheme 1- 2: Proposed synthesis for converting the precursor polymer to a ladder polymer structure

This type of cascade cyclization was completed via nucleophilic attack for the dimer

and trimer model systems previous group members.17_{However, cyclization of molecules}

with multiple nitrile groups has been reported with strong acids. After obtaining evidence of successful conversion, the electrical conductivity of the converted films was probed.

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C. References

(1) Chen, S.; Xing, W.; Duan, J.; Hu, X.; Qiao, S. J. Mater. Sci. 2013, 1 (9), 2941. (2) Sadek, A. S.; Karabalin, R. B.; Du, J.; Roukes, M. L.; Koch, C.; Masmanidis, S. C.

Nano Lett. 2010, 10 (5), 1769–1773.

(3) Hansen, T. S.; West, K.; Hassager, O.; Larsen, N. B. Adv. Funct. Mater. 2007, 17 (16), 3069–3073.

(4) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4 (11), 864–868.

(5) Schneider, G.; Hambach, D.; Niemann, B.; Kaulich, B.; Susini, J.; Hoffmann, N.; Hasse, W. Appl. Phys. Lett. 2001, 78 (13), 1936–1938.

(6) Li, C.; Hu, Y.; Yu, M.; Wang, Z.; Zhao, W.; Liu, P.; Tong, Y.; Lu, X. RSC Adv. 2014, 4 (94), 51878–51883.

(7) Chen, X. L.; Chen, X. L.; Jenekhe, S. a.; Jenekhe, S. a. Macromolecules 1997, 30 (96), 1728–1733.

(8) Pesant, S.; Boulanger, P.; Côté, M.; Ernzerhof, M. Chem. Phys. Lett. 2008, 450 (4-6), 329–334.

(9) Park, S.; Wang, G.; Cho, B.; Kim, Y.; Song, S.; Ji, Y.; Yoon, M.-H.; Lee, T. Nat. Nanotechnol. 2012, 7 (7), 438–442.

(10) Goldfinger, M. B.; Swager, T. M. J. Am. Chem. Soc. 1994, 116 (17), 7895–7896. (11) Miller, T. M.; Kwock, E. W.; Baird, T.; Hale, A. Chem. Mater. 1994, 6 (9), 1569–

1574.

(12) Renak, M. L.; Bazan, G. C.; Roitman, D. Synth. Met. 1998, 97 (1), 17–21.

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126 (10), 6987–6995.

(14) Owens, F. J. Solid State Commun. 2014, 185, 58–61.

(15) Hon, K. K. B.; Li, L.; Hutchings, I. M. CIRP Ann. - Manuf. Technol. 2008, 57 (2), 601–620.

(16) Tolbert, L. M.; Solntsev, K. M. In Hydrogen-Transfer Reactions; 2007; Vol. 1, pp 417–439.

(17) Brannock, M. Investigating the Reactivity of Benzylic Nitriles in the Synthesis of a Multiple Cyano-Containing Polymer, Ph.D., NCSU, Raleigh, NC, 2012.

(18) Xu, J. Investigating the Synthetic Methodology for Fused, Nitrogen-containing Aromatic Ladder Polymer, M.S., NCSU, Raleigh, NC, 2014.

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A. Introduction

1. Basic structure of monomer

The first step in developing the desired ladder polymer was designing a monomer that

can be polymerized, and subsequently cyclized. To this end the precious group members1

looked to the literature for example systems with could be emulated with some modification.

When reviewing the literature it was clear why this type of monomer was selected for

investigation. The examples found that showed ring closer cyclization functionality of cyano

groups2 led down a path utilizing a di-nitrile composed of one benzylic nitrile and one aryl

nitrile. Figure 2-1 shows the type of cyclization we were interested in developing. The most

attractive part of this type of monomer is that the cyclization can be achieved under acidic

conditions. This becomes important when the desired method of cyclization in this system is

the inclusion of a photo-acid in the final thin-film that will be utilized to initiate the cascade

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Figure 2- 1: Example reactions that aided in the development of the selected monomers4

2. AA: BB versus AB: AB monomers

With the basic structure of the monomer in mind, attention turned to the synthetic

approach that would be used for the polymerization itself. Particularly, consideration was

given to whether an AA:BB or AB:AB polymerization would be more attractive.5 Figure 2-2

gives a graphical representation of the differences in the two systems. In an AA:BB setup,

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functionality on either side of the molecule, usually denoted as the electrophilic group of the

reaction. The BB monomer contains the nucleophilic groups, again with one on each side of

the molecule. In an AB system only one monomer unit needs to be synthesized due to the

fact that both the electrophilic species, A, and nucleophilic species, B, on the same molecule.

Figure 2- 2: Graphical representation of A/B polymerizations versus A/A B/B polymerizations

Both of these schemes have pros and cons. In an AA:BB polymerization, one gains a

measure of control over the reaction that is lost in an AB:AB system In an AA:BB system

the activity of the reactive site can be tuned without the influence of the other species. For

example, in a monomer with both the nucleophilic and electrophilic species on the same

molecule the electron rich nucleophilic species could lower the reactivity of the electrophile.

An AB:AB polymerization only requires the synthesis of one monomer instead of two for an

AA:BB system. This makes it more attractive as the resources on the front end of the project

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The structures of each of the monomers that would be needed for either approach are

shown in Figure 2-3. When the synthetic routes for each of these molecules were reviewed, it

was decided that the instillation of three cyano groups on one benzene ring would be

problematic. It was thus decided the AB:AB system would be investigated.

Figure 2- 3: Structures of proposed monomers, A/B, B/B, and A/A

3. Variable portions of the monomer

It was then considered which halogens would work best in the polymerization.

Considering there was interest in investigating both palladium cross-coupling,6,7 and

nucleophilic aromatic substitution chemistries for the preparation of the precursor polymer,

two halogens were considered. Aryl bromides are most efficacious in palladium

cross-coupling,8 and aryl fluorides are most efficacious in nucleophilic aromatic substitution. To

increase the solubility of the precursor polymer, the R group para to the benzyl nitrile could

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Figure 2- 4: Sites on the proposed monomer that can be varied, X= Br, or F, R= H, methyl, or hexyl

B. Results and Discussion

1. Alkyl, bromine monomer

The first monomer that was synthesized was a methyl-substituted benzyl-cyano

bromobenzonitrile (MeM5). Previous group members had studied the polymerization of a

similar monomer with a hexyl group (HxM5) with various amounts of success. Previous

group members had carried out the synthesis of the MeM5 monomer with great success,

though the same could not be said for the polymerization of the monomer. Either of the

monomers can be prepared via a four-step synthesis, shown in Scheme 2-1. The primary

rationale for shifting focus back to the MeM5 monomer was to see if it could be polymerized

with greater efficiency. The HxM5 monomer was also synthesized in order to validate

previous work. For these monomers, the halogen chosen was bromine as there are many

examples in the literature showing that it is effective in palladium cross coupling reactions.6

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Scheme 2- 1: Four-step synthesis of alkyl monomer

Overall, the syntheses of these two monomers, MeM5 and HxM5, followed the same

four steps. The first step involved treating the para-substituted aniline with bromine to give

the dibromide in excellent yields. This was followed by a Sandmeyer reaction,9 in which the

amine was converted into a diazonium salt, which was immediately converted to a cyano

group in the presence of a copper halide. There was a 20% difference in the yields between

the hexyl and methyl molecules, with the reaction on the methyl derived starting material

producing a higher yield. However, it was not clear why the methyl analog was that much

more successful, as the yields reported by previous group members of both molecules are

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The third step of the synthesis involved the nucleophilic aromatic substitution of one

of the bromines by ethyl cyanoacetate.10_{Multiple problems were found with the conditions}

used previously.1 First, the microwave reaction conditions used by previous group members

for this reaction were not working as efficiently as described. When these conditions were

employed, the reaction yielded either unreacted starting material or insoluble material. It was

also observed that the reaction was extremely sensitive to the presence of water or oxygen. It

was found that, if the solvent was distilled directly before use and the reaction vessels were

purged with nitrogen for an extended amount of time before starting the reaction, the desired

product was obtained at an average of 25% yield rather than 3-5%. Another factor that

seemed to be more important than originally realized was the freshness of the t-BuOK. If

these reactions were performed without taking extra care to ensure the t-BuOK was no more

than three months old, the yields were closer to 20-25% rather than the 65-70% reported. The

reported yields took into account that the excess ethyl cyanoacetate was still present in the

product even after two columns.11

The fourth and final step of the synthesis involved the hydrolysis of the ester to a

carboxylic acid by water in DMSO, immediately followed by a decarboxylation. This

reaction had previously been completed for both HxM5 and MeM5 via microwave heating.

However, although multiple variations of the temperature and time of reaction were

examined, it was found that, under microwave heating with either MeM4 or HxM4, either no

reaction occurred or an insoluble product formed. It was found that an overnight reaction at

elevated temperature (80° C) with excess water was extremely efficient (greater than 95%

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showing a breakdown of the reactions attempted to synthesis MeM5 is presented in Table

2-1. The exact amount of water was not explicitly determined, only that 1:10 molar excess was

sufficient where 1:4 excess was not. The excess water posed no problem in the isolation of

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Table 2- 1: Reaction conditions for the hydrolysis / decarboxylation of MeM4 to prepare MeM5

Heating _Type Temperature Power Hold _time

Molar excess

of Water

Recovered Yield

Microwave 110° C 200 W 25 mins 1:2 Insoluble _Material 0%

Microwave 100° C 150 W 25 mins 1:2 _MaterialStarting 0%

Microwave 100° C 175 W 15 mins 1:2 _MaterialStarting 0%

Thermal 25° C / 6 hours 1:4 _MaterialStarting 0%

Thermal 25° C / 12 hours 1:4 Desired _Product 10%

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2. Pyridine monomer

As the polymerization studies were being conducted on the alkyl substituted

benzyl-cyano bromobenzonitrile, consideration was given over the use of a more electron deficient

monomer. If the oxidative addition were the rate-limiting step in the polymerization, which it

typically is in palladium cross-coupling reactions, then the rate would be expected to increase

as the aryl group became more electron deficient. Thus, a pyridyl derivative (PyM5) of the

monomer was prepared. The starting material, 3,5-dibromopyridine-4-carbonitrile was

commercially available, and the monomer could be prepared from it in two reactions, shown

in Scheme 2-2.

Scheme 2- 2: Two-step synthesis of pyridene monomer

The first reaction attempted was the nucleophilic aromatic substitution of one of the

bromines for ethyl cyanoacetate to obtain PyM4. As a more electron deficient substrate, it

was thought the substitution would be more facile.12 Thus, the reaction was closely tracked

by thin layer chromatography (TLC). Indeed, the reaction was complete in only two hours at

room temperature, compared to elevated temperatures and almost 12 hours required for the

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yield (greater than 95%), PyM4 was directly reacted with water in DMSO. This reaction was

carried out under microwave reaction conditions. Similar to the NAS, the decarboxylation

reaction proceeded to produce PyM5 in a nearly quantitative yield.

The decarboxylation of the PyM4 under microwave conditions, when compared to

the alkyl aryl analog, resulted in much better results.13 It is thought that this is due to the fact

that the pyridine derivative required a significantly lower temperature, power, and time for

the reaction. This reaction was successfully run at 70° C at 120 W in five minutes. The

higher temperature and power needed for the alkyl aryl species were problematic for the

microwave to maintain for even short time periods. The lowest temperature, power, and time

needed to see any sign of reaction from MeM4 was 110° C at 200 W for 25 mins. The

non-uniform outcomes of the microwave reaction resulted in the need to return to a thermal

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3. Fluorine monomer

Scheme 2- 3: Two step synthesis of fluorine monomer

In addition to palladium cross coupling, we also wanted to investigate a

polymerization that utilized a nucleophilic aromatic substitution mechanism. Knowing that

fluorine, compared to bromine, is a better halogen to be used under the NAS conditions, a

synthetic route to prepare a fourth monomer was designed. To have a truly comparable

system, a synthesis of methyl-substituted benzyl-cyano fluorobenzonitrile was proposed.

Adding fluorine, or an alkyl group, to benzene is not trivial. Therefore, a commercially

available difluorobenzonitrile, without additional substitution, was used. It was envisioned

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Figure 2- 5: Mass spec of the observed products of the NAS performed between

difluorobenzonitrile and ethyl cyanoacetate

The first step in the synthesis of the fluorine (FM5) monomer was initially performed

following similar reaction conditions that were used previously (e.g. ethyl cyanoacetate and

t-BuOK in DMF under both elevated and room temperature conditions under a nitrogen

atmosphere). When the 1H NMR did not show either the starting material or desired product

the sample was analyzed by mass spectrometry (MS). The results of the MS (Figure 2-5),

showed only one primary product with an m/z of 165. The molecular weight of 165 was five

atomic mass units more than that of the starting material, and 67 less than our desired

product.

151613_A #69-90 RT:0.43-0.54 AV:22 SB:72 0.01-0.21 , 2.78-3.01 NL:4.62E7

T:FTMS + p ESI Full ms [120.00-1000.00]

120 130 140 150 160 170 180 190 200 210 220 230 240 250

m/z 0 10 20 30 40 50 60 70 80 90 100 R e la tive Ab u n d a n ce 165.08231 C9H10N2F

0.35983 ppm

217.10449 183.09271

136.05566 C8H7N F

-0.35621 ppm

150.05868 C8H7N2F

-0.63250 ppm

245.04515 C10H10O6F

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Figure 2-‐ 6: 1_{H-‐NMR of the observed products of the NAS performed between}

difluorobenzonitrile and ethyl cyanoacetate

After this result the 1_{H NMR, Figure 2-6, was reanalyzed to see if a structure with the}

weight of the found product could be determined. The three peaks of the aromatic region

with the appropriate multiplicity of a 1,2,3-trisubstituted benzene ring were observed. What

became difficult to assign was the six peaks at 5.1, 4.3, 3.5, 3.49, 3.1 and 1.3 ppm. A

reaction was proposed that produced two products, each having one of the two cyano groups

reduced to an amine. The integrals of each of the peaks indicated that the two products had

been produced in a 1:1 ratio. In an attempt to confirm these two products, a 13C NMR was

also run (Figure 2-7). When the peak positions were compared to those estimated by

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Figure 2- 7: 13C-NMR of the observed products of the NAS preformed between difluorobenzonitrile and ethyl cyanoacetate

Overall, the interpretation of the data suggested that the NAS occurred followed by

spontaneous decarboxylation and partial reduction of one of the cyano groups to an amine,

when this reaction was run with 2.5 molar equivalents or more of t-BuOK. It is not

understood how the reduction could have occurred. No reaction occurred when fewer

equivalents of base were used in the reaction. Table 2-1 shows the extent of the reaction

conditions attempted to synthesis FM4. The desired product of the nucleophilic aromatic

substitution, FM4, was eventually obtained, but required the use of a weaker base (K2CO3),

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Table 2- 2: Reaction conditions to prepare FM4

Temperature Base Equivalents Recovered Yield

25° C t-BuOK 1.5 _materialStarting 0%

25° C t-BuOK 2 _materialStarting 0%

25° C t-BuOK 2.5 _productsAmine 77%

25° C t-BuOK 3 _productsAmine 73%

80° C t-BuOK 1.5 _materialStarting 0%

80° C t-BuOK 2 _materialStarting 0%

~10° C K2CO3 2 Desired _product 80%

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Due to the difficulty of the preparation of FM4, the decarboxylation reaction was

attempted with care to avoid reaction of the FM5 prepared. The reaction was initially run at

5-10° C, in DMSO with 1:10 molar equivalent of water, but only starting material was

recovered. Repeating the reaction at room temperature under neutral, basic, and acidic

conditions also resulted in no reaction. At that time the reaction was run at elevated

temperatures, 80° C, again under neutral, basic, and acidic conditions for 12 hours. Each of

these reactions produced the FM5 monomer, with the neutral conditions giving the best yield

(97%).

C. Conclusions

In conclusion, four monomers were synthesized. They included both the methyl and

hexyl derivative of the bromide containing monomer, a pyridine derivative, which also had

bromine, and finally a fluorine monomer in which there was no para alkyl group, simply a

proton. Previous group members had synthesized both the hexyl and methyl monomers

containing bromine, while the others were new molecules. However the previous syntheses

needed modification to work efficiently. Each was prepared in gram quantities to facilitate

(45)

D. Experimental

Synthesis of 2,6-dibromo-methylaniline (MeM2), 1. In a 50-mL round bottom flask,

4-methylaniline (1.07 g, 10.0 mmol), methanol (10 mL) and dichloromethane (10 ml) were

added and stirred. In a small vial, bromine (1.28 mL, 25 mmol), methanol (5 mL), and

dichloromethane (5 mL) were added. The bromine solution was added to the stirring solution

in the round bottom flask dropwise with a glass pipet. The solution was stirred for 5 hours at

room temperature. The reaction was then quenched using 20% aqueous NaOH, which was

added until the solution in the reaction flask turned clear. The product was then extracted

with diethyl ether and dried over Na2SO4. Excess bromine was removed via silica plug,

eluting with ethyl acetate. The desired product (2.62 g, 99%) was pure by 1H-NMR (CDCl3):

δ=2.21 (s, 3H), 7.19 (s, 2H).

Synthesis of 2,6-dibromo-4-methylbenzonitrile (MeM3), 2. In a small vial, MeM2 (530

mg, 2 mmol), AcOH (4 mL), and H2SO4 (0.5 mL, 9.6 mmol) were added and stirred. The vial

was then heated to 60° C until MeM2 completely dissolved. While the MeM2 solution was

heating, CuSO4 (445.2 mg, 2.8 mmol) and H2O (2 mL) were added to a 50-mL round bottom

flask and stirred. After the CuSO4 completely dissolved, the round bottom flask was placed

into an ice bath. Then a solution of KCN (650 mg, 10 mmol) in H2O (3mL) was prepared and

added to the round bottom flask dropwise, while adding ice chips to the flask to ensure the

temperature did not exceed 10° C. This was followed by the addition of NaHCO3 (3.36 g, 40

mmol) and hexane (8 mL) to the round bottom flask. The flask was then heated to 55° C,

(46)

10° C to avoid the freezing of the AcOH. In a separate vial, a solution of NaNO2 (165.6 mg,

2.4 mmol) in H2O (4mL) was prepared, stirred until the NaNO2 completely dissolved, and

cooled in an ice bath. This solution was then added to the vial containing the MeM2 solution,

dropwise to minimize foaming. The mixture was allowed to stir for 30 minutes, after which it

was added to the round bottom flask dropwise. The reaction was then stirred for 5 hours at

55° C. The reaction was quenched with a 20% aqueous NaOH solution, extracted with

toluene, rinsed with brine, and dried over Na2SO4. The product mixture was purified by

column chromatography on silica gel (0-30% EtOAc in Hexanes) to get the desired product

(346.5 mg, 64%). 1H-NMR (CDCl3): δ=2.41 (s, 3H), 7.52 (s, 2H).

Synthesis of ethyl 2-(3-bromo-2-cyano-5-methylphenyl)-2-cyanoacetate (MeM4), 3. A

Schlenk flask containing MeM3 (96.3 mg, 0.35 mmol) and a 25-mL round bottom flask

containing t-BuOK (157 mg, 1.40 mmol) were flushed with nitrogen for 30 minutes. DMF (2

mL) was added to the Schlenk flask and the solution was stirred until MeM3 completely

dissolved. A solution of EtO2CCH2CN (166 mg, 1.47 mmol) in DMF (3 mL) was added to

the 25-mL round bottom flask, which was stirred until the t-BuOK completely dissolved. The

contents of the round bottom flask were then added to the Schlenk flask dropwise via a

syringe. The reaction was then heated to 80° C and stirred for 18 hours under a constant

stream of nitrogen. The reaction was then quenched with a 10% HCl solution, extracted with

EtOAc, rinsed with brine, and dried over Na2SO4. The product mixture was purified by

column chromatography on silica gel (0-30% EtOAc in Hexanes) twice to give the desired

product with a small amount of EtO2CCH2CN remaining (75.5 mg, 70%): 1H-NMR (CDCl3):

(47)

Synthesis of 2-bromo-6-(cyanomethyl)-4-methylbenzonitrile (MeM5), 4. In a small vial,

MeM4 (93 mg, 0.3 mmol), H2O (0.5 ml), and DMSO (3 mL) were added. The reaction

mixture was then heated to 80° C for 12 hours. The reaction mixture was then added, via

pipet, to a beaker containing 10% HCl (5 mL) and EtOAc (5 mL) to quench the reaction.

This mixture was then extracted with EtOAc, rinsed with brine and dried over Na2SO4 to

give the desired product (70.5 mg, 99%): 1H-NMR (CDCl3): δ=2.45 (s, 3H), 3.98 (s, 2H)),

7.48 (s, 1H), 7.57 (s, 1H).

Synthesis of 2,6-dibromo-hexylaniline (HxM2), 5. In a 50-mL round bottom flask,

4-n-hexylaniline (1.77 g, 10.0 mmol), methanol (10 mL) and dichloromethane (10 ml) were

added and stirred. In a small vial, bromine (1.28 mL, 25 mmol), methanol (5 mL), and

dichloromethane (5 mL) were added. The bromine solution was added to the stirring solution

in the round bottom flask dropwise with a glass pipet. The solution was stirred for 5 hours at

room temperature. The reaction was then quenched using 20% aqueous NaOH, which was

added until the solution in the reaction flask turned clear. The product was then extracted

with diethyl ether and dried over Na2SO4. Excess bromine was removed via silica plug,

eluting with ethyl acetate. The desired product (3.31 g, 99%) was pure by 1H-NMR (CDCl3):

δ=0.88 (t, 3H), 1.25- 1.28 (m, 6H), 1.53 (m, 2H), 2.45 (t, 2H), 7.19 (s, 2H).

Synthesis of 2,6-dibromo-4-n-hexylbenzonitrile (HxM3), 6. In a small vial, HxM2 (670

mg, 2 mmol), AcOH (4 mL), and H2SO4 (0.5 mL, 9.6 mmol) were added and stirred. The vial

was then heated to 60° C until HxM2 completely dissolved. While the HxM2 solution was

(48)

flask and stirred. After the CuSO4 completely dissolved, the round bottom flask was placed

into an ice bath. Then a solution of KCN (650 mg, 10 mmol) in H2O (3 mL) was prepared

and added to the round bottom flask dropwise, while adding ice chips to the flask to ensure

the temperature did not exceed 10° C. This was followed by the addition of NaHCO3 (3.36 g,

40 mmol) and hexane (8 mL) to the round bottom flask. The flask was then heated to 55° C,

while the small vial containing the HxM2 solution was cooled in an ice bath, to no lower

than 10° C, to avoid the freezing of the AcOH. In a separate vial, a solution of NaNO2 (165.6

mg, 2.4 mmol) in H2O (4 mL) was prepared, stirred until the NaNO2 completely dissolved,

and finally cooled in an ice bath. This solution was then added to the vial containing the

HxM2 solution, dropwise to minimize foaming. The mixture was allowed to stir for 30

minutes, after which it was added to the round bottom flask dropwise. The reaction was then

stirred for 5 hours at 55° C. The reaction was quenched with a 20% aqueous NaOH solution,

extracted with toluene, rinsed with brine, and dried over Na2SO4. The product mixture was

purified by column chromatography on silica gel (0-30% EtOAc in Hexanes) to get the

desired product (286 mg, 64%). 1H-NMR (CDCl3): δ= 0.88 (t, 3H), 1.25-1.34 (m, 6H),

1.57-1.61 (m, 2H), 2.60 (t, 2H), 7.44 (s, 2H).

Synthesis of ethyl 2-(3-bromo-2-cyano-5-n-hexylphenyl)-2-cyanoacetate (HxM4), 7. A

Schlenk flask containing HxM3 (120.5 mg, 0.35 mmol) and a 25-mL round bottom flask

containing t-BuOK (157 mg, 1.40 mmol) were flushed with nitrogen for 30 minutes. DMF (2

mL) was added to the Schlenk flask and the solution was stirred until HxM3 completely

dissolved. A solution of EtO2CCH2CN (166 mg, 1.47 mmol) in DMF (3 mL) was added to

the 25-mL round bottom flask, which was stirred until the t-BuOK completely dissolved. The

(49)

syringe. The reaction was then heated to 80° C and stirred for 18 hours under a constant

stream of nitrogen. The reaction was then quenched with a 10% HCl solution, extracted with

EtOAc, rinsed with brine, and dried over Na2SO4. The product mixture was purified by

column chromatography on silica gel (0-30% EtOAc in Hexanes) twice to give the desired

product with a small amount of EtO2CCH2CN remaining (89.3 mg, 70%): 1H-NMR (CDCl3):

δ= 0.87 (t, 3H), 1.29-1.34 (m, 6H), 1.62 (m, 2H), 2.67 (t, 2H), 4.28-4.32 (m, 2H), 5.13 (s,

1H), 7.45 (s, 1H), 7.55 (s, 1H).

Synthesis of 2-bromo-6-(cyanomethyl)-4-n-hexylbenzonitrile (HxM5), 8. In a small vial,

HxM4 (113 mg, 0.3 mmol), H2O (0.5 ml), and DMSO (3 mL) were added. The reaction

mixture was then heated to 80° C for 12 hours. The reaction mixture was then added, via

pipet, to a beaker containing 10% HCl (5 mL) and EtOAc (5 mL) to quench the reaction.

This mixture was then extracted with EtOAc, rinsed with brine and dried over Na2SO4 to

give the desired product (90.5 mg, 99%): 1H-NMR (CDCl3): δ= 0.89 (t, 3H), 1.30(m, 6H),

1.60-1.64 (m, 2H), 2.67 (t, 2H), 3.99 (s, 2H), 7.41 (s, 1H), 7.50 (s, 1H).

Synthesis of ethyl 2-(5-bromo-4-cyanopyridin-3-yl)-2-cyanoacetate (PyM4), 9. A Schlenk

flask containing 3,5-dibromopyridine-4-carbonitrile (90.3 mg, 0.35 mmol) and a 25-mL

round bottom flask containing t-BuOK (157mg, 1.40 mmol) were flushed with nitrogen for

30 minutes. DMF (2 mL) was added to the Schlenk flask and the solution was stirred until

the starting material completely dissolved. DMF (3 mL) and EtO2CCH2CN (166 mg, 1.47

mmol) were added to the 25-mL round bottom flask, which was stirred until the t-BuOK

completely dissolved. The contents of the round bottom flask were then added to the Schlenk

flask dropwise via a syringe. The reaction was stirred for 2 hours under a constant stream of

(50)

rinsed with brine, and dried over Na2SO4 to give the desired product (99.3 mg, 95%): 1 H-NMR (CDCl3): δ=1.38 (t, 3H), 4.39 (q, 2H), 5.15 (s, 1H), 8.85 (s, 1H), 8.92 (s, 1H).

Synthesis of 3-bromo-5-(cyanomethyl)pyridine-carbonitrile (PyM5), 10. In a microwave

reaction tube, PyM4 (87.3 mg, 0.3 mmol), H2O (0.01 ml), and DMSO (0.5 mL) were added.

The microwave was set to 200 W and a maximum pressure of 150 PSI. The tube was heated

to 75° C over a 3 minute period, and held at that temperature for 5 minutes. The reaction

mixture was then added, via pipet to a beaker containing 10% HCl (2.5 mL) and EtOAc (2.5

mL) to quench the reaction. This mixture was then extracted with EtOAc, rinsed with brine

and dried over Na2SO4 to give the desired product (64.2 mg, 99%): 1H-NMR (DMSO):

δ=4.98 (s, 2H)), 8.82 (s, 1H), 9.08 (s, 1H).

Synthesis of bis(ethyl-2-cyano-2(2-cyano-3-fluorophenyl)acetate) (FM4), 11. A Schlenk

flask containing 2,6-difluorobenzonitrile (48.65 mg, 0.35 mmol) and a 25-mL round bottom

flask containing K2CO3 (96.7 mg, 0.7 mmol) were flushed with nitrogen for 30 minutes.

DMF (2 mL) was added to the Schlenk flask, cooled to 5° C and the solution was stirred. A

solution of EtO2CCH2CN (166 mg, 1.47 mmol) in DMF (3 mL) was added to the 25-mL

round bottom flask. The contents of the round bottom flask were then added to the Schlenk

flask dropwise via a syringe. The reaction was kept between 5-10° C and stirred for 18 hours

under a constant stream of nitrogen. The reaction was then quenched with a 10% HCl

solution, extracted with EtOAc, rinsed with brine, and dried over Na2SO4 to give the desired

product (62.6 mg, 77%): 1H-NMR (DMSO): δ=1.21 (t, 3H), 4.22 (q, 2H), 5.15 (s, 1H), 7.28

(51)

Synthesis of 2-(cyanomethyl)-6-fluorobenzonitrile (FM5), 12. In a small vial, FM4 (70

mg, 0.3 mmol), H2O (0.5 ml), and DMSO (3 mL) were added. The reaction mixture was then

heated to 80° C for 12 hours. The reaction mixture was then added, via pipet, to a beaker

containing 10% HCl (5 mL) and EtOAc (5 mL) to quench the reaction. This mixture was

then extracted with EtOAc, rinsed with brine and dried over Na2SO4 to give the desired

product (44.8 mg, 97%): 1H-NMR (CDCl3): δ=4.01 (s, 2H)), 7.28 (t, 1H), 7.49 (d, 1H), 7.72

(52)

E. References

(1) Brannock, M. Investigating the Reactivity of Benzylic Nitriles in the Synthesis of a

Multiple Cyano-Containing Polymer, Ph.D., NCSU, Raleigh, NC 2012.

(2) Suto, Y.; Kumagai, N.; Matsunaga, S.; Kanai, M.; Shibasaki, M. Org. Lett. 2003, 5

(17), 3147–3150.

(3) Wang, A.; Zhang, H.; Biehl, E. R. Heterocycles 2000, 52 (3), 1133–1141.

(4) Zhao, X. Y.; Wang, M. Z.; Sun, Z. Y.; Niu, C. M.; Xiao, J. J.; Tang, E. J. Plasma

Process. Polym. 2012, 9 (5), 468–472.

(5) Lustoň, J.; Kronek, J.; Markus, O.; Janigová, I.; Böhme, F. Polym. Adv. Technol.

2007, 18 (2), 165–172.

(6) Kambe, N.; Iwasaki, T.; Terao, J. Chem. Soc. Rev. 2011, 40 (10), 4937.

(7) Lang, R. De; Brandsma, L.; Kramer, H.; Seinen, W. Tetrahedron 1998, 54, 2953–

2966.

(8) Bilodeau, F.; Brochu, M. C.; Guimond, N.; Thesen, K. H.; Forgione, P. J. Org. Chem.

2010, 75 (5), 1550–1560.

(9) Harusawa, S.; Yoneda, R.; Omori, Y.; Kurihara, T. Tetrahedron Lett. 1987, 28 (36),

4189–4190.

(10) Stazi, F.; Maton, W.; Castoldi, D.; Westerduin, P.; Curcuruto, O.; Bacchi, S. Synthesis

(Stuttg). 2010, 2010 (19), 3332–3338.

(11) Recio, III, A.; Heinzman, J. D.; Tunge, J. A. Chem. Commun. 2012, 48 (1), 142–144.

(12) You, F.; Twieg, R. J. Tetrahedron Lett. 1999, 40 (I 999), 8759–8762.

(53)

(54)

A. Introduction

1. Types of polymerizations

Polymerizations can be subcategorized in two major types, step growth or chain

growth (Figure 3-1). In a step growth polymerization, as the polymerization begins the

monomers react to form dimers, which then react to form trimers and tetramers and so on.1 The monomer is rapidly used up, and the reaction continues by oligomers reacting with each

other. The average molecular weight of the polymer grows slowly over time. To achieve

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Figure 3- 1: Graphical representation of chain growth polymerization vs. step growth polymerization. The chain growth system shown here is for the case where initiation is slow relative to propagation, thus the time for each chain to ‘start’ and ‘stop’ is fast compared to the rate at which new chains ‘start’.3

A chain growth polymerization begins by reaction between an initiator and the

monomer. The propagation continues as the monomers are added to one end of the chain one

by one. Because only initiators or propagating chains are reactive, the molar mass of the

polymer builds more quickly.4_{In the chain growth depicted in Figure 3-1, slow initiation and}

rapid propagation results in polymer chains with high molecular weights achieved at the

early stages of the reaction, and an average molecular weight that remains fairly unchanged

(56)

Step growth polymerizations can be converted to chain growth polymerizations if the

reaction of monomer + monomer is slower than the reaction of growing chain + monomer.

Systems that can be tailored to behave with chain growth type kinetics have been

reviewed.5,6,7 Generally, some type of initiator is added that reacts faster than the monomer can react with itself. Scheme 3-1 shows a proposed chain growth reaction using the

monomer employed in this work. If both the initiator shown and the growing polymer chain

react with the monomer faster than the monomer reacts with itself, chain growth kinetics

should be observed. This type of polymerization will be explored in this chapter.

Scheme 3- 1: Example of chain growth polymerization

To obtain a relative measure of the initiation rate of a chain growth system various

groups have added a different molar percentage of the initiator to the reaction, ranging from

one to ten percent.8_{In the cases where the initiation is the fastest step, as the initiator to}

monomer ratio is increased, the weight of the polymer will decrease due to more chains being

(57)

started from the outset. However there are cases of chain growth polymerizations that have a

low percentage of initiation or slow initiation relative to propagation.

2. Palladium cross-coupling chemistry

While there are a variety of polymerization routes that have been investigated

throughout the literature, in the project described in this thesis, it was decided to place

significant focus on palladium cross-coupling chemistry. Previous group members were

successful using palladium cross coupling in the synthesis of model oligomers and there was

some putative success in polymerization to form the desired precursor polymer. In reviewing

prior work, however, some inconsistences were discovered. The primary one was a

mathematical error in the analysis of gel permeation chromatography (GPC) data of the

precursor polymer. When analyzing GPC data, a GPC of a known mixture of polystyrenes

with different weights is collected. The time it takes for each of the polymers in the mixture

to come through the GPC column at a particular flow rate and temperature is then plotted

versus the natural log of the weight of each polymer. This plot give a line, the slope of which

is then used to analyze polymer samples of unknown weight. However, it was realized that

previous group members reversed the axes on this plot and were thus calculating molecular

weights incorrectly.9 It was also found that the microwave reactor was not operating under the conditions they reported.10 It seemed as if it was not heating to the temperatures it was set to or not holding at those temperatures. In an attempt to alleviate this other microwave

reactors were used, but this did not result in the obtaining the products that have been

reported by previous group members. Thus, the previously reported work had to be

(58)

In rethinking this project, the literature associated with palladium cross-coupling

reactions was reviewed. There are many known palladium cross-coupling reactions that

utilize zinc, boron, and magnesium metals in the pre-transmetallation of the substrate, such as

the Negishi,11,12 Suzuki,13,14,15 and Kumada16,17 couplings respectively.18,19,20 Examples of each of these reactions are shown in Figure 3-2.

Figure 3- 2: Example palladium cross-coupled reactions

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There are also examples of palladium cross-‐coupling reactions that utilize a

direct deprotonation of the substrate before it enters the catalytic cycle. This type of

reaction requires a reasonably acidic proton on the targeted carbon. Scheme 3-‐2 gives

an example of such a substrate that has been successfully reacted under palladium

coupling conditions, though it requires an elevated temperature and six hour reaction

time to work efficiently.

Scheme 3- 2: Example Pd cross-coupling reaction w/o organometallic reagent21

3. Nucleophilic aromatic substitution reactions

When considering the formation of a bond between an sp3_{carbon and a phenyl}

carbon in a polymerization reaction, one can also contemplate Nucleophilic Aromatic

Substitution (NAS) type reactions.22,23 There are many examples of NAS-based

polymerizations.24 One reaction that has been shown many times is the formation of poly-ethers (Scheme 3-3). These can be considered as templates for developing a NAS that could

(60)

Scheme 3- 3: Example nucleophilic aromatic substitution polymerization

With all of this in mind, the investigation into the polymerization of the

methyl-bromide monomer, MeM5, started by following the microwave reaction conditions

reportedly optimized by previous group members. In the beginning the target was to repeat

the conditions that were found to work most efficiently for the hexyl monomer, HxM5, with

the methyl analog. It became clear that these conditions were not working as reported. At that

point HxM5 was directly synthesized, and attempts to reproduce the polymerizations

previously reported were attempted but yielded oligomers at best.10 After finding these inconsistencies the project transformed into an investigation of the overall polymerization the

precursor polymer. Multiple modifications were made to the polymerization reactions, and

the monomer itself, in an attempt to produce larger polymers. This chapter will discuss the

reasoning behind those modifications, and the results of them.

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B. Results and Discussion

1. Palladium cross-coupling polymerizations of alkyl derivatives

The first polymerizations that were attempted utilized palladium cross coupling

chemistry. Previous reports indicated that HxM5 could be polymerized under microwave

reaction conditions to produce a polymer with a molecular weight of Mn of 2500 and Mw of

3300, with a PDI of 1.31 in a 95 percent yield. It was further reported that synthesis of these

polymers required 2-bromobenzonitrile as an initiator as shown in Scheme 3-4. As described

previously, the initiator 2-bromobenzonitrile was purported to convert this step-growth

polymerization to a chain-growth polymerization. However, it was unclear from previous

reports whether these reactions proceeded best under microwave or thermal heating

conditions.26 Thus, both heating conditions were explored as described below.

a. Microwave conditions

Scheme 3- 4: Initial polymerization optimized by previous group members9,10

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The first reactions attempted were the polymerization of

2-bromo-6-(cyanomethyl)-4-methylbezonitrile (MeM5). These reactions followed the conditions that were reported as

being successful for previous group members synthesizing the polymer derived from the

4-hexyl (HxM5) analog. In the reactions described by previous group members, the catalyst,

ligand and initiator were placed in a microwave reaction tube, and heated to approximately

70° C. In a separate vial, the monomer and base were premixed, then added to the reaction

tube. It was found that the mixture of monomer and base was extremely viscous, resulting in

some of the mixture not transferring to the reaction tube.9 It was found that starting with the pre-mixture of base and monomer in the microwave reaction tube and the catalyst, ligand,

and initiator in a separate vial, led to all of the reagents being present in the correct ratios.

This change is illustrated schematically in Figure 3-4. In addition, there seemed to be quite a

difference between the results of the work with MeM5 when compared to the polymerization

of HxM5 prepared by previous group members. These differences were observed whether

(63)

Figure 3- 3: A schematic representation of the change to the set up for the polymerization reactions. A.) set up from previous group members B.) Updated set up

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Utilizing the updated reaction conditions, a series of reactions were prepared with the

goal of synthesizing the precursor polymer with an average molecular weight of 10,000 to

20,000 or higher. It was thought that some changes to the power (watts), temperature, or time

would result in a more efficient reaction. Changes to these values (Table 3-1) were studied

systematically without altering the percentage of initiator used or the volume of the reaction.

The equivalents of catalyst, ligand, and base were also kept constant. From the data in Table

3-1, it was concluded that these changes to the reaction failed to improve the reaction. The

highest average molecular weight obtained was under 1,000 and the highest yield was a mere

32%. These values were significantly lower than 8,000 average molecular weight polymer

(65)

Table 3-‐ 1: Microwave polymerization of MeM5

Power Temperature Ramp time Hold time Recovereda Yield

(Watts) (°C) (mins) (mins) (%)

300 130 3 10 Low Mw 25

250 130 3 10 Low Mw 20

200 130 3 10 Low Mw 7

300 140 3 10

Insoluble

material N/A

300 130 3 15

Low Mw - Insoluble

material

9

300 130 3 20

Insoluble

material N/A

300 120 3 10 Low Mw 23

300 110 3 10 Starting material N/A

300 120 3 15

Low Mw - Insoluble

material

12

300 120 3 20

Insoluble

material N/A

300 130 5 10 Low Mw 32

300 130 10 10

Insoluble

material N/A

Conditions: 5% Initiator, 150 psi, 1.2 mL; a_{Low Mw= an average Mw < 200}

b. Thermal conditions

With limited success in the preparation of the methyl polymer under microwave

reaction conditions, the focus of the project turned to thermal reaction conditions. Thermal

conditions were tried by previous group members for the HxM5 monomer, but with only

limited variability in reaction conditions. It was decided that a systematic study of reaction

conditions was necessary if a polymer with an average molecular weight of 10,000-20,000

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reaction, temperature, and the volume of the reaction, while the percentage of initiator and

the equivalents of t-BuOK were held constant. Then, the catalyst loading was doubled for a

set of the reactions to determine its effect on the polymerizations.

Knowing that the use of thermal conditions takes significantly longer than that of

microwave reaction, the initial reaction time was set at one hour. However, it took at least

twelve hours at 50° C to see any sign of reaction. Eventually, the time of the reaction was

extended to 24 hours. Using this time of reaction resulted in the largest yield of polymer.

With thermal heating, it was important to determine the effect of temperature, so

reactions were run at multiple temperatures (30° - 90° C). Combining all of the reagents lead

to a darkening of the solution, which could be explained as a sign of transmetallation

occurring, but this was not confirmed. Other than the darkening of the solution, presumably

cause by the deprotonation of the monomer, there were no signs of reaction until the

temperature reached 50° C. It was also found that heating the reaction above 70° C resulted

in the evaporation of the solvents before the reaction was complete. These higher

temperatures also resulted in the formation of insoluble materials that could not be

characterized.

(67)

Table 3- 2: Thermal polymerization of MeM5

Catalyst &

Ligand Temperature

Reaction

time Volume _Recovereda Yield

(Mol eq) (°C) (h) (mL) (%)

0.02 50 1 1.5 Starting Material 0

0.02 50 6 1.5 Starting Material 0

0.02 50 12 1.5 Low Mw - Starting _Material 5

0.02 50 24 1.5 Low Mw - Starting _Material 22

0.02 50 24 1.5 Low Mw 28

0.02 70 12 1.5 Low Mw 30

0.02 70 12 3 Low Mw 51

0.02 70 12 6 Low Mw 47

0.02 70 12 9 Low Mw 42

0.02 70 24 3 Low Mw 62

0.02 70 24 6 Low Mw -

Insoluble Material 25

0.02 70 24 9 Insoluble Material 0

0.04 70 12 1.5 Mw= 543 _{PDI= 3.6} 42

0.04 70 12 3 Mw= 786

PDI= 2.6 49

0.04 70 12 6 Mw= 443 _{PDI= 4.2} 44

0.04 70 12 9 Low Mw 47

0.04 70 24 3 Mw= 1065 _{PDI= 2.3} 65

(68)

One of the major disadvantages of the microwave reaction was the limited amount of

solvent that could be used in the reaction tubes. If the solubility of the polymer is at all a

factor to the limited extent of reaction, then increasing the volume of the reaction could have

an effect on the extent of the polymerization. This was shown to be true when the volume of

the reaction was raised from 1.2-1.5 mL to 3 mL. However, increasing the volume past that

point seemed to create other problems in the reaction, leading to higher PDIs, lower yields, or

the formation of lower molecular weight polymer. The most likely cause of these difficulties

is the slowing reaction rate as the concentration of the reagents was decreased.

1_{H NMR was used to determine if the polymerization of the monomer had occurred.}

If the polymerization occurred, the appearance of a peak for the methine proton (5.8 ppm) on

the carbon linking the two monomers was expected and was observed (Figure 3-5). The

additional peaks in the aliphatic region were due to the difficulty in removing the ligand,

PCy3, from the reaction. However, the spectrum overall is consistent with the presence of the

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Figure 3- 4: Example 1H NMR of the desired product from the polymerization of MeM5.

Example IRs of both MeM5 monomer and polymer are shown in Figure 3-6. These

IRs fit the expected spectrum of the desired polymer, except for the smaller than expected

signal for the cyano stretch (typically between 2100 and 2300 cm-1), which are usually stronger in other polymers.10 The IRs of both the monomer and polymer display two distinct cyano stretches as expected from one cyano on the benzene ring and the other on the benzylic

carbon. When the cyano stretches observed for the polymer are compared to those of the

monomer, they are blue shifted, of weakened intensity, and broadened.

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Figure 3- 5: IRs of MeM5 monomer (blue) and polymer (red); Arrows show effect of the polymerization on the cyano stretches.

c. Effect of Initiation

After finding the best reaction conditions for the polymerization, the importance or

value of including the initiator was examined. A series of reactions were prepared keeping all

conditions the same except the percentage of initiator added. Previous work by other group

members concluded that 5% initiator gave the overall largest molecular-weight polymer with

a reasonable poly-dispersity index (PDI). There was evidence that when the percentage of

initiator was lower, the molecular weight increased, but the PDI was higher than desirable (>

3).10,9 When the percentage of the initiator was increased, the product contained a lower molecular weight polymer. To confirm the previous data, reactions were conducted with

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showing the correspondence between the percentages of initiator and the Mn/Mw of the

collected product is presented in Figure 3-7. Each reaction produced oligomers of effectively

the same size. It was concluded the inclusion of the initiator had little to no effect on the

reaction, other than limiting the yield of the oligomers that were collected.

Figure 3- 6: Graphical depiction of the percentage of initiator added and its effect on the molecular weight of the polymer obtained.

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Upon the observation that the presence of an initiator made little to no difference in

the reaction, a small set of reactions were prepared to ensure that the best conditions for the

polymerization were being used. The conditions altered were the time and volume of the

reaction. While each reaction, with the exception of the one prepared with a 9 mL volume,

produced a polymer, the largest polymer and greatest yield were observed with a 24 hour

reaction time in 3 mL of solution.

Table 3- 3: Thermal polymerization of MeM5 w/o initiator

Reaction

time Volume Recovereda Yield

(h) (mL) (%)

12 3

Mw= 685

PDI= 3.4 53

12 6

Mw= 438

PDI= 4.8 41

12 9 Low Mw 47

24 3

Mw= 1103

PDI= 2.2 70

Conditions: 70° C, 0.04 equivalents of Catalyst and ligand

a_{Low Mw= average Mw < 200}

To further understand what was occurring during these reactions, three additional

reactions were run. Two reactions were performed under thermal conditions; one with 5%

initiator while the other had no initiator. The third reaction, which also included 5% initiator,

was reacted via microwave conditions. After the reactions were complete, the solvent was

removed, and Mass Spectrometry (MS) examined the entire remaining reaction mixture, the

results of which can be observed in Table 3-4. From these data, two observations were