Author Manuscript
Title: Orthogonal Cationic and Radical RAFT Polymerizations to Prepare Bottle-brush Polymers
Authors: Wei You, Dr.; Joji Tanaka; Satu H ¨akkinen; Parker Boeck; Sergei Sheiko; Sebastien Perrier; Yidan Cong
This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofrea-ding process, which may lead to differences between this version and the Version of Record.
To be cited as: 10.1002/anie.202000700
Orthogonal Cationic and Radical RAFT Polymerizations to
Pre-pare Bottlebrush Polymers
Joji Tanaka,
[a]Satu Häkkinen,
[b]Parker T. Boeck,
[a]Yidan Cong,
[a]Sébastien Perrier,
[b][c][d]Sergei S.
Sheiko,
[a]and Wei You*
[a]Dedication Orthogonal Polymerization
[a] Dr. J. Tanaka, P. T. Boeck, Y. Cong, Prof. S. Sheiko, Prof. W. You Department of Chemistry
University of North Carolina at Chapel Hill Chapel Hill, North Carolina, 27599-3290, U.S.A. E-mail: [email protected]
[b] S. Häkkinen, Prof. S. Perrier Department of Chemistry University of Warwick
Gibbet Hill Road, Coventry, CV4 7AL, U.K. [c] Prof. S. Perrier
Warwick Medical School University of Warwick
Gibbet Hill Road, Coventry, CV4 7AL, U.K. [d] Prof. S. Perrier
Faculty of Pharmacy and Pharmaceutical Sciences, Monash University
381 Royal Parade, Parkville, VIC 3052, Australia
Supporting information for this article is given via a link at the end of the document.
Abstract: We report a novel orthogonal combination of cationic and radical RAFT polymerizations to synthesize bottlebrush polymers using two distinct RAFT agents. Selective consumption of the first RAFT agent is used to control the cationic RAFT polymerization of a vinyl ether monomer bearing a secondary dormant RAFT agent, which subsequently allows side-chain polymers to be grafted from the pendant RAFT agent by a radical-mediated RAFT polymerization of a different monomer, thus completing the synthesis of bottlebrush polymers. The high efficiency and selectivity of the cationic and radical RAFT polymerizations allow both polymerization to be conducted in one-pot tandem without intermediate purification.
Reversible Addition Fragmentation chain Transfer (RAFT) has grown in the last 20 years as one of the most versatile polymerization techniques, enabling the control of molecular weight distribution, branched architecture, and chemical functionality for a wide range of polymeric systems.[1] Based on the RAFT versatility, recent years have seen an expansion of using various RAFT agents beyond traditional thermally initiated radical polymerizations.[2] One emerging direction is the use of
RAFT agents that facilitate other classes of polymerization such as Anionic Ring Opening Polymerization (AROP)[3] and cationic RAFT polymerization,[4] in synergy with radical mediated RAFT polymerization, to enable dual copolymerization strategies [4b, 4d,
5] and access unique copolymer compositions.[3c, 4b] Another
emerging research area is the exploitation of the versatile photochromic behavior of RAFT agents for photo-controlled polymerization upon direct photo-fragmentation of the R-group of the RAFT agent by UV-Vis irradiation (following the iniferter mechanism)[6] or through a photocatalyst (photoinduced-electron/energy transfer).[7] In particular, Xu and Boyer demonstrated the wavelength dependency of selectively photo-activated RAFT agents[6b, 8] to allow selective RAFT control for
Through selective photo-fragmentation, Matyjaszewski, Boyer and coworkers elegantly demonstrated an orthogonal
iniferter-RAFT polymerization.[9b] In their case (Fig. 1a), a RAFT
agent-bearing methacrylate monomer was first polymerized orthogonally to produce a linear polymeric chain with pendant RAFT agents remaining intact during the polymerization of the methacrylate unit. The pendant RAFT agents were then used to initiate a second monomer and form grafted chains, thus leading to bottlebrush polymers (following the so-called ‘grafting-from’ approach).[9b] However, this approach requires thorough
purification of intermediate products to remove unreacted monomers that bear RAFT agents prior to the second RAFT polymerization. In the current literature, orthogonal polymerizations through selective RAFT processes have only been demonstrated with photo-controlled methacrylate polymerizations where RAFT agents bearing acrylic-based R-group remain dormant during the first polymerization.[9] Moreover, the photo-control relies on irradiation by different wavelengths, which are prone to undesirable ‘cross-over’ due to overlapped photoresponse from different RAFT agents that co-exist in the system.[11]
Figure 1. Generalized structure of a RAFT agent and comparison of two-step syntheses of bottlebrush macromolecules via orthogonal RAFT in (a) previous literature[9b] and (b) this work.
We envision that an alternative approach to enable orthogonal RAFT polymerizations is through selective cationic and radical chain transfer using disparate RAFT agents (e.g., Fig. 1b). We noticed that in the literature reports where one common RAFT agent was used to facilitate both cationic and radical chain transfer to allow interconvertible cationic and radical RAFT copolymerizations,[2a, 4d, 11b] a lack of cationic RAFT chain extension was typically observed from the acrylic terminal RAFT polymer. This can be explained by incompatible cationic fragmentation of the R-group which bears an electron withdrawing group (EWG) (Scheme 1).[5b, 5c] In contrast, EWGs
can favor radical fragmentation as they (EWGs) increase radical stabilization energy.[5b, 12]
These fundamental insights inspired us to design a novel selective RAFT process to achieve orthogonal polymerizations without relying on an external control (e.g., light). Our design employs one RAFT agent to selectively control the cationic polymerization of a vinyl ether monomer bearing a second RAFT agent which remains dormant during cationic chain transfer events (Fig. 1b). Subsequently, the pendant RAFT agent enables the successive radical-centered RAFT polymerization of another monomer, furnishing the structurally well-defined bottlebrush polymers. Furthermore, due to the high efficiency and selectivity of the cationic and radical RAFT polymerizations, we can conduct both polymerizations as one-pot tandem without intermediate purification.
Experimentally, the dual selective cationic and radical RAFT polymerization system was realized by employing
1-isobutoxyethyl diethylcarbamodithioate (IBDTC) to control the
cationic polymerization of 4-(vinyloxy)butyl 2-(((butylthio)carbonothioyl)thio)propanoate (VBBTP), followed by the radical polymerization of acrylic monomers controlled by the RAFT agent embedded in the VBBTP side chains (Fig. 2). IBDTC was selected as the cationic RAFT agent due to its favorable fragmentation of the carboxonium R-group and excellent cationic chain transfer properties of dithiocarbamates arising from the Z-group.[4d] VBBTP was designed to combine
high reactivity of trithiocarbonates towards radical addition[13] and radical stabilizing electron-withdrawing R-group for highly effective radical fragmentation,[14] but incompatible for cationic fragmentation (Scheme 1);[5b, 5c] the latter is essential to achieve the orthogonal RAFT polymerizations.
3
Keeping close to the previously reported condition for cationic RAFT polymerization,[4d] VBBTP was polymerized by a cationic process in dichloromethane (DCM) at0 – 48 °C using TfOH as the initiator at fixed molar concentrations ([VBBTP] = 0.5 M, [TfOH] = 1 mM). The following degrees of polymerization (DPs) of PVBBTPn was targeted: n = 10, 24, 45, 143 by
changing the molar ratio of [VBBTP]/[IBDTC], taking into the account chains generated from TfOH initiation (Fig. 2, Table 1). Completion of the polymerization was confirmed after 10 minutes by 1H-NMR spectroscopy from the disappearance of vinylic proton (δ = 6.41 – 6.50 ppm, Fig. S1), indicative of quantitative monomer conversion in all cases. For work up, the polymerization was first quenched with a mixture of methanol and triethylamine and precipitated into methanol. Pleasingly, Size Exclusion Chromatography (SEC) of PVBBTPn shows
appreciably narrow monomodal molar mass distributions (Ð = 1.14 – 1.37, Table 1) and a shift towards higher molar masses with increasing [VBBTP]/[IBDCT] ratio (Fig 2). Even though the SEC analysis relative to polystyrene standards does not provide accurate information about the targeted theoretical molar mass (Mn,th), the systematic difference in the measured number
average molar mass (Mn,SEC) is a good indication of molar mass
control. Nevertheless, the quartet signal in 1H NMR analysis at δ = 4.76 - 4.85 ppm, which corresponds to CH of the R-group adjacent to the trithiocarbonate (Hh, Fig. S1, Fig. 3), has the
expected 1:2 ratio (by integration) with VBBTP CH2 signal at δ =
4.09-4.22 ppm (Hf, Fig. S1, Fig. 3), thus confirming complete
absence of chain transfer of VBBTP side chains during the cationic polymerization. Furthermore, end group signals from carboxonium initiation (Hp, Fig. 3) and the terminal vinyl-ether
next to the dithiocarbamate (Hb*, Fig 3) is indicative of the
selective consumption of IBDTC.
Figure 2. Differential molar mass distributions (THF-SEC, PS standards) of PVBBTPn (n = 10, 24, 45, 145 Table 1) prepared by selective cationic RAFT of
VBBTP controlled by IBDTC.
Table 1: PVBBTPn obtained from selective cationic RAFT polymerization
Entrya [VBBTP] : [IBDTC] : [TfOH]b Mn,th (kg mol-1) c Mn,SEC (kg mol-1) d Ð d PVBBTP10 10 : 1 : 0.02 3.6 2.7 1.14 PVBBTP24 25 : 1 : 0.05 8.3 4.8 1.25 PVBBTP45 50 : 1 : 0.1 16 7.3 1.37 PVBBTP143 200 : 1 : 0.4 48 11 1.34 a.
DP of PVBBTPn determined by p × [VBBTP]/([TfOH] + [IBDTC]), where p
corresponds to monomer conversion determined by 1
H-NMR. b.
relative equivalence with respect to fixed monomer initiator concentrations ([VBBTP] = 0.5 M, [TfOH]= 1 mM). c.
Mn,th calculated from DPPVBBTP × MWVBBTP + MWIBDTC,
where DPPVBBTP, MWVBBTP and MWIBDTC corresponds to degree of
polymerization of VBBTP and molar mass of VBBTP and IBDTC respectively.
d.
Figure 3. 1H-NMR (CDCl3) VBBTP monomer (black), PVBBTP10 (red) and
PVBBTP10-g-PMA8 (blue). Changes in chemical shifts are highlighted by
asterisks.
Figure 4. Differential molar mass distribution (dRI, THF-SEC, PS) of PVBBTPn-g-PMAm brush polymers (Table S1) prepared by grafting from
VBBTP side chains from respective PVBBTPn backbone.
Poly(methyl acrylate) (PMA) side chains were then grafted from the PVBBTPn by radical RAFT polymerization from the
VBBTP side chains. DP of the PMA side chains were estimated by the monomer conversion and the initial molar ratio of MA to the VBBTP side chains (DPPMA = MA conversion ×
[MA]0/[VBBTP]sc). Polymerizations were carried out at 70 °C,
using AIBN as the radical initiator ([VBBTP]sc/[AIBN]0 = 20). To
prevent the formation of intermolecular brush-brush by bimolecular radical combination of growing side chains, polymerizations were stopped at low monomer conversions (Fig. S2, p < 62 %, Table S1)[15] and relatively low initiator consumption to minimize termination events (typically < 1 % terminated grafts).[16] In all cases, SEC analysis showed a clear
evolution of monomodal molar mass distribution PVBBTPn
-g-PMAm from the respective PVBBTPn scaffolds (Fig. 4).
Figure 5. A) Molar mass distribution of PVBBTP143 (red) and PVBBTP143
-g-PBA15 (gold). B) AFM micrograph of PVBBTP143-g-PBA15 LB monolayers on a
mica substrate, scale bar = 100 nm
PVBBTP10-g-PMA8 bearing short PMA side chains (DP = 8)
was synthesized and isolated by precipitation into hexane for end group analysis by 1H-NMR spectroscopy (Fig. 3).
Quantitative grafting efficiency from VBBTP side chains was evident from the complete disappearance of the aforementioned quartet C-H peak corresponding to the R-group (Hh, Fig. 3)
replaced by terminally inserted MA at δ = 4.95 - 4.87 ppm (H u-terminal, Fig. 3). Importantly, the retention of terminal vinyl-ether
C-H signal (C-Hb*terminal, Fig 3, δ = 6.02 - 6.15 ppm) indicates the lack
of chain transfer of the backbone terminal RAFT end group during the radical polymerization of MA. This observation contrasts the previously reported photo-mediated orthogonal RAFT system (Fig. 1a), as the terminal trithiocarbonate on the
for AFM were prepared by the Langmuir Blodget (LB) technique that yielded monolayers of densely packed worm-like macromolecules (Fig. 5B). From molecular images, we measured a number average contour length of Ln= 17 nm,
corresponding to a DP of 68 for the backbone (DP= Ln/0.25 nm),
smaller than expected (DP of 143). We attribute this as an inherent limitation of cationic polymerization to generate long linear chains compared to other techniques,[17] On the other hand, the distance between neighbouring macromolecules (brush width) d = 16.8 nm is consistent with the targeted DP of side chains Nsc =15.[18] These data provide a direct evidence for
the successful synthesis of bottlebrush polymers via selective
5
Given the excellent ‘orthogonality’ observed for our system, tandem dual cationic-radical RAFT polymerization was then carried out in one pot, by sequential monomer addition (Fig. 6B) and as a one-shot selective polymerization of VBBTP and MA (Fig. 6C).[4c, 12, 19] Final molar concentrations of
[MA]0/[VBBTP]0/[IBDTC]0 = 6/0.25/0.01 M were used in both
cases to target the same backbone and side chain DP for comparison. To accommodate reactions for the dual cationic-radical RAFT system, toluene was used as a compatible solvent. In both cases, the initial cationic RAFT polymerization of VBBTP was quenched with MeOH/TEA after 20 minutes prior to the thermally initiated radical RAFT polymerization at 70 °C for 1 hour. In the case of sequential addition of MA, no additional solvent was added expect for AIBN in its stock solution. Successful formation of PVBBTP45-g-PMA17 by the tandem
radical RAFT from a crude post-cationic RAFT reaction was indicated by SEC chromatograms, showing a clean shift from the PVBBTP45 backbone (Fig. 6B, Mn,SEC = 7.5 kg mol-1 Ð =
1.18) towards a higher molar mass (Fig. 6B, Mn,SEC = 15 kg mol-1,
Ð = 1.15, p = 69 %), with no significant difference from
PVBBTP45-PMA18 prepared by prior isolation of the PVBBTP45
scaffold (Fig. 6A, Mn,SEC = 20 kg mol-1, Ð = 1.20, p = 75 %). On
the other hand, one-shot tandem cationic-radical RAFT copolymerization of VBBTP and MA resulted PVBBTP45 having
comparatively slightly lower Mn,SEC (Mn,SEC = 5.1 kg mol-1, Ð =
1.23) in spite of a quantitative monomer conversion (Fig. S5). The presence of small molecular weight species in the SEC chromatograms indicates early termination during the cationic RAFT polymerization of VBBTP, possibly due to the contamination of water arising from the MA as the comonomer mixture ([MA]0 = 60 % vol), as an aldehyde peak at 9.81 ppm
was observed by NMR (Fig. S5). Subsequently grafting PMA sidechains (p = 74 %) resulted in a bimodal distribution, with a shift towards a higher molar mass (Fig. 6C). Comparatively lower Mn,SEC was obtained from deconvoluting the molar mass
distribution (Fig. 6C, *Mn,SEC = 10.5 kg mol-1, Ð = 1.10), due to a
shorter PVBBTP backbone and the consumption of MA by early terminated PVBBTP species; the latter was observed as smaller broad PMA species (*Mn,SEC = 2.8 kg mol-1, Ð = 2.35).
In conclusion, we demonstrate a new form of orthogonal polymerizations by selective cationic RAFT control of monomers bearing a second RAFT agent which remains dormant during the cationic RAFT process. This allows a second monomer to be grafted from the pendant RAFT agents via a radical RAFT process to synthesize bottle brush polymers. Although the backbone is limited in length and monomer selection, the compositions of the grafts is less limited and can be further expanded by changing the Z-group of the pendent RAFT agents, as the selectivity arises from the R-group. In the future this approach can be explored further with other selective polymerisation to simplify complex macromolecular designs.
Acknowledgements
This work was financially supported by the National Science Foundation (NSF) under Award CHE-1808055 (J.T., P.B.T. and W.Y.) and DMR-1921835 (Y.C. and S.S.). S.H. and S.P. acknowledge financial support from Royal Society Wolfson Merit Award (WM130055, SP) and Lubrizol (SH). We want to thank Dr.
Marc A. ter Horst for help with NMR characterization, Sally Lewis and Dr. Aaron Teator for help with SEC characterization, and Prof. Frank Leibfarth and Travis Varner for valuable advice on cationic polymerizations.
Keywords: Orthogonal polymerization, Selective RAFT process, cationic RAFT, bottlebrush polymers
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Entry for the Table of Contents
Here, selective chain transfer between two RAFT agents has enable orthogonal cationic and radical RAFT polymerizations to synthesize bottlebrush polymers.
