Synthesis of Block Copolymers by Combination of ATRP and Visible Light-

The possibility of using Mn₂(CO)₁₀ combination with suitable co-initiators such as CH₂Cl₂ in the presence Ph₂I⁺PF₆^- as a novel initiating system for visible light induced cationic polymerization [425] prompted us to employ the same redox process for the mechanistic transformation. Accordingly, block copolymers were obtained via two discrete steps. In the first step, a series of halide end-functional PSt with different molecular weights was simply synthesized by ATRP (Table 3.1). In the second step, free radical promoted cationic photopolymerization of CHO or IBVE initiated by the photolysis of Mn2(CO)10 in the presence of the obtained macro-coinitiators resulted in formation of the corresponding block copolymers, PSt-b-PCHO or PSt-b-PIBVE, respectively (Table 4.11). Apparently the molecular weights increased after

succesfull growth of the second segment (PCHO) (Figure 4.27).The overall process is presented on the example of CHO polymerization (Figure 4.28).

Table 4.11 : Properties of the block copolymers synthesized by visible light induced free radical promoted cationic photopolymerization^a of CHO or IBVE (0.3 mL).

Polymer Conv. (%)^b Mn (g·mol^-1)^c PDI^c BC^d PSt (%)^e

PSt(1)-b-PCHO 24.7 5200 1.07 30.3 85.7

PSt(2)-b-PCHO 34.7 6720 1.09 41.0 88.9

PSt(3)-b-PCHO 33.7 9990 1.13 40.2 83.3

PSt(2)-b-PIBVE 20.0 5420 1.13 - -

a[Mn₂(CO)₁₀] = 7.210^-2 mol·L^-1; [Ph₂I⁺PF₆^-] = 3.310^-3 mol·L^-1; [PSt] = 3.610^-2 mol·L^-1.

bConversion of each monomer was determined gravimetrically before extraction of the homo-PCHO with hexane. ^cMolecular weights and PDIs were determined with GPC. ^dBlock copolymer content (BC, % w/w) was determined gravimetrically after the extraction. ^ePSt content (% mol) of the block copolymers was determined by ¹H-NMR analysis.

Figure 4.27 : GPC traces of PSt-b-PCHO block copolymers synthesized by visible light induced free radical promoted cationic photopolymerization of CHO (0.3 mL) using Mn2(CO)10 (7.210^-2 mol·L^-1) in the presence of Ph2I⁺ PF6- (3.310^-3 mol·L^-1) and PSt (3.610^-2 mol·L^-1) with various molecular weights (see Table 3.1) in toluene.

As can be seen from Table 4.11, the molecular weight of the block copolymers increases with increasing molecular weight of PSt, however, the contents of PSt segments in the block copolymers are not high accordingly. This is probably due to the steric hindrance, since when the PSt chain gets longer, transformation of bromo functionalities to radicals and consequently cations becomes difficult, and eventually addition of monomers to the chain end will be hard.

Figure 4.28 : Synthesis of PSt-b-PCHO by combination of ATRP and visible light-induced free radical promoted cationic polymerization.

The structure of the precursor polymer and the resulting block copolymers was confirmed by ¹H-NMR spectral analysis. The NMR spectrum of the block copolymer displays signals at 0.8-2.2 ppm CH2, CH (PSt, PCHO), 3.43 ppm OCH (PCHO), and 6.5-7.2 ppm Ph (PSt) (Figure 4.29).

Unimodal GPC traces of precursor PSt(2), PSt(2)-b-PCHO, and unreacted PSt recovered from the control experiment completely correlate with the ¹H-NMR results. Expectedly, no polymerization took place in the control experiment in which Mn2(CO)10 was missing since Ph2I⁺ PF6-

was transparent under the incident light. The identical GPC chromatograms of the precursor (Figure 4.30A) and the polymer from the control experiment (Figure 4.30B) approve that Mn2(CO)10 is necessary for the formation of polystyryl radical which is then oxidized to initiating end-chain polymeric cation by Ph₂I⁺ PF₆^-. Apparently, when Mn₂(CO)₁₀ was added to the polymerization system, a shift to higher molar mass was observed indicating the central role of Mn₂(CO)₁₀ for free radical promoted cationic polymerization and consequently the formation of the block copolymer (Figure 4.30C). This result is in accordance with the proposed mechanism and with the report [425] in which control experiments omitting any components of the photoinitiating system failed to produce polymer.

Figure 4.29 : ¹H-NMR spectra of bromo functional PSt (Mn: 2250 g·mol^-1) (A), PSt-b-PCHO (B) and PSt-b-PIBVE (C) obtained by combination of ATRP and visible light induced cationic photopolymerization.

It should be pointed out that the GPC traces of the crude block copolymers before extraction with hexane (solvent for PCHO) showed a shoulder at higher molecular weight elution volumes. Such homopolymer formation is unavoidable since phenyl radicals formed concomitantly from the decomposition of diphenyliodonium salt participate in further redox reactions to generate initiating cations (Figure 4.31) [395]. In this way, the initiation of growing (cationic) chains is multiplied. These additional chains do not consist of initial PSt segments. Notably, presence of the very small shoulder at higher elution volume is probably due to lose of bromo functionality of the fraction of PSt chains during the ATRP or photolysis processes.

Figure 4.30 : Gel permeation chromatograms of bromo functional polystyrene (M_n: 3250 g·mol^-1) (A) and resulted polymers after irradiation of block copolymerization system with (C) or without (B) Mn2(CO)10.

Figure 4.31 : Generation of additional cationic species.

Further thermal and spectroscopic analyses also confirmed the successful formation of block copolymers. Glass transition temperature (T_g) of the PSt-b-PCHO investigated by DSC was observed around 82 C, between the Tg values of the precursor segments; homo-PCHO (65 C) and homo-PSt (100 C). This result obviously is not surprising for the block copolymer consisted of PCHO and PSt segments which are miscible. In FT-IR spectrum of the precursor PSt (Figure 4.32), characteristic bands such as aromatic C=C stretch bands around 1400-1600 cm^-1 were observed; whereas, spectrum of the block copolymer contained additional band at 1077 cm^-1 corresponding to typical etheric C-O-C stretch band.

Figure 4.32 : FT-IR spectra of precursor PSt (Mn: 3250 g·mol^-1) and PSt(2)-b-PCHO block copolymer synthesized by visible light induced free radical promoted cationic photopolymerization of CHO (0.3 mL) using Mn₂(CO)₁₀ (7.210^-2 mol·L^-1) in the presence of Ph₂I⁺ PF₆^- (3.310^-3 mol·L^-1) and PSt (3.610^-2 mol·L^-1) in toluene.

To extend applicability of the approach, copolymerization of another cationically polymerizable monomer, IBVE, was also investigated. ¹H-NMR spectra of the resulting PSt-b-PIBVE revealed that the initiating system is also effective in the polymerization of vinyl type monomers. The expected molecular weight shifts were observed also in this case (Figure 4.33).

It must be pointed out that chain transfer is an important process as far as cationic polymerization of alkyl vinyl ethers is concerned. In our experiments, we have employed thiolane to prevent formation of initiating species resulting from chain transfer that are free of initial PSt segments [426, 427]. However, GPC traces indicate that polymer chains with different chain lengths are formed since propagating cations can be created not only by direct oxidation of the terminal polystyryl radical but also the radicals obtained by the addition of vinyl ether monomer (Figure 4.34).

Figure 4.33 : Gel permeation chromatograms of bromo functional PSt (Mn: 2250 g·mol^-1) (A), PSt-b-PCHO (B) and PSt-b-PIBVE (C) obtained by combination of ATRP and visible light induced cationic

photopolymerization.

Figure 4.34 : Possible pathways for the production of cationic species during the block copolymer formation.