An unexpected stereochemical bias in the RAFT syntheses of styrene/p-bromostyrene copolymers uncovered by the Kerr effect

- In the RAFT of S/pBrS copolymers, the enchainment of r pBrS-pBrS diads is preferred.
- The enchainment of r pBrS-pBrS diads is not favored in the ATRP syntheses of S/pBrS copolymers.
- This conclusion is reinforced by their Kerr effect and thin film dewetting studies.
- 13C-NMR is not able to distinguish these micro- and macrostructural details in S/pBrS copolymers.

During our recent investigations of the viability of using the observed contributions synthetic polymers make to the birefringence of their dilute solutions when subjected to strong electric fields, i.e., their Kerr effects, to characterize the complete architectures or macrostructures of their chains, we discovered a surprising result. The Kerr constants measured for styrene/p-bromostyrene (S/pBrS) copolymers synthesized by controlled RAFT copolymerization could only be reproduced by their predicted calculated values when we assumed that pBrS─pBrS diads were enchained with a strong preference for the racemic (r) stereosequence, while at the same time S─S diads show no such stereochemical preference. Gradient and random S/pBrS copolymers made by RAFT showed similar Kerr constants, while those made by FRP and ATRP had Kerr effects different by a factor of two or more and were similar to those shown by samples with resultant blocky and random comonomer sequences made previously by BR of a-PS in poor and good solvents, respectively. By comparison to the Kerr effects observed for S/pBrS copolymers with similar compositions and comonomer sequences, but obtained by bromination (BR) of atactic polystyrene (a-PS) or uncontrolled free-radical (FRP) and controlled ATRP syntheses, which are well known to produce random atactic vinyl homo- and copolymers, we were able to confirm the stereochemical bias introduced during our controlled RAFT copolymerizations first suggested by our Kerr effect observations. Our Kerr effect observations and the conclusions drawn from them received further confirmation from the dewetting behaviors of their thin films observed from silicon wafers when they were heated well above their Tgs. For example, thin atactic S/pBrS films made by uncontrolled FRP and controlled ATRP free-radical polymerizations with random commoner sequences dewetted rapidly from silicon wafers when heated, while thin films made from atactic S/pBrS with random comonomer sequences, but obtained by controlled RAFT syntheses were “stickier”, and only dewet from silicon wafers after much longer annealing times. We suggest this increased “stickiness” of the RAFT synthesized S/pBrS copolymers is attributable to pBrS─pBrS diads with predominantly racemic structures that adopt preferred trans─trans conformations. In trans─trans racemic pBrS─pBrS diads, neighboring side chains are placed on opposite sides of the copolymer backbone in position for their p-Brs to strongly interact with the silicon wafer surfaces.