Polymerization rate considerations for high molecular weight polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) triblock polymers synthesized via sequential reversible addition-fragmentation chain transfer (RAFT) reactions

The reversible addition-fragmentation chain transfer (RAFT) polymerization mechanism is a powerful technique for synthesizing functional block polymers for myriad applications. Most kinetic studies regarding the RAFT mechanism have focused on low molecular weight homopolymer and block polymer syntheses using a dithiobenzoate chain transfer agent (CTA). Here, the polymerization kinetics are evaluated for a high molecular weight A-B-C triblock polymer system, polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) (PI-PS-PDMA), using a trithiocarbonate agent for application of these types of polymers. Importantly, it is demonstrated that the polymerization of polyisoprene is the step that generates the block with the largest dispersity for high molecular weight PI-PS-PDMA polymers. As such, the kinetics of isoprene polymerization must be altered systematically for desired nanostructures to be formed. In addition, it is established that the PS and PDMA block additions exhibit polymerization rate retardation, which is due to slow chain fragmentation of the CTA, as demonstrated by the magnitudes of the equilibrium constants for both the styrene and N,N-dimethylacrylamide reactions, and as calculated using ab initio modeling. This elucidation of the nature of the controlled RAFT mechanism provides a critical handle for the more precise design and control of other next-generation high molecular weight block polymer systems that are polymerized using the RAFT mechanism. Reaction design considerations for the reversible addition-fragmentation chain transfer-mediated polymerization of polyisoprene-b-polystyrene-b-poly(N,N-dimethylacrylamide) at high molecular weights are established. Controlling these conditions illustrates that there is an upper limit on the rate of polymerization for the isoprene monomer used. Furthermore, the second and third block polymerizations show slow fragmentation rate retardation and slow initialization using a trithiocarbonate chain transfer agent.