(670e) Effect of Monomer Sequence on Polymer Solution Phase Behavior

Monomer sequence of both synthetic and biological soft matter has been shown to play a critical role in material phase behavior. In synthetic systems, tapering or reverse tapering of interfaces between block copolymers has been shown to improve polymer compatibility, which affects bulk polymer morphology. In biological systems, the sequence of intrinsically disordered proteins (IDPs) directly influences liquid-liquid phase separation and the formation of biocondensates. Recent work has used coarse-grained molecular simulations to demonstrate that small changes in sequence of IDPs at fixed overall chain composition, particularly at the end of the chain, can profoundly impact the solution phase behavior of these molecules. Here, we aim to make connections between the liquid-liquid phase behavior of IDPs and that for synthetic macromolecules. Anionic polymerization of styrene and isoprene was selected as it is an ideal method for synthesis of bulk quantities of polymers with controlled chain sequence: initiation occurs rapidly resulting in low polydispersity. There is no termination if species reactive with alkylithiums are excluded, so it is possible to synthesize complex polymers with multiple, well-defined blocks. Therefore, using this technique we synthesized copolymers of isoprene and styrene with similar molecular weight, polydispersity, and overall composition but small changes (~5%) in monomer sequence. The overall molecular weight was held constant at 100 kg/mol and the overall composition was held constant at a 50/50 weight fraction of styrene to isoprene. The sequence of the polymers was altered by placing 5 kg/mol blocks of polystyrene or polyisoprene homopolymer at the end or middle of a random copolymer chain. The phase behavior of the various polymer sequences in hexane was studied using dynamic light scattering. The presence of small homopolymer blocks at the end of the chains significantly shifts the critical solution temperature and, in some cases, leads to the formation of micellar aggregates prior to macroscopic phase separation. Conversely, homopolymer blocks in the center of the chain have a minimal effect.