Does co-monomer sequence in random copolymers matter?*

“Random” copolymers are synthesized by either direct copolymerization of the individual monomers or by post-polymerization modification (PPM) of a homopolymer with a reactive chemical species. It is a great challenge to characterize properly the “degree of randomness” using existing experimental tools. In our presentation we will provide evidence that Kerr effect (not the conventionally-employed NMR) is the method of choice for this daunting task. In our work, we prepare “random” copolymers by PPM of polystyrene with bromine that leads to the formation of poly(styrene-co-4-bromostyrene) (PBr_xS), where x is the mole fraction of 4-bromostyrene units (4-BrS) . We will document that the distribution of the distribution of the 4-BrS units depends intimately on the solvent quality during the bromination reaction. We will also discuss that in some cases (i.e., RAFT) direct copolymerization of styrene and 4-BrS leads to PBr_xS with a gradual variation of chemical composition and tacticity. We will provide overview of previous work in our group that aimed at determining the effect of co-monomer sequences on adsorption of “random” copolymers on surfaces form melts and from solutions. We will also review computational methods to monitor the formation and behavior of such “random” copolymers in solution and in bulk. Specifically, using turbidity measurements and small angle-neutron scattering experiments we will establish that while phase separation of PBr_xS comprising “truly random” distribution of styrene and 4-BrS segments resembles closely that of homopolymers, PBr_xS that possess “random-blocky” co-monomer distribution exhibits “pretransitional clouding” at temperatures above the phase separation. The temperature at which this “pretransitional clouding” occurs varies with cooling rate. We attribute this behavior to the formation of micro-aggregated domains of PBr_xS in solution due to limited solubility of the 4-BrS in cyclohexane.