A fascinating and unique aspect of polymer science is molecular structure. Polymer chains can loop, branch, and cross-link, and they have pronounced molecular weight dispersity and often unusual chain architecture. In small molecule chemistry, a molecule is a molecule. Every molecule of, say, cyclohexane, is identical to every other molecule of cyclohexane. Polymers never have that certainty. Even a polymeric molecule as “simple” as polyethylene can vary widely from another polyethylene molecule. Chain length, branching, material density – all of these can be different for the same chemical species. In fact, only in the extraordinarily unlikely scenario of of polymers with perfect monodispersity will every chain be identical to another. And despite decades of study, new insights are still being made into how these factors affect polymer materials.
Two reports published in Polymer this week explore aspects of molecular weight dispersity and polymer architecture. Iedema and co workers apply a Monte Carlo simulation to further understand the effects of common chemical modifiers on the branching and molecular weight distributions of polyethylene.1 By fitting a scission mechanism of vinyl end group reaction with a secondary in-chain radical coupling site, their simulation succeeded in completely reproducing subtleties in the processed polymers’ weight distribution. Extension of the model was shown to be able to predict branching topologies, which is useful for predicting material properties like radius of gyration and macroscale rheology. In another study, Nardai and Zifferer modeled the dynamics of interchain contact between a general polymer mixture of linear and star-branched chains.2 The study extends their previous work by modeling reactive group shielding across a range of concentrations. Among their findings is the interesting observation that for long linear chains, shielding is less pronounced in the bulk than in dilute theta solutions, contrasting to short linear chains where bulk and dilute theta solution environments behave identically. In addition to theoretical interests, this system has direct application to certain RAFT-based polymerizations where growing linear chains interact with star-shaped stabilizing agents.
References
1. Iedema PD, Remerie K, van der Ham M, Biemond E. “Development of MWD and branching during peroxide modification of High-Density Polyethylene by SEC-MALS and Monte Carlo simulation.” Polymer. 2013; 54: 4093-4104.
2. Nardai MM, Zifferer G. “Shielding effects in polymer-polymer reactions. V. Concentration dependence of contact formation between star branched and linear chains.” Polymer. 2013; 54: 4183-4193.