Molecular weight and long chain branch distributions of branch-block olefinic thermoplastic elastomers

olefinic thermoplastic elastomers can be prepared by incorporating semi-crystalline macromonomers (e.g. isotactic or syndiotactic poly(propylene), high-density polyethylene) onto amorphous backbones (e.g. atactic poly(propylene), ethylene/alpha-olefin copolymers). The macromonomer incorporation reaction can be carried out in semi-batch reactors by adding previously synthesized macromonomers to the reactor (ex situ approach), or by generating and incorporating the macromonomers in a single step (in situ approach). The differences in the microstructre of copolymers synthesized by in situ and ex situ techniques are explored herein through a mathematical model that can predict the concentration of linear and branched chains, their average molecular weights, polydispersity indices, and molecular weight distributions. In both cases linear chains predominate, but the ex situ approach produces a larger amount of branched chains with thermoplastic elastomer properties. Furthermore, for the in situ strategy, a significant amount of branched chains is only formed after the macromonomer concentration reaches a critical value.