Chapter 19 Polymerization of rodlike molecules

Abstract

Polymers with a rodlike conformation have assumed considerable technological importance as they can be processed into heat resistant fibres of ultra-high modulus which find application, for example, in the manufacture of light weight composites, cables and bullet proof vests. The synthesis of most of the commercially important rodlike molecules is by step growth polymerization, and the process is qualitatively different from step growth polymerization of flexible polymeric molecules. Molecular dynamics plays a much greater role in the case of rodlike molecules since a requirement for reaction is near parallel orientation of the racting oligomers. The sharp decrease in rotational diffusivity with molecular length results in diffusion control at relatively low molecular weights; further, orienting flow fields can have a significant impact on the course of the polymerization. We present here a review of the theoretical and computational studies of the polymerization of rodlike molecules. Results are presented of analyses of diffusion controlled polymerization based on Smoluchowski's approach, multiparticle Brownian dynamics and pairwise Brownian dynamics, which is a hybrid method combining the Smoluchowski approach and Brownian dynamics. The dynamics of rodlike molecules in solution, and the experimental results for the polymerization of rodlike molecules are briefly reviewed first. The implications of the role of molecular dynamics for manufacture of such rodlike polymers are discussed.