An investigation into the origins and limits of "livingness" within the quasiliving carbocationic polymerization of isobutylene

Christopher Lewis Curry


The quasiliving polymerization of isobutylene (IB) has been the subject of extensive investigations within the carbocationic polymerization community for the past twenty years, and is a primary focus of research within our laboratories. Throughout the past, traditional gravimetric analysis has been employed to determine polymerization kinetics of IB polymerizations; furthermore, due to the laborious nature of such analytical techniques, reliable kinetic data for the polymerization of low-molecular weight materials was impossible. The first objective of this research was to apply real-time in situ monitoring via ATR-FTIR spectroscopy for the determination of polymerization kinetics of IB polymerizations that generate low-molecular weight polyisobutylenes, and consequently display extremely fast rates of polymerization. Additionally, recent kinetic investigations of such IB polymerizations and unimolecular carbenium ion rearrangement are the dominant chain-interrupting events within these quasiliving processes. These detrimental side reactions limit the "livingness" in these systems, and in turn, may impede their use in the engineering of specific macromolecular architectures. A second objective of this research was to determine the structure and reactivity of the propagating chain end groups in quasiliving IB polymerizations at very high monomer conversions ( i.e. , monomer starvation conditions), in the presence of an active Lewis acid with the goal of characterizing the dominant termination processes in these systems. IB polymerization kinetics were monitored in real time using mid-infrared FTIR-ATR spectroscopy, with diamond-composite insertion probe and light conduit technology. The polymerizations investigated employed the 5-tert -butyl-1,3-di(2-chloro-2-propyl)benzene (t -Bu-m -DCC)/TiCl4 initiating system in a 60/40 (v/v) hexane/methyl chloride solvent mixture, with [TiCl 4 ] = 0.12 or 0.24 M, and [t -Bu-m -DCC] = 1.19 × 10-2 M, with temperatures ranging from -40 to -80°C ([IB]0 = 1.0 M). Polymerizations were performed in the presence of 2,4-dimethylpyridine (DMP), 2,6-di- tert -butylpyridine (Dt BP), tetra-n -butylammonium chloride (n -Bu4 NCl), and/or pyridine hydrochloride (Pyr-HCl), and control polymerizations were conducted without any additive. With two exceptions, polymerizations conducted in the presence of any combination of DMP, Dt BP, or n -Bu4 NCl provided polymers with targeted molecular weights, very narrow molecular weight distributions (MWD) that narrowed with increasing temperature, and polymerization reactions that proceeded in the apparent absence of irreversible chain termination. (Abstract shortened by UMI.)