Title

Synthesis, characterization, and kinetic studies of novel polyisobutylene-based block copolymers

Date of Award

1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

First Advisor

Robson F. Storey

Advisor Department

Polymers and High Performance Materials

Abstract

The synthesis of advanced materials such as thermoplastic elastomers based on poly(isobutylene) (PIB) was accomplished by simple addition of a second monomer, typically styrene, to living PIB chains under conditions optimized for PIB synthesis. However, a dearth of information exists on the living cationic polymerization behavior of styrene itself. The objective of this research was to investigate the living cationic polymerization of styrene and elucidate optimal conditions for the synthesis of advanced materials based on PIB and styrene. Composition of poly(styrene-b -isobutylene-b -styrene) (PS-PIB-PS) block copolymers initiated from an [Special characters omitted.] -[Special characters omitted.] telechelic tert -chloride terminated PIB in either MCHex/MeCl or hexane/MeCl 60/40 (v/v) cosolvents at -80°C was studied. Both cosolvent systems yielded materials characterized by a minor fraction of coupled species caused by alkylation of a phenyl ring within a PS block by a growing polystyryl carbocation. Coupled product formed as early as 40% conversion of the initial styrene charge; thus, stopping polymerization short of complete styrene conversion did not prevent formation of chain-coupled species. Next, the kinetics of living/controlled polymerization of styrene were monitored in real-time by mid-infrared ATR-FTIR spectroscopy. Polymerizations were initiated by either 2-chloro-2,4,4-trimethylpentane (TMPCl) or 5- tert -butyl-1,3-di(2-chloro-2-propyl)benzene (bDCC) at -80, -70, -60, and -50°C. In all cases TiCl4 was the coinitiator and 2,4-dimethylpyridine (DMP) was employed as electron donor. With either initiator, the polymerizations displayed an initial period of rapid monomer consumption (RMC) followed by a slower, first-order consumption of monomer. The RMC was observed to be very short-lived for bDCC initiated polymerization compared to TMPCl initiated polymerization. The RMC as well as overall rate of polymerization were observed to decrease with increasing reaction temperature. The energy of activation was calculated to be -6.4 kcal/mol for styrene polymerization in the cosolvent system investigated over the temperature range -80°C to -50°C. Finally, Poly([Special characters omitted.] -caprolactone) (PCl) and PCl-poly(isobutylene)-PCl (PCl-PIB-PCl) block copolymers were synthesized in anhydrous toluene by in situ conversion of either 2-methyl-1-propanol (2M1P) or [Special characters omitted.] -[Special characters omitted.] telechelic PIB diol, respectively, to the corresponding aluminum alkoxide by reaction with a stoichiometric amount of triethylaluminum (TEA) followed by addition of [Special characters omitted.] -caprolactone. Structural characterization of the homo-PCl by gel permeation chromatography (GPC) and matrix assisted laser desorption ionization - time of flight (MALDI-TOF) mass spectrometry demonstrated the presence of cyclic oligomers. The origin of the carboxylic acid termini is suggested to be fragmentation of the initiator residue from the chain end during MALDI analysis. For PCl-PIB-PCl block copolymers, two PIB di-alcohols were used as initiators, one derived from allyl and one from isopropenyl terminated PIB. Block copolymers possessed molecular weight distributions [Special characters omitted.] 1.4. Structural analysis indicated that the PCl block ends were severed from the crude block copolymer during MALDI analysis; for both allyl and isopropenyl-derived materials. For allyl-derived materials the PCl blocks were found to uniformly carry a C2 residue at the point of detachment of the PIB block; however, the isopropenyl-derived block copolymers showed a complex mixture of different residues suggesting a complex fragmentation mechanism during loss of the PIB block. (Abstract shortened by UMI.)