Polyisobutylene chain end transformations: Block copolymer synthesis and click chemistry functionalizations

Andrew Jackson David Magenau

Abstract

The primary objectives of this research were twofold: (1) development of synthetic procedures for combining quasiliving carbocationic polymerization (QLCCP) of isobutylene (IB) and reversible addition fragmentation chain transfer (RAFT) polymerization for block copolymer synthesis; (2) utilization of efficient, robust, and modular chemistries for facile functionalization of polyisobutylene (PIB). In the first study block copolymers consisting of PIB, and either PMMA or PS block segments, were synthesized by a site transformation approach combining living cationic and reversible addition-fragmentation chain transfer (RAFT) polymerizations. The initial PIB block was synthesized via quasiliving cationic polymerization using the TMPCl/TiCl 4 initiation system and was subsequently converted into a hydroxylterminated PIB. Site transformation of the hydroxyl-terminated PIB into a macro chain transfer agent (PIB-CTA) was accomplished by N,N'-dicyclohexylcarbodiimide/dimethylaminopyridine-catalyzed esterification with 4-cyano-4-(dodecylsulfanylthiocarbonylsulfanyl)pentanoic acid. In the second study another site transformation approach was developed to synthesize a novel block copolymer, composed of PIB and PNIPAM segments. The PIB block was prepared via quasiliving cationic polymerization and end functionalized by in-situ quenching to yield telechelic halogen-terminated PIB. Azido functionality was obtained by displacement of the terminal halogen through nucleophilic substitution, which was confirmed by both 1 H and 13 C NMR. Coupling of an alkyne-functional chain transfer agent (CTA) to azido PIB was successfully accomplished through a copper catalyzed click reaction. Structure of the resulting PIB-based macro-CTA was verified with 1 H NMR, FTIR, and GPC; whereas coupling reaction kinetics were monitored by real time variable temperature (VT) 1 H NMR. In a third study, a click chemistry functionalization procedure was developed based upon the azide-alkyne 1,3-dipolar cycloaddition reaction. 1-(ω-Azidoalkyl)pyrrolyl-terminated PIB was successfully synthesized both by substitution of the terminal halide of 1-(ω-haloalkyl)pyrrolyl-terminated PIB with sodium azide and by in situ quenching of quasiliving PIB with a 1-(ω-azidoalkyl)pyrrole. GPC indicated the absence of coupled PIB under optimized conditions, confirming exclusive mono-substitution on each pyrrole ring. In a fourth study, radical thiol-ene hydrothiolation "Click" chemistry was explored and adapted to easily and rapidly modify exo -olefin PIB with an array of thiol compounds bearing useful functionalities, including primary halogen, primary amine, primary hydroxyl, and carboxylic acid. The thiol-ene "click" procedure was shown to be applicable to both mono and difunctional exo -olefin polyisobutylene. Telechelic mono- and difunctional exo -olefin PIBs were synthesized via quasiliving cationic polymerization followed by quenching with the hindered amine, 1,2,2,6,6-pentamethylpiperidine. Lower reaction temperatures were found to increase exo -olefin conversion to near quantitative amounts. In the fifth study, thiol-terminated polyisobutylene (PIB-SH) was synthesized by reaction of thiourea with α,ω-bromine-terminated PIB in a three step one-pot procedure. First the alkylisothiouronium salt was produced using a 1:1 (v:v) DMF:heptane cosolvent mixture at 90°C. Hydrolysis of the salt by aqueous base produced thiolate chain ends, which were then acidified to form the desired thiol functional group. An extension of this reaction was performed by a sequential thiol-ene/thiol-yne procedure to produce tetra-hydroxy functionalized PIB. 1 H NMR was used to confirm formation of both alkyne and tetrahydroxyl functional species. Further utility of PIB-SH was demonstrated by base catalyzed thiol-isocyanate reactions. A model reaction was conducted with phenyl isocyanate in THF using triethylamine as the catalyst. Last, conversion of PIB-SH directly into a RAFT macro-CTA was accomplished, as shown by 1H NMR, by treatment of PIB-SH with triethylamine in carbon disulfide and subsequent alkylation with 2-bromopropionic acid. (Abstract shortened by UMI.)