Synthesis of Polyisobutylene-Based Miktoarm Star Polymers from a Dicationic Monoradical Dual Initiator

Abstract

A dicationic monoradical dual initiator 3-[3,5-bis(1-chloro-1-methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropionate (DCCBMP) was designed for the preparation of A 2 B type miktoarm star copolymers, where A is a polyisobutylene (PIB)-based homo or block copolymer that is produced by living carbocationic polymerization (LCP), and B is a polyacrylate or other polymer block produced by atom transfer radical polymerization (ATRP). DCCBMP was obtained by chlorination of 3-[3,5-bis(1-hydroxy-1-methylethyl)phenyl]-3-methylbutyl 2-bromo-2-methylpropionate (DCOHBMP), which was synthesized in a pure form via aerobic oxidation of 3-(3,5-diisopropylphenyl)-3-methylbutyl 2-bromo-2-methylpropionate using N -hydroxyphthalimide (NHPI)/Co(OAc) 2 ·4H 2 O catalyst system. Initiation efficiency of DCCBMP was 0.89–0.98 for LCP of isobutylene (IB) at −70 °C targeting molecular weights of 5K, 10K, and 20K g/mol, which was comparable to the standard dicationic initiator 5- tert -butyl-1,3-(1-chloro-1-methylethyl)benzene ( t- Bu- m -DCC). Sequential monomer addition under LCP conditions produced narrow-polydispersity poly(styrene- b -isobutylene- b -styrene) (PS–PIB–PS) triblock copolymers. These polymers possessed a 2-bromo-2-methylpropionate (bromoester) ATRP initiating site in the approximate middle of PIB segment and two sec -benzylic chloride groups at the PS chain ends, which were also effective in initiating ATRP. With these PS–PIB–PS macroinitiators, poly( tert -butyl acrylate- b -styrene- b -isobutylene) 2 - s -poly( tert -butyl acrylate) [(P t BA–PS–PIB) 2 - s -P t BA] miktoarm star polymers were readily synthesized via ATRP of tert -butyl acrylate ( t BA). The sec -benzylic chloride functionality was alternatively eliminated at high temperature (180–200 °C) under vacuum, yielding PS–PIB–PS macroinitiators containing only the bromoester site (thermolyzed PS–PIB–PS). Size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy showed that thermolysis removed all sec -benzylic chlorides without destruction of bromoester moieties. Thermolyzed PS–PIB–PS allowed synthesis of poly(styrene- b -isobutylene) 2 - s -poly( tert -butyl acrylate) [(PS–PIB) 2 - s -P t BA] star polymers with the potential hydrophilic segment attached only to the core of PS–PIB–PS. For both star terpolymer configurations, the P t BA blocks were converted to PAA via thermolysis at 150 °C to produce amphiphilic PIB-based miktoarm star polymers with narrow polydispersity indices (<1.1).