Synthesis of polyisobutylene-based miktoarm star polymers from a dicationic monoradical dual initiator

摘要

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 _2B 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 _2O 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) [(PtBA-PS-PIB) _2-s-PtBA] miktoarm star polymers were readily synthesized via ATRP of tert-butyl acrylate (tBA). 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-PtBA] star polymers with the potential hydrophilic segment attached only to the core of PS-PIB-PS. For both star terpolymer configurations, the PtBA blocks were converted to PAA via thermolysis at 150 °C to produce amphiphilic PIB-based miktoarm star polymers with narrow polydispersity indices (<1.1).