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

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Polymers and High Performance Materials


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 A2B 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)2·4H2O 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(tertbutyl 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)

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