Synthesis, Characterization and Properties of Octa-Arm Polyisobutylene-Based Star Polymers

Abstract

This review concerns the synthesis, characterization, and select properties of a series of novel eight-arm polyisobutylene (PIB)-based stars. An octafunctional initiator containing 2-methoxyisopropyl groups 1 was synthesized by quantitative functionalization of 5,11, 17,23,29,35,41,47-octaacetyl-49,50,51,52,53,54,55,56-octamethoxy-calix[8]arene and used to prepare the stars. Stars with PIB arms, terminally functionalized PIB arms, poly(styrene-b-isobutylene) (PSt-b-PIB) arms, and poly(p-chlorostyrene-b-isobutylene) (PpClSt-b-PIB) arms were synthesized. Polymerizations of isobutylene (IB) to prepare well-defined PIB stars were effected by the use of octa-functional initiator with BCl3-TiCl4 coinitiators in two stages. Stage I was carried out in the presence of a fraction of the required amount of IB in CH3Cl and polymerization was induced by the addition of BCl3. Stage II was induced by the addition of TiCl4 and the balance of IB, and the polymerization was carried to completion. The relative concentrations of BCl3 and TiCl4 were critical for the synthesis of well-defined stars. By the use of well-chosen BCl3-TiCl4 concentrations, added in sequence in two stages, narrow dispersity stars, Mw / Mn = 1.11, were prepared. Gel permeation chromatography (GPC), with on-line RI, UV, and laser light scattering (LLS) detectors, was used to determine the molecular weights and compositions of linear and octa-arm star polymers. The number of arms of the stars was determined by 1H NMR spectroscopy and by selective core destruction. Both studies indicated that the number of arms was close to the theoretical value of eight. The mechanism of polymerization was studied by the use of a model monofunctional analogue of the octa-functional initiator, 2-(p-methoxyphenyl)-2-methoxypropane.