Carbocationic polymerizations in supercritical carbon dioxide
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The first carbocationic polymerization of isobutylene (IB) in supercritical carbon dioxide (SC·CO2) has been accomplished. It was demonstrated that in CO2 at 32.5°C and ∼ 120 bar the 2-chloro-2,4,4-trimethyl-pentane (TMPCl)/SnCl4 and TMPCl/TiCl4 initiating systems lead to ∼30% IB conversion, and gave polyisobutylenes (PIB) with Mn∼2000 and Mw/Mn∼2.0. This is the highest temperature IB was ever polymerized to reasonably high molecular weight products. Polymerizations at 32.5 °C under similar but conventional (non-living) conditions in the absence of SC·CO2 would yield only very low molecular weight oligomers (∼tetramers). The structure of the PIBs obtained in SC·CO2 is virtually identical to those obtained at much lower temperatures in conventional liquid-phase systems indicating the presence of chain transfer to monomer in both systems. In contrast to TMPCl initiated polymerizations, the 1,3-bis-(2-hydroxy-2-propyl)-5-tert-butylbenzene (HPBB) initiator in conjunction with BCl3 and SnCl4 yields only oligomers (Mn∼500) in SC·CO2.
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