Effect of Reaction Conditions on Synthesis and Properties of Multiarm Star-Branched Polyisobutylenes

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Polymers and High Performance Materials


A series of multiarm star-branched polyisobutylenes was synthesized from narrow polydispersity arms with molecular weights ranging from 12,000 to 60,000 g/mol, via living carbocationic polymerization using the cumyl chloride/TiCl4/pyridine initiating system and divinylbenzene(DVB) as core-forming comonomer. The effect on star development of arm molecular weight, temperature, solvent composition, and DVB concentration was studied. The rate of star formation and the weight-average number of arms per star polymer, (N) over bar(w), were found to scale inversely with arm molecular weight; (N) over bar(w), = 60 was attained for 13,100 g/mol arms, but (N) over bar(w) = 2.5 for 60,000 g/mol arms. It was established that star formation was much faster at -80 degrees C compared to 23 degrees C, regardless of solvent composition. For hexane : methyl chloride (MeCl) solvent compositions containing from 40 to 60 vol % MeCl, star-star coupling was observed at -80 degrees C, but not at 23 degrees C, even after 312 h; for the most polar 40 : 60 hexane : MeCl composition, star-star coupling was so extensive at -80 degrees C that gelation was observed after only 44 h. The rate of star formation was found to be substantially higher in 60 : 40 hexane : MeCl compared to 60 : 40 hexane : methylene chloride (MeCl2). Some reactions containing MeCl were immediately warmed to 23 degrees C after DVB addition, and the MeCl thus volatilized was replaced with either MeCl2 or hexane for the duration of the star-forming reaction. Slightly higher rates were consistently observed when MeCl2 was the replacement solvent. The strong influence of initial MeCl content on rate of star formation was found to persist throughout the star-forming reaction, even when the solvent was immediately converted to 100% hexane. The fraction of arms that remained unlinked into stars was found to be higher at the higher temperature and at lower solvent polarity. Regardless of solvent or temperature, the residual arm fraction was approximately the same at a given stage of star development as measured by the average number of arms per star. One star sample was produced with the W-transparent 2-chloro-2,4,4-trimethylpentane initiator; analysis showed that the residual arm fraction had approximately the same UV absorbance as the star fraction, indicating efficient crossover to DVB and the potential for approximately quantitative arm incorporation given sufficient time. (C) 1998 John Wiley & Sons, Inc.

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Journal of Polymer Science Part A: Polymer Chemistry





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