The polymerization of propylene oxide with zinc and tin xanthates was studied. Polymerization with both systems was found to be a zeroth-order, nonterminating process, where the molecular weight was controlled by transfer reactions. It is discussed that these observations were consistent with a mechanism in which the rate-determining step was the addition of monomer on the growing chain rather than the coordination of the monomer to active sites. These catalysts turned out to be quite stable, even under semiclosed conditions. With zinc isopropyl xanthate, high conversions in short polymerization times could be obtained. The product had a very broad molecular weight distribution and could be split into crystallizable and amorphous fractions. The crystallizable fractions consisted of stereoregular segments separated from each other by stereoirregularities or regioirregularities. The degrees of polymerization of stereoregular segments (S1's) were estimated from melting point measurements. It was found that the melting points and, hence, average S1 values changed in different fractions. It was concluded that there was only one type of active center; however, the rates of wrong additions (e.g., head-to-head addition) of incoming monomer controlled the average S1 value of the chain. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011
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