Propagation statistics

As an example, polybutylene is a commercially inq ortant polymer made with Ziegler-Natta catalysts. Such catalysts frequently produce more than one catalytic site, and the resulting polymer is a blend of several homopolymers differing only in propagation statistics. Previously, the two-site E/B model has been used to analyze the tacticity of this polymer.(11,12) Reasonably good fits with experimental data were observed. 

Because of facile cross-propagation, statistical (or nearly random) copolymerization is very easily achieved in free-radical systems, in contrast to ionic reactions. The reactivity of many comonomers are relatively similar. For example, in RP, methacrylates have similar reactivity to styrene and are 3 times more reactive than acrylates. However, in anionic polymerization acrylates are 100 times more reactive than methacrylates and the latter much more reactive than styrene. In cationic polymerization the opposite reactivity order is observed. The copolymerization of monomers with similar reactivity should result in statistical copolymers with no compositional variation during the pol5unerization. This has been observed for copolymerization of the same type of monomers such as various styrenes, various methacrylates, and various acrylates. This is the case for both CRP and conventional RP. However, in batch copol5unerizations of different classes of comonomers there is a continuous change of residual monomer composition in the reaction because one comonomer reacts faster than the other one. 

It is well-known that the polymerisation of vinyl chloride proceeds under the control of the Bernoullian statistical model (selection between meso and racemo). Table 6.3 shows the comparison of the observed relative areas of methine pentad and methylene peaks with those calculated by Bernoullian propagation statistics. Relative areas of observed peaks are determined by the curve resolution method. The relative areas of observed pentad peaks agree well with calculated values, indicating the validity of their pentad assignments by the method of Dong and co-workers [61]. As for the methylene peaks, observed areas (except peak 6 ) agree well with calculated values. Assuming... 

Calculated by Bemoullian propagation statistics. Source Author s own files ... 

It is evident that for polymer B (VA = 0.31), the Bemoullian model can be abandoned and that a significant amelioration of the results is achieved by going from first-order Markov to second-order Markov analysis. Thus the propagation statistics of polymer B is adequately described by a second-order Markov process. — G. van der Velden [24]... 

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