Enantiomorphic site control statistical model

Statistical analysis of the stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also supports the enantiomorphic site control model. 

The polymer stereosequence distributions obtained by NMR analysis are often analyzed by statistical propagation models to gain insight into the propagation mechanism [Bovey, 1972, 1982 Doi, 1979a,b, 1982 Ewen, 1984 Farina, 1987 Inoue et al., 1984 Le Borgne et al., 1988 Randall, 1977 Resconi et al., 2000 Shelden et al., 1965, 1969]. Propagation models exist for both catalyst (initiator) site control (also referred to as enantiomorphic site control) and polymer chain end control. The Bemoullian and Markov models describe polymerizations where stereochemistry is determined by polymer chain end control. The catalyst site control model describes polymerizations where stereochemistry is determined by the initiator. 

This model can be applied to evaluate the pentad distribution from NMR spectra of metallocene-based isotactic polypropenes in which overlapping with peaks from end group or regioirregular units (2.1 and 3.1) occurs. In this case only the peaks of mmmm, mmmr, mmrr, and mrrm pentads can be obtained from direct spectrum integration. Furthermore. the mmmr peak overlaps with the mmmm base, and the mmrr has to be correct by subtracting the contribution from the 2,1-erythro unit. The total pentad distribution is calculated under the hypothesis that the polymerization statistic follows a pure enantiomorphic site control, using the expressions reported in Table 13 as follows ... 

The polymerization conditions leading to the three sPP microstructures formed by syndlospecific metallocenes cannot be adequately described with previous statistical models that were derived for Isospeclfic polymerizations. Indeed, Furukawa has shown that the enantiomorphic site control model cannot account for polymers with greater than 25 % racemic triads. 

An excellent way to treat such data is to use reaction probability models.(1,2) In the NMR analysis of tacticity, it is frequently possible to distinguish whether the configuration is chain-end controlled or catalytic-site controlled during polymerization. Various statistical models have been proposed. The chain-end controlled models include Bemoullian (B), and first- and second-order Markovian (Ml and M2) statistics.(1) The simplest catalytic-site controlled model is the enantiomorphic site (E) model.(3) The relationship between the chain-end and catalytic-site controlled models and possible hybrid models have been delineated in a recent article.(4)... 

Statistical modeling of pentad distributions of polypropenes prepared with these catalyst systems can be satisfactorily done using a two-site modeF - based on a mixing of a chain-end-controlled site (to model the atactic blocks) and an enantiomorphic site (for isotactic blocks). ... 

As reported in section II.E, the two main mechanisms of stereocontrol in 1-olefin polymerization arise from the chirality of the catalytic site enantiomor-phic site control) and from the chirality of the last methine in the polymer chain (chain-end control). Two statistical models, based on these basic mechanisms, have been developed and used by different authors and are known as the enantiomorphic site modeP and the Bernoullian modeB ... 

Enantiomorphic Site with Chain-End Control. In the case of less stereoselective Cz-symmetric metallocene catalysts, the magnitude of chain-end control can be comparable to that of site control. In this case, obviously, the former has to be added to the model using Markovian statistics. The probability parameters are the same found for pure chain-end control p si re), i.e., the probability of insertion of a si monomer enantioface after a monomer inserted with the re face, p re si), p si si), and p re re). In this case, the metallocene chirality prevents the equiprob-ability of the si olefin insertion after a re inserted monomer (see structure on the left in Scheme 36) and re olefin insertion after a si inserted monomer (see structure on the right in Scheme 36). 

By determining which statistical model is followed in a polymerization, such as Bemoullian, or Markov, or other, it should be possible to understand better the mechanism of steric control. Thus the Bernoulli model describes those reactions in which the chain ends determine the steric arrangement. These are polymerizations that are carried out under conditions that yield mostly atactic polymers. The high isotactic sequences follow the enantiomorphic site model and the high syndio-tactic ones usually follow the Markov models. 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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