Enantiomorphic sites model

One component obeys the Bemoullian model the other two obey the enantiomorphic- site model. Similarly, the NMR data of fractionated copolymers can be used to demonstrate the presence of multiple components in the copolymers. An example is shown of ethylene-propylene copolymers where the NMR/fractionation data are used to show the presence of two or three catalytic sites. 

With the experimental techniques available at present, rate constants of diad formation cannot be determined directly. There is however a way to calculate the rate constants from the experimentally determined triad, diad, etc. fractions if the rate constant of propagation and the statistical model are known (e.g., a one-way mechanism, a two-way mechanism, enantiomorphic site model) (9, JO). Very few rate constants of propagation are available, however. 

As in Ziegler-Natta polymerization, steric control of the propagation step may involve either the interaction of the monomer with a chiral metal centre (enantiomorphic sites model), or the interaction of the monomer with the chiral centres in the repeating unit(s) adjacent to the metal centre (chain-end model). (The relationship to Ziegler-Natta polymerization will be considered further in Section VIII.C.3.)... 

Barriers to rotation about Mt=C have been measured by observation of NMR coalescence temperatures123,330,331. In some cases these are sufficiently high that epimerization by rotation about Mt=C is unlikely to be important, but in other cases such a process may be as fast as or faster than the propagation step. More detailed considerations show that when both cis and trans double bonds are formed in accordance with the enantiomorphic sites model then the cis junctions will always be associated with r dyads, and trans junctions with m dyads27,332. This model thus correctly predicts the observed tacticities in the first, third and fifth groups of results listed in Table 7. Cases of intermediate tacticity can also be interpreted in terms of this model if it is modified to include partial epimerization of P/ and Pr between propagation steps. 

It has been suggested recently [410] that, also in the case of heterogeneous isospecific Ziegler-Natta catalysts, the stereoregulation mechanism assuming a growing chain orientation is more reasonable than that predicted by the enantiomorphic site model [411,412],... 

It is known that in propylene polymerization, both with conventional and supported Ziegler-Natta catalysts, at least two types of active centers can be distinguished. Such species can be associated with the so-called isotactic and atactic polymeric fractions, which have different configurations and may be separated by simple extraction with boiling heptane. Based on the 13C NMR analysis of the microstructure of the atactic and isotactic fractions, Inoue 1451 has recently proposed a two site model. At one site the stereospecific polymerization proceeds according to the Bernouillian model, and at the other it proceeds according to the enantiomorphic site model. However, it is understood that a two site model is an oversimplification. As a matter of fact, the crude polypropylene can usually be separated into several fractions having different tacticity 51>. 

Determined assuming the enantiomorphic site model, on primary insertions only see ref 232. Total seco ndary insertions ... 

Polymerization conditions 1-L stainless steel autoclave, 0.4 L of propene, 50 °C, 1 h, zirconocene/MAO aged 10 min. Determined assuming the enantiomorphic site model, on primary insertions only see ref 232. Determined as described in ref 232 end groups not included. In liquid monomer, hobs b. Average values. End groups included. Tp = 70 °C. 

The use of statistical models to interpret (and to rationalize) NMR tacticity and sequence data is well established (97,98). In this volume the enantiomorphic-site model has been used by Segre et al. in their studies of polypropylene at high fields (55). A two-site model has been en loyed by Shimozawa et al. to observe the effects of internal donors m propylene polymerization (56). Other models for polyolefins have been reported in the literature, e.g., the multi-site model (99), the dual catalytic-site/chain-end model (100), the perturbed model (101), the consecutive two-site model (102), the four-component model (103), and the chain end model (104). 

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. 

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)... 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

The enantiomorphic site control model attributes stereocontrol in isoselective polymerization to the initiator active site with no influence of the structure of the propagating chain end. The mechanism is supported by several observations ... 

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. 

The steric triad distributions of polypropylene with structure (IS) are consistent with an enantiomorphic-site propagation model based on stereochemical control by the chirality of the active center on the catalyst 132,133). It should be noted that isotactic polypropylenes are formed along both propagation models, enantiomorphic-site control and chain-end control. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

In (53) PJ and Pg represent chains with active L and D ends in the case of an end-controlled polymerization such as the one we are considering (or, in the case of the enantiomorphic catalyst site model, catalyst sites preferring L- and D-monomers, respectively), P L, P D etc. are the... 

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 ... 

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 ... 

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). 

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