Growing polymer chain, 238 hypothesis

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

In order to find evidence for this hypothesis we decided to prepare a polyro-taxane that would mimic the chain dimensions and therefore not only the electronic but also steric factors imposed by the growing polymer chain via a post-threading route. As the simplest model we selected a poly(hexamethylene amine) and its corresponding protonated form (Fig. 1.39). ... 

According to the second hypothesis, the a-olefin is adsorbed on a transition metal atom and the polymer chain grows on an other metal atom included in the catalytic center (88,90,91). In this case the asymmetry of the catalyst may be due either to the presence of an asymmetric alkyl group (R ) bound to the transition metal, which determines the preferential complexation of one of the a-olefin antipodes (XXXVIII), or to the arrangement of the two metal atoms and of the last carbon atom... 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The above findings suggest the hypothesis that in the case of Ti catalysts, the monomer coordinates to the active center by the vinyl group only. This hypothesis seems quite reasonable in the case of butadiene and isoprene since 1,2 and 3,4 polymers, respectively are formed from these monomers. The situation could seem different in the case of pentadiene which yields cis-1,4 polymers. On could suppose that in this case the formation of cis-1,4 units is caused by the fact that pentadiene coordinates to Ti by the two double bonds in the cis conformation before it is incorporated into the growing chain. 

All the findings reported under 1,2, and 3 suggest the hypothesis that, in the case of Co catalysts, the monomer coordinates to the active center in the cis conformation before it is incorporated into the growing chain. This hypothesis easily explains why the cis isomer of pentadiene is not polymerized, the cis conformation being prohibited for this isomer. It also explains why all the common conjugated hydrocarbon diolefins yield predominantly cis-1,4 polymers by these catalysts. In fact, if all the different diolefins coordinate to Co predominantly in the cis conformation (by the two double bonds), predominantly cis-1,4 polymers will be obtained in every case. 

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