Syndioselective Propagation

The initiator formed from VCLt and A1(C2H5)2C1 is one of the most efficient means for syndioselective polymerization of propene, especially in the presence of a Lewis base such as anisole (methoxybenzene) [Doi, 1979a,b Natta et al., 1962 Zambelli et al., 1978, 1980], Other vanadium compounds such as vanadium acetylacetonate and various vanadates [VO(OR)xClp x), where x — 1,2,3] can be used in place of VCI4 but are more limited in their stereoselectivity [Doi et al., 1979]. Trialkylaluminum can also be used as a coinitiator, but only for VCI4. Syndiotacticity increases with decreasing temperature most of these syndioselective polymerizations are carried out below —40°C and usually at —78°C. The initiators must be prepared and used at low temperatures since most of them undergo decomposition at ambient and higher temperatures. There is considerable reduction of V(III) to V(II) with precipitation of ill-defined products that are low in activity and do not produce syndiotactic polymer, when the initiators are prepared at or warmed to temperatures above ambient. 

In summary, syndioselective initiators exaggerate the inherent tendency toward syndiotactic placement by accentuating the methyl-methyl repulsive interactions between the propagating chain end and incoming monomer. Isotactic placement occurs against this inherent tendency when chiral active sites force monomer to coordinate with the same enantioface at each propagation step. 

The polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

Not all syndioselective polymerizations proceed with polymer chain end control. Some metallocene initiators yield syndioselective polymerization through catalyst site control (Sec. 8-5). 

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All aspects of syndioselective propagation with ( -symmetric catalysts, the influence of ligand,794,797-807 metal,806,808,809 and counteranion100,104 variations, as well as the influence of the polymerization conditions,805,810 have been studied in detail and reviewed.162,181,209,288,811... 

Styrene unit and the incoming styrene molecule. In Scheme 14.5, the incorporation of a coordinated styrene molecule is fulfilled via two simultaneous steps cis-migratory insertion, and a nucleophilic substitution of the phenyl moiety of the last unit of the growing polymer chain end by the newly incorporated unit. Sequentially after incorporating this unit, a new styrene coordinates enantioselect-ively to the metal center, leading to a mirror related active species that implies the syndioselective propagation of the polymer chain. ... 

Syndioselective polymerizations of propene are somewhat less regioselective than the isoselective reactions, with the typical highly syndiotactic polymer showing a few percent of the monomer units in head-to-head placement [Doi, 1979a,b Doi et al., 1984a,b, 1985 Zambelli et al., 1974, 1987]. The mode of insertion is secondary, contrary to what is expected for a carbanion propagating center. Apparently, steric requirements imposed by the counterion derived from the initiator force propagation to proceed by secondary insertion. 

Of all the hafnium compounds listed in Figure 2.7, none excels their zirconium counterparts in syndioselectivity. This observation is sometimes attributed to hafnium s decreased rate of bimolecular propagation relative to unimolecular site epimerization. Lower overall activities with hafnium catalysts corroborate this argument. The most syndioselective hafnium catalyst appears to be Cl-symmetric i-92-Hf, which produces syndiotactic polypropylene with [r] = 92.2% and... 

The polymerization is assumed to proceed via (1) nucleophilic attack of diazobearing carbon of EDA to the Rh(I)-center of propagating chain end, (2) formation of Rh(in)-carbene complex after ehmination of N2, and (3) inserhon of the carbene into Rh-C bond regenerating the Rh(I) propagating species (Scheme 28). DPT calculation fohowing this mechanism indicated the preference for the syndioselective insertion step in chain end control. 

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