Polymerization isotactic

Figure 10.11. Single mistake during a chain-end controlled isotactic polymerization and schematic 13C NMR spectrum of the methyl region (assuming that chain continues in m mode)...

Figure 10.14. Simplified view of site-controlled isotactic polymerization...

Polymers with at least partial helical conformation, synthesized by isotactically polymerizing an achiral monomer using a chiral catalyst96-99, have also been used as CSPs. This is an interesting concept (see Figure 11) with considerable future potential. 

Fig. 33. Values of the ratio between the rate of the chain-transfer processes depending on the catalyst concentration and the rate of the chain-transfer process with the monomer (plus the rate of the spontaneous termination process) (I = 70°, pc,ut = 950 mm. Hg, ground a-TiCU sample A). The values were calculated assuming for the isotactic polymeric fraction x = Kih] = KiC. ...

Yerusalimskii has demonstrated a similar mechanism for acrylonitrile polymerization initiated by Grignard reagents (13). We were unable at the time to investigate the isotactic polymerization initiated by t-BuMgBr in toluene as it was too fast for classical techniques. We have now developed techniques which can follow this reaction. 

Preliminary results on the kinetics of the polymerization and the efficiency of initiation of the isotactic polymerizations initiated by t-BuMgBr in toluene solution are consistent with the Bateup mechanism proposed for the stereoblock and syndio-tactic-like polymerizations initiated by n-BuMgBr in THF-rich solution — a mechanism which involves initiation and propagation through monomer — active centre complexes (5,8). 

The optimum conditions for isotactic polymerization require a trace of THF in excess of the mono-etherate stoichiometry. 

Isotactic polymerization requires the presence of halide, and is critically dependent on the nature of the alkyl or aryl group. At least one of each of these groups must be present in the vicinity of the active centre. The requirement of a minimum amount of THF for the formation of an isotactic growth site implies that this must also be coordinated at this site. We have evidence that monomer is complexed at the active centre, which has been postulated for many years (10,15). 

Isotactic polymerization occurs with practically complete regioselectivity, that is, with a continuous head-to-tail addition, which is a prerequisite to stereoregularity. 

It was demonstrated by spectroscopic studies (IR, NMR) that in isotactic polymerization of propylene a so-called primary or type 1-2 insertion... 

Studies on stereoselective polymerization of racemic olefins also support this view.338 Polymerization of 3,7-dimethyl-l-octene (the chiral center is in a position to the double bond) took place with 90% stereoselectivity yielding an equimolar mixture of homopolymers of the two enantiomers. No stereoselectivity was observed in the polymerization of 5-methyl-1-heptene (the chiral center is in y position to the double bond). The conclusion is that the ability of a catalytic center to distinguish between the two enantiomers of a monomer required for stereoselective polymerization must arise from its intrinsic asymmetry. The first-ever chiral polypropylene synthesized using a chiral zirconium complex with aluminox-ane cocatalyst is the latest evidence to testify the role of the catalyst center in isotactic polymerization.339... 

The addition of the monomer362 is syn, but in contrast to isotactic polymerization, secondary or type 2-1 insertion is characteristic of syndiotactic polymerization 358,359... 

Fig. 1. The proposed model of stereoregulation in the propagation process of isotactic polymerization of acetaldehyde by organoaluminum catalyst [Natta...

Fig. 11. The proposed model of the initiation of isotactic polymerization of acetaldehyde [Yasuda et cd. (42j ...

It is not yet determined whether a cocatalyst acts simply as a cocatalyst or both as a cocatalyst and as a reactant. We feel that the latter possibility is more plausible for the isotactic polymerization. 

Unfortunately, no catalyst is found which gives an isotactic polymer in quantitative yield. This fact obstructs the determination of the structure of the real active species for isotactic polymerization and the collection of unequivocal information about the mechanism of stereoregulation. Formation of the highly isotactic polymer which is cleanly separable from the atactic polymer indicates the existence of a highly stereospecific catalyst species in the polymerizing system. In order to answer to these unsolved problems, it will be necessary to do more experiments by utilizing new ideas or by more ingenious experimental techniques. 

After the discovery of isotactic polymerization of propylene using zirconocene catalysts, structurally analogous hafnium catalysts produced from hafnium tetrachloride [15499-05-5] were found to produce high yields of high molecular weight polypropylene (55), but not enough to lead to commercial development. 

The cationic nature of the polymerization of various alkyl vinyl-ethers has been studied by Iwasaki, Fukutani and Nakano (JO). They conclude that magnesium salts, being weak Lewis acids, are mildly effective for the isotactic polymerization of alkyl vinylethers. 

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