Stereospecific Placement

The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond of the monomer together with an electrophilic attack by the cationic counterion (G) on the alkene tt-electrons. The catalyst fragment acts essentially as a template or mold for the orientation and isotactic placement of incoming successive monomer units. Isotactic placement occurs because the Initiator fragment forces each monomer unit to approach the propagating center with the same face. This mechanism is referred to as catalyst site control or enantiomorphic site control. 

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The homogeneous catalysts can range in efficiency from very high to very low. Usually, a much narrower molecular weight distribution is obtained in polymers formed with these catalysts than with the heterogeneous ones. In addition, syndiotactic poly(a-olefins) form at low temperatures (-78 C). The amount of stereospecific placement, however, is usually not as great as is the isotactic placement with heterogeneous catalysts. 

Stereospecific placement appears to result from steric interactions between the substituents on the ultimate units of the growing chains and the incoming monomers. NMR spectral evidence shows that the double bond opening is also cis, as with the heterogenous catalysts. 

Several reaction mechanisms were also proposed to explain stereospecific placement with insoluble catalysts. Furukawa [46] suggested that here the mechanism for cationic polymerization of vinyl ethers depends upon multicentered coordinations. He felt that coordinations of the polymeric chains and monomers with the catalysts are possible if the complexed counteranions have electrically positive centers. This can take place in the case of aluminum alkyl and boron fluoride ... 

The amount of stereospecific placement, however, is usually not as great as is the isotactic placement with heterogeneous catalysts. 

In copolymerization of ethylene with different class monomers, all the foregoing aspects are superimposed various chemical comonomer sequences can be generated, with various mutual stereospecific placements. Chemical comonomer sequences can be analyzed by statistical methods completely analogous to those described in Section IV (Tables 3 and 4), but practical analysis may be complicated by superposition of stereosequence statistics. Short-chain branching in longer polyethylene sequences is an additional factor to be considered. All these aspects can be demonstrated on the example of the ethylene-vinyl alcohol E/V copolymer prepared by a high pressure, free radical process, which has been characterized in great detail by modern NMR methods. 

In an ionic polymerization the strong electrostatic field of the ion pairs should have a pronounced effect on the ratio of the probabilities of the two placements. Furthermore, solvation of an ion pair is much stronger than of a neutral radical, hence the influence of a solvent on stereospecificity of addition is expected to be much more pronounced in an ionic polymerization than in a radical polymerization. The nature of the gegen ion represents still another factor which is of extreme importance in determining the stereospecificity of the polymerization. 

The structure of the chain, i.e., whether it is a helix or a random coil, might influence not only the rate but also the stereospecificity of the growing polymer. For example, it is plausible to expect that in normal vinyl polymerization helix formation might favor specific placement, say isotactic, while either placement would be approximately equally probable in a growing random coil. Formation of a helix requires interaction between polymer segments, and this intramolecular interaction is enhanced by bad solvents particularly those which precipitate the polymer. 

With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

The isotacticity is higher in poly[(R)-Q-methylbenzyl methacrylate] prepared by BuLi in toluene than in the polymer of the racemic monomer. However, the stereoregularity of poly[(S)-a-methylbenzyl methacrylate-co-methyl methacrylate] is mostly the same as that of poly((RS)-a-methylbenzyl methacrylate-co-methyl methacrylate], regardless of their compositions except for low methyl methacrylate contents. This indicates that the isotactic placement of the (R)- or (S)-monomer to the growing chain ending in the antipode monomer unit is less favorable than to die anion of the same monomer unit, while the stereospecificity in the addition of a-methylbenzyl methacrylate to methyl methacrylate unit should be the same between the (R)- and (S)-monomers. ... 

Ionic polymerizations yield highly stereoregular polymers when control is exercised over monomer placement. The earliest stereospecific vinyl polymerizations were observed in preparation of poly isobutyl vinyl ether) with a BFa-ether complex catalyst at -70 °C. An isotactic polymer formed. °The same catalyst was employed later to yield other stereospecific poly(vinyl ether)s. " The amount of steric placement increases with a decrease in the reaction temperature, and, conversely, decreases with an increase in the temperature. ... 

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