Steric Control in Free-Radical Polymerization

Polymerizations in thiourea canal complexes yield high melting crystalline rra/w-1,4-polybutadiene, 2,3-dimethylbutadiene, 2,3-dichlorobutadiene, and 1,3-cyclohexadiene. Cyclo-hexadiene monoxide, vinyl chloride, and acrylonitrile also form stereoregular polymers. On the other hand, polymerizations of isobutylene and of vinylidine chloride fail to yield stereospecific polymers. 

Sodium montmorilonite can also be used to polymerize polar monomers between the lamellae. Here, too, the organization of monomer molecules within the monolayers influences the structure of the resultant polymers. Poly (methyl methacrylate) formed in sodium montmorilonite is composed of short, predominantly isotactic stereosequences. The percent of isotactic component 

Perhydrotriphenylene also forms channel-like inclusions with conjugated dienes. Polymerization of these dienes yield some steric control. 

There were attempts at controlling steric placement by a technique called template polymerization. An example is methyl methacrylate polymerization in the presence of isotactic poly(methyl methacrylate). The presence of template polymers, however, only results in accelerating the rates of polymerizations. 

Sodium montmorillonite can also be used to polymerize polar monomers between the lamellae. Here too, the organization of monomer molecules within the monolayers influences the structure of the resultant polymers [212, 213]. Poly(methyl methacrylate) formed in sodium montmorillonite is composed of short, predominantly isotactic stereosequences [211]. The percentage of isotactic component increases with an increase in the ion exchanging population on the surface of the mineral and is independent of the temperature between 20 and 160°C. In this way, it is possible to vary the population of isotactic triads at will up to 50% composition [205]. 

Homogenous controlled/ living free radical polymerizations are based, therefore, on the reversible deactivations of growing radicals. Early, Matyjaszewski divided such polymerizations into three classes [240, 241]. These were  

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How can canal complexes be used for steric control in free-radical polymerization Give examples. 

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