Steric Control in Free-Radical Polymerization

In free-radical polymerization reactions, the propagating radical chain has a great amount of freedom. Atactic polymers, therefore, are usually formed. Some control that the reaction conditions exercise over the propagating species increases at lower temperatures due to lower mobilities. This leads to increased syndiotactic placement, as was discussed in the section on propagation. Special techniques, however, such as the use of canal complexes can be employed to form stereoregular polymers by free radical mechanism. Urea and thiourea were used originally for such purposes . Monomers such as butadiene or others form complexes within the voids, or canals of the crystal lattices of these compounds. Brief exposure to high energy radiation initiates chain growth. In the canals, the monomer molecules are held in fixed positions, so chain growth is restricted in one direction only. Steric control is exercised because in these fixed positions the monomer molecules tend to align uniformly. It was suggested that in the canal complexes the monomers are not just lined up end to end, but packed in an overlapping arrangement. For molecules such as isobutylene or vinylidene chloride, it may be possible for the monomers to lie directly on top of each other, resembling a stack of coins. Such stacking greatly facilitates reactions between guest molecules . Polymerizations in thiourea canal complexes yields high melting crystalline trans-1,4 polybutadi ene, 2,3-dimethylbutadiene, 2,3-dichlorobutadiene, and 1,3-cyclohexadiene. Cyclohexadiene mon oxide, vinyl chloride, and acrylonitrile also form stereoregular polymers. On the other hand, polymerizations of isobutylene and of vinylidene chloride fail to yield stereospecific polymers. 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 . Poly(methyl methacrylate) formed in sodium montmorillonite is composed of short, predominantly isotactic stereosequences . 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 160C. In this way, it is possible to vary the population of isotactic triads at will up to 50% composition . Perhydrotriphenylene also forms channel-like inclusions with conjugated dienes. Polymerization of these dienes yields some steric control . Uemura and coworkers carried out radical polymerizations of vinyl monomers (styrene, methyl methacrylate, and vinyl acetate) within various nanochannels of porous coordination polymers. They studied the relationships between the channel size and polymerization behaviors, such as monomer reactivity, molecular weight, and stereo structures. They reported that in these polymerization systems, the polymer-growing radicals were remarkably stabilized by efficient suppression of the termination reactions within the channels, resulting in relatively narrow molecular weight distributions. A significant nanochannel effect on the polymer stereoregularity was also seen, leading to a clear increase of isotactic placement in the resulting polymers. 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). Thus template polymerization is a process of polymerizing a monomer in the presence of a polymer, usually from a different monomer. The presence of template polymers, however, only results in accelerating the rates of polymerizations .

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مواضيع ذات صلة

The rmal Polymerization

Inhibition and Retardation

The Termination Reaction

Ring Forming Polymerization

Polymerization of Monomers with Multiple Double Bonds

Methods for Measuring Molecular Weights of Polymers

Radius of Gyration

Solutions of Polymers

Orientation of Polymers

The Mesomorphic State, Liquid Crystal Polymers

Kinetics of Crystallization

Thermodynamics of Crystallization