Steric effect chain polymerization

These alkoxides are liquids or sublimable solids and, unless the steric effects of the alkyl chain prevent it, apparently attain octahedral coordination of the titanium by polymerization (Fig. 21.6). The lower alkoxides are especially sensitive to moisture, hydrolysing to the dioxide. Application of these organic titanates (as they are frequently described) can therefore give a... 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

A wide variety of homogeneous and hetrogeneous systems are effective for polymerizing various monomers to isotactic structures. Therefore the question must be raised as to whether macro-surface theories have any required validity for steric control but whether some other factor is important. Fig. 10 summarizes the monomers polymerized to isotactic polymers and shows that all propagate with an asymmetric center at the end of the chain. 

Cp-substituted reactants. It is not clear why such long chain lengths are required to alleviate steric interactions in the Humphrey-Lucas molecules, but similar steric effects may be acting in the polymerizations of complex 3, where two molecules of 3 and a catalyst molecule all need to interact to carry out the polymerization. Nevertheless, despite this one setback, ADMET remains an attractive method for synthesizing new polymers with metal-metal bonds along their backbones. 

Clearly the benzylic H-atoms attached to the polystyrene backbone are not as labile as in cumene. This is likely due to the steric effect of the coil configuration of the polymer chain which blocks access of the /er/-butoxy radicals. Nonetheless, some backbone H-atom abstraction from the polystyrene backbone does occur during radical polymerization of styrene. The extent of abstraction is proportional to the concentration of peroxide initiator added to the process. Typically, in commercial continuous bulk polymerization processes the concentration of peroxide initiator is kept below 500 ppm. Also a few percent of a solvent having some chain transfer activity (ethylbenzene) is added to the styrene feed. This is done so that the extent of branching is small. If the concentration of initiator is increased to >500 ppm and/or the chain transfer solvent falls below a certain level, the extent of branching can increase to a level where gels began to appear in the product. The mechanism of... 

This probably cannot be accounted for on the basis of steric effects, but may have its origin in greater solubility in the case of the stilbazole as opposed to vpy or BPE-containing polymeric metal complex chains, which results in much lower surface coverages. 

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