Ziegler-Natta catalyst, vinyl chloride polymerization

The Ziegler-Natta catalyst trimethylammonium o-methyl-1-(2-hydroxy Icy do hexyl)-carborane zirconium chloride has been prepared and affixed to a Merrifield resin. When used as a polymerization catalyst for vinyl chloride, t-butyl acrylate, styrene, or ethylene, oligomers with molecular weights <6000 daltons were obtained. 

The polymer is commonly obtained from vinyl chloride with a peroxide initiator such as peroxydicarbonates, fert-butylperpivalate, benzoyl or lauroyl peroxide, acetyl cyclohexylsulfonyl peroxide, or azobis(2,4-dimethylvaleronitrile). The polymerization can be done by suspension, emulsion or solution techniques. Low polymerization temperatures are used when high MW material is required. Suspension polymerization employs water suspension agents, such as poly(vinyl alcohol) or methylcellulose. The resulting polymer Is a partially syndiotactic material but with low crystallinity. The macromolecules typically have head to tail linkages (H-T) and a small proportion (less than 1.5%) of branching. Ziegler-Natta catalysts are not used to produce PVC. 

Polymerizations initiated by Ziegler-Natta catalysts may also proceed by a radical rather an a coordinated anionic mechanism [199]. In the polymerization of vinyl chloride with a catalyst system comprising tetra-butoxytitanium, triethylaluminum, and epichlorohydrin, the crystallinity of the resulting PVC was similar to that obtained by radical polymerization. In heptane, the process proceeded at one-half to one-third the rate as when the solvent was absent [199]. 

Attempts have been made to prepare stereoreguiar polymers from polar monomers (e.g. vinyl chloride, methyl methacrylate) using modified Ziegler-Natta catalysts, but without success. When polymerization does occur it yields non-stereospecific polymer and is thought to proceed by free-radical mechanisms. 

In heterogeneous polymerizations in bulk, the formed polymer is insoluble in its monomer and the polyreaction is performed below the softening point of the polymer. On an industrial scale, this type of process is especially utilized for chain polymerizations, for example, the radical polymerization of liquid vinyl chloride, the polymerization of liquid propylene with Ziegler-Natta or with metallocene catalysts, and the polymerization of molten trioxane. 

Silylene 1 is an unusually versatile catalyst for alkene and alkyne polymerization. The list of compounds polymerized by 1 includes ethene, propene, 1-hexene, styrene, dimethylbutadiene, vinylidene chloride, vinyl ethyl ether, methyl methacrylate, and phenylacetylene. The polymerization does not seem to take place by any of the usual mechanisms, anionic, cationic or free-radical. Instead it somewhat resembles coordination polymerization, as observed for Ziegler-Natta type catalysts. Silylene 2 also catalyzes the polymerization of 1-hexene, but the polymerization is 10 to 100 times slower than with 1. 

Polymerization of Vinyl Chloride with a Ziegler-Natta Type Catalyst. 408... 

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