ISSN:
0022-3832
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Recent results of investigations of stereospecific polymerizations are reported. Butadiene polymers very high in cis-1,4 content have been obtained through polymerization in the presence of soluble catalysts prepared from compounds of group VIII metals, cobalt in particular, and from dialkylaluminum monohalides. The polymers thus obtained have higher melting temperatures and better elastomeric properties than those of cis-1,4 polybutadienes of lower steric purity.Two types of butadiene polymers having 1,2 enchainment and isotactic or syndiotactic structure may be prepared in the presence of soluble catalytic systems prepared from trialkylaluminum and complexes of chromium or vanadium. A great number of nonhydrocarbon monomers have also been polymerized stereospecifically, in the presence of catalysts acting in homogeneous phase. Metallorganic compounds of magnesium act as stereospecific catalysts for the polymerization of 2-vinylpyridine, with production of crystalline isotactic polymers. Crystalline isotactic polymers of acetaldehyde have been prepared by polymerization at low temperature with the use of metallorganic compounds such as alkylaluminum and alkylzinc as catalysts. Aldehydes higher than acetaldehyde have also been polymerized stereospecifically with catalysts that act usually with a cationic mechanism, such as aluminum bromide. The crystalline structure and the crystalline helical chain conformation of a great number of these polyaldehydes have been determined. A chemical mechanism for this polymerization, which makes clear the stereospecificity of the process even in the presence of soluble catalysts, has been proposed. Stereospecific catalysts have been prepared from compounds of amides with very electropositive metals. In the presence of TiCl3, these catalysts promote the stereospecific polymerization of propylene to isotactic polymer, whereas, as such, they furnish crystalline polymers of acrylates, methacrylates, and 2-vinylpyridine. The nature of the terminal groups of the polymers establishes the fact that the polymerization occurs with an anionic mechanism. The mechanism of the stereospecific polymerization of α-olefins is discussed, and further evidence is given confirming the previously hypothesized anionic coordinated mechanism. The factors determining stereospecificity are also discussed; it is concluded that when a group rich in electrons in addition to the polymerizable double bond is present in the monomer, stereospecific polymerization may take place in the presence of catalysts acting in homogeneous as well as heterogeneous phase. In these cases, stereospecificity must be ascribed to the coordination of the monomer to the catalyst, which results in a constant mode of presentation of the monomer to the growing chain, and to a constant type of opening, cis or trans, of the polymerizable double bond. For the polymerization of propenyl ethers to di-isotactic polymers, we established that this opening is of the cis type.
Additional Material:
3 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1960.1204815023
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