Polymerization Reactions

The tremendous growth of polymer production since the middle of the twentieth century has been intimately connected with the development of new types of catalysts, A notable example in this regard is the field of poly olefin materials, While catalysts are also used in the production of other types of polymers, the properties of most of these materials are not particularly dependent on the type of catalyst employed, Many polycondensation reactions, e, g, the formation of polyesters, polyamides or urea-formaldehyde resins, are speeded up by addition of some Br0nsted or Lewis acids. Since relevant properties of these polymer products, such as their average chain lengths, are controlled by equilibrium parameters, primarily by the reaction temperatures and molar ratios of the monomers employed, and since their linkage patterns are dictated by the functional groups involved, addition of a catalyst has little leverage on the properties of the resulting polymer materials, For typical polyolefin materials, on the other hand, the most relevant properties depend in addition to the types and molar ratios of the monomers used quite critically on the catalyst used for their production, This is due to the large numbers of different structural elements which can be formed with practically equal free energies by polymerization reactions even of simple olefins such as ethylene and/or propylene (Figure 1), The proportions with which each of these concatenation patterns occurs in a particular polymer product are thus controlled by the relative rates of their formation, i,e, by the selectivity with which these patterns are produced in the course of the polymerization process employed, rather than by any equilibrium parameters.