The properties of different polymers may be brought together by different ways. Random copolymer is one method. More strikingly new and improved properties are achieved by graft or block copolymerization, with respective examples of ABS and impact modified polypropylene. Specific process is required to prepare each such copolymer system. This is illustrated by the synthesis of thermoplastic elastomeric polypropylene, comprising of alternating blocks of isotactic polypropylene (I-PP) and atactic polypropylene (a-PP) . It is prepared with a specially designed asymmetric (C_1) metallocene catalyst, rac-ethylidene( 1-η~5- tetramethylcyclopentadienyl)( 1-η~5-indenyl) titanium dichloride, which functions by switching between two catalytic states to promote alternatively stereoselective and nonselective polymerization of propylene. A general method to obtain new products and to expand the market is by the physical mixing together (blending) of existing products into polymeric alloys or blends. No new polymer need be synthesized. Since two polymers are usually immiscible for well-known thermodynamic reason, alloy is best formed between polymers with comparable cohesive energy densities. The process can be promoted by compatibilizing agent. Such system is thermodynamically unstable. However, mixing at phase boundary may persist sufficiently long for it to have useful applications. The most effective compatibilizing agent is usually a block copolymer of the polymers to be blended. However, the synthesis of any block copolymer is not a simple matter (vide supra). Polyolefins form a large family of materials possessing every known property. However, it is very difficult to prepare blends of polyolefins. We disclose here a method to prepare novel "naturally" compatible polyolefin alloys by binary single-site Ziegler catalysts.
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