Polypropylenes stereo-block

Semicrystalline polyolefin blends and method of their preparation were described. The blends were reported to show enhanced inter-spherulitic and interlamellar strength. The first polymer should have higher crystallinity and crystallization temperature than the second. Thus, 50-99.9 wt% PP was blended with ethylene-a-olefin copolymers, either a stereo block polypropylene or an ethylene-propylene copolymer, EPR... 

In chain reactions the different types of monomers can be added subsequently to an active chain end. The most important techniques here are sequential living polymerization techniques, such as anionic or cationic polymerization. Certain metallocenes can be used in coordination polymerization of olefins leading to stereo block copolymers, like polypropylene where crystalline and amorphous blocks alternate with each other due to the change of tacticity along the chain [34]. In comparison to living polymerization techniques, free radical and coordination polymerization lead to rather polydisperse materials in terms of the number of blocks and their degree of polymerization. 

The most recent developments in this area relate to custom-made zir-conocenes for specific applications. The stereo-electronic environment at the zirconium center can be varied in a very subtle manner and this can lead to the formation of specific polymers. Thus, a bridged metallocene in combination with MAO can lead specifically to isotactic polypropylene (Fig. 2.26) [26]. Similarly, variation in the metallocene structure can lead to specific formation of a syndiotactic polypropylene. Even a polypropylene containing alternating isotactic and syndiotactic blocks can be prepared by modifying the metallocene. 

A direct proof that a certain sample of polypropylene contains stereoblock chains is NMR identification of the (stereo)chemical junctures between the blocks. In most cases, though, this is very difficult to achieve, if not impossible. 

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