Polypropylene (PP): Synthesis

An important concept in understanding the link between the structure of Polypropylene (PP) and its properties is tacticity. The relatives orientation of each methyl group relative to the methyl groups on neighbouring monomers has a strong effect on the finished polymer's ability to form crystals, because each methyl group takes up space and constrains backbone bending. Like most other vinyl polymers, useful polypropylene cannot be made by radical polymerization due to the higher reactivity of the allytic hydrogen (leading to dimerization) during polymerization. Moreover, the material that would result from such a process would have methyl groups arranged randomly, so called atactic Polypropylene (PP). The lack of long range order prevents any crystallinity in such a material, giving an amorphous material with very little strength and only specialized qualities suitable for niche end uses. A Ziegler-Natta catalyst is able to limit incoming monomers to a specific orientation, only adding them to the polymer chain if they face right direction. Most commercially available Polypropylene (PP) is made with such Ziegler-Natta catalysts, which produce mostly isotactic polypropylene (the upper chain in the figure above). With the methyl group consistenly on one another to form the crystals that give commercial polypropylene many of its desirable properties. More precisely engineered Kaminsky catalysts have been made, which offer a much greater level of control. Based on metallocene molecules, these catalst use organic groups to control the monomers being added, so that a proper molecules, these catalysts use organic groups to control the monomers being added, so that a proper choice of catalyst can produce isotactic, syndiotactic, or atactic polpropylene, or even a combination of these. Aside from this quantitative control, they allow better quantitative control, with a much greater ratio of the desired tacticity than previous Ziegler-Natta techniques. They also produce narrower molecular weight distributions than tradisional Ziegler-Natta catalyst, which can further improve properties. Tproduce a rubbery polypropylene, but with the organic groups that influence tacticity held in place by a relatively weak bond. After the catalyst has produced a short length of polymer which is capable of crystallization, light of the proper frequency is used to break this weak bond, and remove the selectivity of the catalyst so that the remaining length of the chain is atactic. The result is a mostly amorphous material with small crystals embedded in it. Since each chain has one end in a crystal but most of its length in the soft, amorphous bulk, the crystalline regions serve the same purpose as vulcanization.