Tacticity
Tacticity refers to the relative orientation of each methyl group to the neighboring methyl groups affecting local chain symmetry which affects crystallization and packing. Each methyl group takes up space and constrains backbone bending. Isotactic refers to methyl groups on one side only; atactic refers to random location of the methyl groups along the chain backbone and syndiotactic refers to methyl groups on alternate sides of the chain backbone. -CH2-CH - CH2- CH - CH2 - CH - CH2 – CH -CH2 – CH - CH2 - CH - CH3 CH3 CH3 CH3 CH3 CH3 (http://en.wikipedia.org/wiki/Polypropylene; 1/29/2010)
More on Tacticity Like most other vinyl polymers, useful polypropylene cannot be made by radical polymerization due to the higher reactivity of the allylic 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 PP. The lack of long-range order prevents crystallinity resulting in an amorphous material with very little strength. A Ziegler-Natta catalyst limits incoming monomers to a specific orientation, only adding them to the polymer chain if they face the right direction. Most commercially available polypropylene is made with such Ziegler-Natta catalysts, which produce mostly isotactic polypropylene (the upper chain in the figure above). With the methyl group consistently on one side, such molecules tend to coil into a helical shape; these helices then line up next to 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 catalysts use organic groups to control the monomers being added, so that a proper choice of catalyst can produce isotactic, syndiotactic, or atactic polypropylene, or even a combination of these. Aside from this qualitative 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 traditional Ziegler-Natta catalysts, which can further improve properties. To produce a rubbery polypropylene, a catalyst can be made which yields isotactic 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. (http://en.wikipedia.org/wiki/Polypropylene; 1/29/2010)
Recent US Patents
To go to the text version of the patent, click on US Patent Number Search and enter the patent number in the search box.
Recent Journal Articles
To get to the abstract or article, copy the citation and paste it into favorite search engine (ex. Google). The date is the date the entry was found
Review Articles
To get to the abstract and the article, open up your internet search engine and copy the title to the search box. Once on the Journal's website, find the year and issue. Then scroll to the article. The date is the date of the entry made on this page.
Editor's Notes
As the literature is reviewed, and items of interest concerning this topic are found. These may be added in an abbreviated form with the reference. Readers and contributors are invited to add their own notes. Contributors may add them directly and other readers can simply send their notes to the editor, Roger Corneliussen at rcorneliussen@4spe.org. He may edit and add them to this page at his discretion.