Coordination polymerization syndiotactic polypropylene

The Dow corporation has recently developed constrained geometry addition polymerization catalysts (CGCT), typically Me2Si(C5Me4)(NBut)MCl2 (M = Ti, Zr, Hf) (141) activated with MAO. The homo-polymerization of a-olefins by CGCT afford atactic or somewhat syndiotactic (polypropylene rr 69%) polymers. The metal center of the catalyst opens the coordination sphere and enables the co-polymerization of ethylene to take place, not only with common monomers such as propylene, butene, hexene, and octene, but also with sterically hindered a-olefins such as styrene and 4-vinylcyclohexene [202]. 

Propylene monomer, like ethylene, is obtained from petroleum sources. Free-radical polymerizations of propylene and other a-olefins are completely controlled by chain transferring. It is therefore polymerized by anionic coordination polymerization. At present, mainly isotactic polypropylene is being used in large commercial quantities. There is some utilization of atactic polypropylene as well. Syndiotactic polypropylene, on the other hand, still remains mainly a laboratory curiosity. 

Coordination polymerization also produces high sterospecificity in the polymerization of alkenes. Isotactic and syndiotactic polymers can be obtained by appropriate choice of the catalyst components although such polymers are not useful as elastomers. However, Ziegler-Natta catalysts are used to produce EPR and EPDM rubbers. (Coordination polymerization is important for the synthesis of linear polyethylene and isotactic polypropylene which find extensive utility as plastics.) The Symposium paper by Su and Shih describes the synthesis of propylene-l-hexene block copolymers using several catalysts based on titanium and aluminum components. 

Free-radical polymerization arranges functional groups, such as alkyls, in a random manner, whereas coordination polymerization can exercise stereochemical control over functional groups. With the proper choice of experimental conditions, such as temperature, solvent, and catalyst, monomers can polymerize to any of three arrangements isotactic, syndiotactic, and atactic. (For polyethylene, there are no such stereoisomers, since the monomeric units are identical, —CH2—.) Isotactic and syndiotactic polypropylenes are highly crystaUine atactic polypropylene is a soft, elastic, and rubbery material. Following are the three stereoisomers of polypropylene ... 

Polypropylene (PP) is a semicrystalline commodity thermoplastic produced by coordination addition polymerization of propylene monomer [197]. Most frequently, stereospecific Ziegler-Natta catalysts are used in industrial processes to produce highly stereospecific crystalline isotactic (iPP) and syndiotactic (sPP) polymer with a small portion of amorphous atactic PP as a side product. Polymerization of non-symmetrical propylene monomer yields three possible sequences however, the steric effect related to the methyl side group highly favors the head-to-tail sequence. The occurence of head-to-head and tail-to-tail sequences produces defects along the PP chain [198]. Presence of such defects affects the overall degree of crystallinity of PP. 

Polypropylene (PP) with the formula [-CH2CH(CH3)-] , CAS 9003-07-0, is a common polymer that is usually obtained by coordination catalyst polymerization. This polymer can be made in isotactic, syndiotactic or atactic forms, typically head to tail. Most practical uses are known for isotactic polypropylene, which is linear and highly crystalline. As indicated in Section 1.3, the linearity of the isotactic polymer implies in fact that the backbone forms a regular spiral that in case of polypropylene has three units per turn, as schematically shown below ... 

Further support for this mechanism was provided by Ewen in the form of a catalyst which polymerizes propylene to hemiisotactic polypropylene. The metallocene shown in Scheme IV has two different coordination sites, one which is isospecific and one which is aspecific.51 When used for propylene polymerization, the alternation between iso- and aspecific sites results in a hemiisotactic polymer (Scheme IV). The polymer was readily characterized due to the pioneering work of Farina, who independently prepared this material previously. The rational synthesis of isotactic, syndiotactic, and hemiisotactic polyolefins represents a crowning achievement in the application of transition metal catalysts in stereocontrolled reactions. 

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