Syndiotactic polypropylene polymerization

Bis(phenoxy-imine) catalysts in olefin polymerization, 4, 1096 for syndiotactic polypropylene polymerization, 4, 1115 Bis(phenoxy) imine-donor complexes, with Zr(IV) and Hf(IV), 4, 814... 

When propene is polymerized under free radical conditions the polypropylene that results IS atactic Catalysts of the Ziegler-Natta type however permit the preparation of either isotactic or syndiotactic polypropylene We see here an example of how proper choice of experimental conditions can affect the stereochemical course of a chemical reaction to the extent that entirely new materials with unique properties result... 

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

Figure11-1. Propylene can undergo polymerization in three different ways to form atactic (a), isotactic (b), or syndiotactic polypropylene (c). " ...

Fig. 10. 220 MHz proton NMR spectra of solutions of polypropylene, (a) Isotactic polypropylene broad bands between 8.62 and 8.8 r, others hidden by CH3 resonances in the region of 9.0 and 9.25 r (85). (b) Syndiotactic polypropylene a single set of peaks between 8.8 and 9.0 r (85). (c) Soluble polypropylene obtained by polymerization with Zr(benzyl)4 (38).

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]. 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

Syndiotactic polymerization, 76 99-102 Syndiotactic polymethacrylate esters, glass transition temperatures of, 76 273t Syndiotactic polypropylene, 20 524 Syndiotactic polystyrenes (SPS), 23 365 Syndiotactic polystyrene, 70 180-183 applications for, 70 183 properties of, 70 182... 

The low-temperature polymerization of propylene has been performed with various types of vanadium compounds instead of V(acac)3. The results are summarized in Table 1. Vanadium compounds such as VC14, VOCl3, VO(acac)2 and VOCl2(acac) afford syndiotactic polypropylenes with rather broad MWDs, close to the most probable distribution (Mw/lVln = 2.0). At present, V(acac)3 is the sole vanadium compound being effective for the living polymerization of propylene. 

Tian J, Coates GW, Development of a diversity-based approach for the discovery of stereoselective polymerization catalysts Identification of a catalyst for the synthesis of syndiotactic polypropylene, Angew. Chem. Int. Ed. Eng., 39 3626-3629, 2000. 

The next important step was made using the ansa metallocenes synthesized by Brintzinger et al. in 1982 [28] for the stereospecific polymerization of propene. Ewen et al. [29] succeeded 1988 in synthesizing a Cs-symmetric zireonoeene ([Me2C(Flu)(Cp)]ZrCl2) which produces syndiotactic polypropylene in high quantities. Since 1985, a rapid, worldwide industrial and academic development began in the field of metallocene catalysts which continues today. 

The Ziegler-Natta catalysts that are used most industrially are solids that are suspended in the reaction medium. The polymerization reaction that is favored depends on the catalyst components and on their state of aggregation and other details of their preparation. Heterogeneous catalytic systems appear to be necessary for the production of isotactic polyolefins, but soluble catalysts have been used for the synthesis of polyethylene and syndiotactic polypropylene. 

Examples of homogeneous catalytic systems in propylene polymerization are very few. Zambelli et al. obtained syndiotactic polypropylene by working at low temperatures with catalytic systems based on some vanadium compounds and aluminum alkyls, while Giannini et al. were the first to prepare isotactic polypropylene with some benzyl derivatives of titanium or zirconium. 

MWD variation of polypropylene with time has been investigated by many authors. Doi et al. found no dependence of MWD on polymerization time (in the range 2 to 10 hours) for syndiotactic polypropylene obtained at —78 C with VCl — A1(CjH5)2C1 homogeneous catalytic system. 

---