Metallocene-catalyzed propylene polymerization

SCHEME 1.2 General mechanism of chain propagation for metallocene-catalyzed propylene polymerization (L = cyclopentadienyl or similar hgand). 

One very important feature of metallocene-catalyzed propylene polymerization is that more than one polymer chain is produced by each metal site. This is possible through mechanisms of chain release which follow monomer insertion, both 1,2-insertion and 2,1-insertion. Scheme 1.5 depicts the two most common chain release mechanisms f)-hydride transfer to the metaP (with or without associative displacement by incoming monomer) and -hydride transfer to the monomer. Both processes result in identical chain endgroups (vinylidene and n-propyl), but can be studied by examining the rate law for chain release. 

Guo, Z. Swenson, D. C. Jordan, R. F. Cationic zirconium and hafnium isobutyl complexes as models for intermediates in metallocene-catalyzed propylene polymerizations. Detection of an a-agostic interaction in (C5Me5)2Hf(CH2CHMe2)(PMe3)" ". Organometallics 1994, 13, 1424-1432. 

In metallocene-catalyzed propylene polymerization, propagation proceeds via 1,2-insertion of the monomer. 2,1-Insertion gives rise to a secondary alkyl species. This species is known to be much less active for the next insertion and tends to be involved in chain transfer or isomerization into 1,3-inserted species. As shown in Scheme 4, 0-hydrogen elimination followed by rotation and re-in-... 

Allyl PS macromonomers, which were synthesized by the ATRP of styrene with CuBr/bipyridine, have been used as comonomers in metallocene-catalyzed propylene copolymerizataions using Me2Si(2-Me-4,5-BzInd)2ZrCl2/ MAO [110]. It has been found that the incorporation of the PS macromonomers increases with a decrease in molar mass of the macromonomer and propylene concentration and increasing polymerization temperature. The highest comonomer incorporation (10.8 wt%) was achieved in the copolymerization at 70 °C. 

There is a large amount of literature and many patents in this area, as well as many good reviews and books [8,9,10,11,12,13,14,15,16,17,18,19]. The recent review by Coates [10] describing stereoselective polymerization overlaps considerably with this chapter, and is recommended for consultation. In this chapter, metallocene-catalyzed olefin polymerization is discussed, focusing on the synthesis of stereoregulated polymers. The aim of this review is not to be a complete survey the outline and some recent topics in polymerization of propylene, higher a-olefins, styrene, acrylate esters such as methyl methacrylate (MMA), 1,3-butadienes, and cycloolefins will be described. Polyethylene is one of the most important commercially manufactured polymers. The homopolymer, as well as the copolymer with ethylene and other olefins, is an important subject in the polyolefin industry. However, it will be only briefly mentioned because the stereochemistry is less involved. 

Chien JCW, Llinas GH, Rausch MD, Lin YG, Winter HH, Atwood JL, Bott SG (1992) Metallocene catalysts for olefin polymerizations. XXIV. Stereoblock propylene polymerization catalyzed by rac-anri -ethylidene(l-T 5-tetramethylcyclopentadienyl)(l-r 5-indenyl) dimethyltitanium A two-state propagation. J Polym Sci A 30 2601-2617... 

Metallocene-Catalyzed Polymerization of Propylene to Highly Isotactic Polypropylene in Organic Suspension... 

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

Natta postulated that for the stereospecific polymerization of propylene with Ziegler-Natta catalysts, chiral active sites are necessary he was not able to verify this hypothesis. However, the metallocene catalysts now provide evidence that chiral centers are the key to isotacticity. On the basis of the Cossee-Arlman mechanism, Pino et al. (164,165) proposed a model to explain the origin of stereoselectivity The metallocene forces the polymer chain into a particular arrangement, which in turn determines the stereochemistry of the approaching monomer. This model is supported by experimental observations of metallocene-catalyzed oligomerization. 

In the early 1980s, Kaminsky and Sinn discovered an efficient way to activate homogeneous metallocene catalysts with methylaluminoxane (MAO). Titanocene and zirconocene complexes activated with MAO exhibited very high activity for ethylene polymerization these early systems, however, still had low activity for propylene polymerization and formed atactic polypropylene [5]. Met-allocene/MAO systems containing stereospecific ligands could be used to catalyze the polymerization of prochiral olefins (a-olefins) through the use of catalysts with well-defined active sites [6]. Later, Brintzinger [7] and Ewen... 

Chien, J. C. W. Elinas, G. H. Rausch, M. D. Lin, Y. G Winter, H. H. Atwood, J. L. Bott, S. G. J. Metallocene catalysts for olefin polymerization catalyzed by rac-[anft -ethylidene(l-r -tetramethylcyclopentadienyl)(l-r -indenyl)]dimethyltitanium A two-state propagation. XXIV. Stereoblock propylene polymerization. J. Polym. ScL, Part A Polym. Chem. 1992, 30, 2601-2617. 

Hydrosilylation ofmethyldiundecenylailane (68) yielded hyperbranched polymer (69) having terminal olefins [40]. Subsequent hydroboration with bis(pentafluoroph-enyl) borohydride (70) gave the polymeric borane catalyst (71) [41] (Scheme 19.17). The polymeric borane was tested as cocatalyst in the metallocene-catalyzed polymerization of propylene. Activation through the polymeric borane (71) led to higher activities compared to that of B(C6Fs)3. 

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