Propylene syndiotactic polymerization

In contrast to the case of Cp2ZrX2/MAO giving atactic poly(alkene)s, Cp MCl2/MAO, M = Zr (139) and Hf (140), are the catalyst precursors of the syndiotactic polymerization of 1-butene and propylene [176]. Triad distribution indicated that this is chain-end controlled syndiospecific polymerization. The syndiospecificity is attributed to the increase of steric encumbrance around the metal center. Thus, Cp HfX2 is the most effective syndiospecific catalyst component in this system. 

It must be noted that the approach of the monomer to the reactive metal-carbon bond should occur from the less hindered side. The greater stability of the trans complex should arise because the substituted carbon atom of the last unit is involved in the formation of the four-membered ring, so that reaction is hindered. The structure of copolymers of deuteropropylene from cis- and trans-(U,)-propylene proved that, even in syndiotactic polymerization, cis-addition to the double bond occurs, because with 1,2-disubstituted monomers, structures of ery-thro and threo di-syndiotactic polymers degenerate (see Fig. 4). 

In 1988, Ewen and Razavi developed a catalyst for the syndiotactic polymerization of propylene based on Cs-symmetric metallocenes (Table VI)... 

According to quantum-chemical calculations the formation of the 2,4-dimethyl-cyclopentane intermediate is favored. However, Zambelli et al. reported that the formation of a metal-cyclopentane intermediate cannot explain the isotactic and syndiotactic polymerization on the base of the same type of mechanism. With some assumptions (for example, about the chirality of metal environment) the metal-cyclopentane mechanism can explain the formation of isotactic propylene. However, the driving force of the formation of the syndiotactic polymer is not evident at all. 

With the replacement of Cp by a 9-fluorenyl moiety, the metallocene promotes syndiotactic polymerization of propylene under site control. Syndiotactic PP with (rrrr) as high as 0.77 was obtained with Me2Si(9-Flu) (N-t-Bu)ZrCl2/MAO catalyst. ° The stereoselectivity is due to the (pseudo)Cs-symmetry of the catalytic complex, and the stereocontrol mechanism is analog to that for the Cs-symmetric awM-metallocenes. 

Zambelli, A. Sessa, L Grisi, F. Fusco, R. Accomazzi, P. Syndiotactic polymerization of propylene single-site vanadium catalysts in comparison with zirconium and nickel. Macromol. Rapid Commun. 2001, 22, 297-310. 

Recent studies (86) on the mechanism of syndiotactic polymerization of propylene showed that in the presence of homogeneous... 

Both a and insertion lead to head-to-tail structures. A distinction can be made between these two mechanisms when the reaction is carried out with an excess of metal alkyl and analyzing the metal alkyl produced after killing the active centers with methanol. For example, the isotactic polymerization of propylene with TiCU/AlRs can be definitely stated to occur via a insertion, whereas the syndiotactic polymerization of the same monomer with VCU/(C2H5)2AlCl/anisole very probably occurs via a P insertion. 

Because of its high tensile strength and stress corrosion resistance, poly-(butene-1) is used for pipes and packaging film. Atactic poly(butene-l) (APB) is mostly produced by direct polymerization. It has similar properties to atactic poly (propylene). Syndiotactic poly (butene-1) is obtained by hydrogenating 1,2-poly (butadiene) however, it has no commercial significance. 

For stereoregular insertion there are two modes to consider—cis insertion and trans insertion. For both isotactic and syndiotactic production, the cis mechanism has been determined to be in operation. This was established by polymerizing with cis-, and trans-l-deuteriopropylene or related monomers. The expected stereochemistry was demonstrated when deuteriopropylene was polymerized. The cis monomers produce erythro monomer imits whereas the trans monomer yields the threo units when cis- and tra/is-l-d-propylene is polymerized. In some cases the nomenclature appearing in the literature can be confusing and contradictory, but all indicate cis insertion. To be specific, as defined below, stereochemical structures from cis and trans addition to the double bond of cis-(l-di) and trans-(l-di)-propylene to isotactic polypropylene are as follows (229) ... 

The issues raised in Section 7.2 are best illustrated by considering several real-world examples. Isotactic polymerization is considered first, followed by syndiotactic polymerization. These classic examples are followed by two studies on novel polymerization systems. The third case study concerns a single-site metallocene system that produces a stereoblocky material with novel elastomeric properties. The last example involves an ethylene-propylene copolymerization catalyst that exhibits nonrandom monomer incorporation. Common themes of these four examples include monomer-polymer chain control, van der Waals attraction, and the importance of the counteranion. 

One of the key features of the stereoselective coordination catalysts for propylene oxide polymerization is that, while they can give Isotactic polymer with a very high ratio of Isotactic to syndiotactic sequences (e.g. > 370) (34). in the same reaction mixture a large part of the polymer is amorphous. Even vdien R-propylene oxide is used as starting material, much of the product is amorphous polymer of low optical rotation containing many S-propylene oxide units (30). 

A comparative study was made for the stereoregularities of the polypropylene and the polystyrene formed by various metallocene catalysts is studied (Table 7) [101]. When the chiral metallocene was used, stereoregular polymers, IPP and SPS, were produced. In the syndiotactic polymerization of styrene, the secondary insertion occurred. On the other hand, in the case of isospecific polymerization of propylene. 

The presence of head-to-head" and tail-to-tail units in addition to the head-to-tail ones in propylene homopolymers (11,32), and of a number of even methylene sequences in ethylene/propylene copolymers (11,31,33-35) denotes lack of regiospecificity. As a consequence the syndiotactic polymerization of propylene is conveniently described as a binary copolymerization (11, 31, 36) between head-to-tail" and "tail-to-he ad oriented monomer molecules ... 

It is known that both the isotactic and syndiotactic polymerizations of propylene with Ziegler-Natta catalysts occur by the cis opening of the double bond. Figure 4.1 shows the relationship between two kinds of copolymers obtained from deuterium-substituted styrene monomer and the conformation of their vicinal protons. 

P. Sacchi, M. C, Rigamonti, E., Zambelli, A. Syndiotactic polymerization of propene regiospecificity of the initiation step. Macromolecules, 17,123 (1984). (d) Zambelli, A., Tosi, C., Sacchi, M. C. Polymerization of propylene to syndiotactic polymer. VI. Monomer insertion. Macromolecules, 5,649 (1972). 

Vinyl monomers, such as styrene, and alkenes with a side group, such as propylene, can polymerize in several molecular forms whose crystallization behaviors are quite different from each other. If the side groups are all on one side of the backbone, the structure is called isotactic, and if they are on alternating sides, it is called syndiotactic. If they are distributed in a random fashion, the polymer is said to be atactic. The isotactic and syndiotactic forms are crystallizable, often in a helical structure, while the atactic form does not crystallize and solidifies only at its glass transition temperature. Figure 2.3 illustrates the tacticities mentioned above for the case of polypropylene. It has been found that polypropylene tacticity can also have an important effect on chain dimensions [10] and on the rheological behaviour of the melt [11]. 

Polypropylene. There is an added dimension to the catalytic polymerization of propylene, since in addition to the requirement that the catalyst be sufficiently active to allow minute amounts of catalyst to yield large quantities of polymer, it must also give predominantly polypropylene with high tacticity that is, a highly ordered molecular stmcture with high crystallinity. The three stmctures for polypropylene are the isotactic, syndiotactic, and atactic forms (90) (see Olefin polya rs, polypropylene). 

---