Propylene stereochemistry

Zambelli et al. have studied the effect of incorporating small amounts of ethylene on the stereochemistry of polypropylene prepared with stereoregular catalysts, concluding that for an isotactic catalyst, insertion of ethylene has no effect on the propylene stereochemistry, whereas for a syndiotactic catalyst, the stereochemistry alters. For the isotactic case therefore, the stereochemistry is controlled by the catalyst, a conclusion also reached by Sanders and Komoroski. However Tonelli has questioned the deduction of Zambelli et al., on the basis that chemical shift calculations suggest that the chemical shift of the central ethylene unit in a PPEPP sequence used by Zambelli et al. is in fact independent of the adjacent propylene stereochemistry, and cannot give information on the mechanism. 

When poly(propylene) was first made, it was found to exist in two possible forms. One was similar to poly(ethylene), but had greater rigidity and hardness the other was found to be amorphous and of little strength. The first of these is now known to be isotactic, that is with a regular stereochemistry at each alternating carbon atom. The other is now known to be atactic, that is with a random distribution of different stereochemical arrangements... 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

Many of the physical properties of propylene produced in this way depend on the stereochemistry of these stereocenters. 

Isomerization of butene via a 7r-allyl species introduces an added dimension to the stereochemistry. The 7r-allyl species from propylene is presumed to be planar with its plane approximately parallel to the surface. Since it is attached to the electropositive zinc, it may have considerable carbanion character. A corresponding structure for adsorbed butene would lead to two isomeric forms, viz ... 

The fact that Schrock s proposed metallocyclobutanes decomposed to propylene derivatives rather than cyclopropanes was fortunate in that further information resulted regarding the stereochemistry of the olefin reaction with the carbene carbon, as now the /3-carbon from the metal-locycle precursor retained its identity. The reaction course was consistent with nucleophilic attack of the carbene carbon on the complexed olefin, despite potential steric hindrance from the bulky carbene. Decomposition via pathways f-h in Eq. (26) was clearly confirmed in studies utilizing deuterated olefins (67). 

Contents G. Henrici-Olive, S. Olive Oligomerization of Ethylene with Soluble Transition-Metal Catalysts. A. Zambelli, C. Tosi Stereochemistry of Propylene Polymerization. C.-D.S. Lee, W.H. Daly Mercaptan-Containing Polymers. Yu. V. Kissin Structures of CopolymerS of High Olefins. 

Turning to propylene, cis addition was found also for syndiotactic polymers (4(X), 401). This result deserves additional comment. It is known that only one disyndiotactic polymer is obtained from a CHA=CHB olefin (see Sect. II-B) but this is no longer true when one considers the syndiotactic copolymers between two differently labeled monomers. The syndiotactic copolyriKr between perdeuteropropylene and propylene-l-d, can have either of the two structures 99 Ot 199. Hew 9ve motvomet mil deti ixv% from Ihe second mononvei (present in small quantity) can be clearly identified as to its stereochemistry. 

The same conclusion is reached by considering the response of the system when an ethylene molecule is inserted instead of propylene. The effect of the chain end containing an ethylene unit [—CH(CH3)—CHj—CHj—CH2— catalyst] on the stereochemistry of a new propylene unit should be much weaker than that existing in homopolymerization [—CH(CH3)—CH2— catalyst], since it corresponds to a 1,5- and not to a 1,3-asymmetric induction. The new propylene unity should have 0 or / configuration with almost the same probabUity. In contrast, stereochemical control tied to the catalytic center should impose the same configuration on the propylene unit before and after the introduction of the error. This is precisely what does happen with common iso-specific catalysts (407). 

The stereochemistry of ring-opening polymerizations has been studied for epoxides, episul-fides, lactones, cycloalkenes (Sec. 8-6a), and other cyclic monomers [Pasquon et al., 1989 Tsuruta and Kawakami, 1989]. Epoxides have been studied more than any other type of monomer. A chiral cyclic monomer such as propylene oxide is capable of yielding stereoregular polymers. Polymerization of either of the two pure enantiomers yields the isotactic polymer when the reaction proceeds in a regioselective manner with bond cleavage at bond 1. 

Zambelli, A. and Tosi, C. Stereochemistry of Propylene Polymerization. Vol. 15, pp. 31-60. Zucchini, U. and Cecchin, G. Control of Molecular-Weight Distribution in Polyolefins Synthesized with Ziegler-Natta Catalytic Systems. Vol. 51, pp. 101-154. 

By means of the 13C NMR technique Zambelli et al. proved311 that in polymerization of propylene catalyzed by TiCl3 + (I3CH3)2A1I the stereochemistry of the very first insertion is the same as that of subsequent units despite the lack of asymmetry in the original alkyl group of the activator. 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

The same concept is presented by Tsuruta, Inoue, Ishimori and Yoshida (95) in their study of propylene oxide polymerization. They concluded that the stereochemistry of the monomeric units in the isotactic structure is regulated by the asymmetric center of the proceeding monomer unit. 

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