Stereoselective polymerization, propylene

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

By assuming that stereoselectivity is connected to the regular heterogeneous catalyst surface, Natta introduced violet T1CI3 and used preformed heterogeneous catalysts in further studies.234,240 These complexes may act as stereoselective catalysts and polymerize propylene to crystalline stereoregular polymers. 

Inoue,S., Tsukuma.I., Kawaguchi,M., Tsuruta,T. Synthesis of optically active polymers by asymmetric catalysts. VI. Behavior of organozinc catalyst systems in the stereoselective polymerization of propylene oxide. Makromol. Chem. 103,151 (1967). 

A final example of a stereoselective heterogeneous catalytic system is the work of Laycock, Collacott, Skelton and Tchir.17 Layered double hydroxide (LDH) synthetic hydrotalcite materials were used to stereospecifically polymerize propylene oxide [PO] to crystalline isotactic and liquid atactic poly(propyleneoxide) [PPO]. These authors suggest that the LDH surface acts as other inorganic or organometallic coordination initiators or catalysts by providing specific surface orientations for propylene oxide monomer. X-ray powder diffraction showed some loss of crystallinity after calcination and X-ray photoelectron spectroscopy showed an enhancement of Mg/Al content due to restructuring of the Mg and A1 surface atoms. The surface was also rich in Cl ... 

Most stereoselective coordination catalysts polymerize propylene oxide to yield polymers that contain high ratios of isotactic to syndiotactic sequences. Laige portions of amorphous materials, however, are also present in these same products. These amorphous portions contain head-to-head units that are imperfections in the structures. For every head-to-head placement, one (R) monomer is converted to an (S) unit in the polymer. This shows that at the coordination sites abnormal ring openings occur at the secondary carbon with an inversion of the configuration and result in head-to-head placements. Also, erythro and threo isomers units are present. The isotactic portion consists almost exclusively of the erythro isomer while the amorphous fraction contains 40-45% erythro and 55-60% threo ... 

Special relationships apply, however, when configurationally different but constitionally identical monomers are copolymerized. An example of this is the polymerization of D- and L-propylene oxide mixtures. Here, it is important to distinguish sharply between stereospecific and stereoselective polymerizations. The meaning of these two terms is not the same as the corresponding meaning of stereospecific and stereoselective reactions in low-molar-mass organic chemistry. 

A major consequence of this pathway, also known as migratory insertion, is that the growing chain sweeps from one side to another with every addition of monomer. This is a generalization, for some authors have explained the loss of stereoselection in metallocene polymerizations of propylene, for example, at low monomer concentrations to the action of a windshield wiper isomerization, in which the polymer chain and open coordination site switch places without the benefit of monomer insertion. At low insertion rates, this site inversion phenomenon may become competitive with insertion and thus render ineffective any substituent influences which differ between the two faces of the catalyst site. With appropriate ligand design, different or enantiomeric steric environments may be created for the two sides of the active site. This makes possible stereoselective polymerization of propylene and higher a-olefins, as will be seen below. 

Razavi, A. Peters, L. Nafpliotis, L. Geometric flexibility, ligand and transition metal electronic effects on stereoselective polymerization of propylene in homogeneous catalysis. J. Mol. Catal. A Chem. 1997,115,129-154. 

Other chapters of this book discuss the manner in which well-defined stereorigid metal complexes with chirotopic sites can be converted into highly efficient catalysts for the stereoselective polymerization of propylene. For anifl-metallocenes, in particular, the relationship between precursor symmetry and catalyst stereoselectivity has been deeply studied and is now well-understood. ... 

Hasebe, Y. Tsuruta, T. Mechanism of stereoselective polymerization of propylene oxide with [ Me0CH2CH(Me)0Zn0CH(Me)CH20Me 2 EtZn0CH(Me)CH20Me 2] as initiator. Makromol. Chem. 1988,189, 1915-1926. 

The chirality (asymmetry) of the methlne carbon In propylene oxide and In Its polymer has provided many extra experimental parameters to use In studies of the mechanism of stereoselective polymerization by coordination catalysts. The first proposal of the coordination polymerization scheme to explain stereoselective or-olefln and olefin oxide polymerization arose from the propylene oxide studies (26.27). 

Another man of genius, G. Natta at Milano Polytechnic, soon extended that type of catalysis to the stereoselective polymerization of propylene into isotactic (and also syndiotactic) stereoregular polypropylene chains (highly cristalline fiber-forming material, m.p. close to 170 °C). 

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