Diene polymerization, electron donors

The stereochemistry of the polymerization of dienes is most conveniently discussed in two sections (a) polymerization in hydrocarbon solvents and (b) polymerization in the presence of amines, ethers and other electron donors. 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

The rate of polymerization of polar monomers, for example, maleic anhydride, acrylonitrile, or methyl methacrylate, can be enhanced by coraplexing them with a metal halide (zinc or vanadium chloride) or an organoaluminum halide (ethyl aluminum sesqui-chloride). These complexed monomers participate in a one-electron transfer reaction with either an uncomplexed monomer or another electron-donor monomer, for example, olefin, diene, or styrene, and thus form alternating copolymers (11) with free-radical initiators. An alternating styrene/acrylonitrile copolymer (12) has been prepared by free-radical initiation of equimolar mixtures of the monomers in the presence of nitrile-coraplexing agents such as aluminum alkyls. 

Cooper, W. Eaves, D. E. Vaughan, G. Electron donors in diene polymerization. Chem. Sen 1966,... 

The intramolecular Diels-Alder reactions of the Fischer carbene complexes, (L)(0C)4W=C(0R)CH=CH(CH2) CH=CHCH=CH2, (L = CO, PPh3 R = Me or Pri n = 3 or 4) are compared with known reactions of the analogous methyl esters. The stereoselectivites are comparable to those of the Lewis acid catalysed reactions but are more tolerant of functionalised diene units." The reaction between Mo(CO)6 and All(H)N(CH2)2NHAll/CH(NMe2)(OMe)2 (All = allyl) in methylcyclohexane at 1(X) C affords the chelated cyclic carbene complex 122, while polymeric carbene materials 123 are available from electron rich alkene precursors containing pendant phosphine donor groups such as 124. ... 

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