Polymerization stereochemical control

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

An obvious extension of the studies on photodimerization of crystalline olefins is to solid-state vinyl polymerization (with light, if absorbed, or y-irradiation), with the aim of achieving stereoregular polymers. In fact, an immense effort has been made in this direction, but with singular lack of success. The explanation is that, for various reasons, the lattice in the vicinity of the chain front becomes progressively more damaged as polymerization proceeds, so that after relatively few steps there is loss of stereochemical control. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

Analysis of the poly(methyl methacrylate) sequences obtained by anionic polymerization was undertaken at the tetrad level in terms of two different schemes (10) one, a second-order Markov distribution (with four independent conditional probabilities, Pmmr Pmrr, Pmr Prrr) (44), the other, a two-state mechanism proposed by Coleman and Fox (122). In this latter scheme one supposes that the chain end may exist in two (or more) different states, depending on the different solvation of the ion pair, each state exerting a specific stereochemical control. A dynamic equilibrium exists between the different states so that the growing chain shows the effects of one or the other mechanism in successive segments. The deviation of the experimental data from the distribution calculated using either model is, however, very small, below experimental error, and, therefore, it is not possible to make a choice between the two models on the basis of statistical criteria only. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Stereochemical Control of Polymerization Ziegler-Natta Catalysts... 

J. A. Ewen, Mechanisms of Stereochemical Control in Propylene Polymerizations with Soluble Group 4B Metallocene/Methylalumoxane Catalysts, J. Am. Chem. Soc. 106, 6355-6364 (1984). 

Production problems with current metallocene-aluminoxane catalysts reflect the high aluminoxane/metal ratio needed for good catalyst productivity and stereochemical control. Aluminoxane production is slow and relatively inefficient and the large amounts used make post-polymerization catalyst removal processes necessary. These deficiencies are expected to be remedied by different cocatalysts, which are being disclosed in the patent literature. 

Atomic Structure. The control of atomic structure is fundamental to any system, and an incomplete understanding of atomic structure can limit advancement. For example, our understanding of preceramic polymers, up through the formation of networks, is improving but the full exploitation of this chemistry is still limited by the lack of detailed knowledge of the structure of the resulting ceramic at the atomic level. Even with more familiar silicone polymer systems, synthetic barriers are encountered as polymers other than poly(dimethylsiloxane) are used. Stereochemical control is inadequate in the polymerization of unsymmetrical cyclic siloxanes to yield novel linear materials. Reliable synthetic routes to model ladder systems are insufficient. 

Polymerization with Ziegler-Natta catalysts has two important advantages over free-radical polymerization (a) it gives linear polymer molecules and (b) it permits stereochemical control. 

Coordination catalysts also permit stereochemical control about the carbon-carbon double bond. By their use, isoprene has been polymerized to a material virtually identical with natural rubber c/j-I,4-polyisoprene. (See Sec. 8.25.)... 

As discussed so far in this section, the helical polymethacrylates are synthesized predominantly using anionic polymerization techniques. However, recently, more versatile, inexpensive, and experimentally simple free-radical polymerization has been proved to be an alternative, effective way to prepare helical polymethacrylates from some monomers. Although the stereochemical control of radical polymerization is generally more difficult compared with that in other types of polymerization,69 an efficient method would make it possible to synthesize helical, optically active polymers having functional side chains by direct radical polymerization without using protective groups. In the radical polymerization of bulky methacrylates, helix-sense selection is governed by the chirality of a monomer itself or an additive. 

Stereochemical control in the polymerization of 1,2-disubstituted olefins susceptible to cationic polymerization, such as propenyl ethersand methylstyrene, continues to be investigated with particular emphasis on the effect of the reaction solvent. 

Ewen, J.A. Elder, M.J. Jones, R.L. Haspeslagh, L. Atwood, J.L. Bott, S.G. Robinson, K. Metallocene/polypropylene structural relationships implications on polymerization and stereochemical control mechanisms. Macromol. Symp. 1991, 48, 253. 

The Nature of Stereochemical Control in Metal-Catalyzed Butadiene Polymerization... 

The stereosequences indicate site stereochemical control with chain migratory insertions, which result in site isomerization and occasional reversal in diastereoface selectivity. With the zirconocene, the activity was found to be 56 kg PP/mmol Zr. Even cyclic olefins such as cyclobutene, cyclopentene, or norbomene could be polymerized with the chiral catalysts to high melting polymers (mp > 400°C) . ... 

It is infonnative to consider how tacticity arises in terms of the mechanism for propagation. The radical center on the propagating species will usually have a planar sp configuration. As such it is achiral and it will only be locked into a specific configuration after the next monomer addition. This situation should be contrasted with that which pertains in anionic or coordination polymerizations where the active center is pyramidal and therefore has chirality. I his explains why stereochemical control is more easily achieved in these polymerizations. 

Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

Biocatalysis is a key route to both natural and non-natural polysaccharide structures. Research in this area is particularly rich and generally involves at least one of the following three synthetic approaches 1) isolated enzyme, 2) whole-cell, and 3) some combination of chemical and enzymatic catalysts (i.e. chemoenzymatic methods) (87-90). Two elegant examples that used cell-fi-ee enzymatic catalysts were described by Makino and Kobayashi (25) and van der Vlist and Loos (27). Indeed, for many years, Kobayashi has pioneered the use of glycosidic hydrolases as catalysts for polymerizations to prepare polysaccharides (88,91). In their paper, Makino and Kobayashi (25) made new monomers and synthesized unnatural hybrid polysaccharides with regio- and stereochemical-control. Van der Vlist and Loos (27) made use of tandem reactions catalyzed by two different enzymes in order to prepare branched amylose. One enzyme catalyzed the synthesis of linear structures (amylose) where the second enzyme introduced branches. In this way, artificial starch can be prepared with controlled quantities of branched regions. 

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