Vinyl polymerization, stereospecific

The structure of the chain, i.e., whether it is a helix or a random coil, might influence not only the rate but also the stereospecificity of the growing polymer. For example, it is plausible to expect that in normal vinyl polymerization helix formation might favor specific placement, say isotactic, while either placement would be approximately equally probable in a growing random coil. Formation of a helix requires interaction between polymer segments, and this intramolecular interaction is enhanced by bad solvents particularly those which precipitate the polymer. 

Consistent with the discussion on alkali metal alkyls, the least stereospecific catalysts for vinyl polymerizations should be those which are derived from the least electronegative metals having the weakest p or d bonding orbitals. On this basis, one expects increasing stereospecificity for making isotactic or cis-1,4 products in the order Ba < Sr < < Ca Mg Be, with some variations due to monomer structure. 

Zinc and cadmium alkyls have not been successful as stereospecific catalysts in the absence of co-catalysts, presumably because they do not complex strongly enough with the monomer and the metal-carbon bonds are too covalent. Cadmium alkyls were first reported by Furukawa and coworkers (260) to induce vinyl polymerization, but it was shown later (267, 262) that oxygen was a co-catalyst and the reactions were free radical in nature. Similar free radical results were obtained with zinc alkyls (261—263) and vinyl monomers. However, with more basic and more easily polarized monomers, such as olefin oxides and aldehydes, the zinc catalysts operate by a coordinated anionic mechanism (250). 

Ionic polymerizations yield highly stereoregular polymers when control is exercised over monomer placement. The earliest stereospecific vinyl polymerizations were observed in preparation of poly isobutyl vinyl ether) with a BFa-ether complex catalyst at -70 °C. An isotactic polymer formed. °The same catalyst was employed later to yield other stereospecific poly(vinyl ether)s. " The amount of steric placement increases with a decrease in the reaction temperature, and, conversely, decreases with an increase in the temperature. ... 

Needless to say, vinyl polymerization is one of the most important methods for polymer synthesis. A variety of carbon-carbon (C-C) main chain polymers have been prepared by the vinyl polymerization of monomers with diverse substituents, via radical, cationic, anionic, or coordination mechanism. Furthermore, with the technological achievement such as living and stereoselective (or stereospecific) polymerizations, fine-tuning of the polymer structure with respect to molecular weight and tacticity has been realized in a number of examples. In particular, polymers obtained with vinyl polymerization (vinyl polymer) as represented by polyethylene, polypropylene, polystyrene, and poly(methyl methacrylate) have contributed to the progress of modern society in various aspects as useful synthetic materials. 

Copper(II) triflate has also been used for the carbenoid cyclopropanation reaction of simple olefins like cyclohexene, 2-methylpropene, cis- or rran.y-2-butene and norbomene with vinyldiazomethane 2 26,27). Although the yields were low (20-38 %), this catalyst is far superior to other copper salts and chelates except for copper(II) hexafluoroacetylaeetonate [Cu(hfacac)2], which exhibits similar efficiency. However, highly nucleophilic vinyl ethers, such as dihydropyran and dihydrofuran cannot be cyclopropanated as they rapidly polymerize on contact with Cu(OTf)2. With these substrates, copper(II) trifluoroacetate or copper(II) hexafluoroacetylaeetonate have to be used. The vinylcyclopropanation is stereospecific with cis- and rra s-2-butene. The 7-vinylbicyclo[4.1.0]heptanes formed from cyclohexene are obtained with the same exo/endo ratio in both the Cu(OTf)2 and Cu(hfacac)2 catalyzed reaction. The... 

The molecular design of stereospecific homogeneous catalysts for polymerization and oligomerization has now reached a practical stage, which is the result of the rapid developments in early transition metal organometallic chemistry in this decade. In fact, Exxon and Dow are already producing polyethylene commercially with the help of metallocene catalysts. Compared to the polymerization of a-olefins, the polymerization of polar vinyl, alkynyl and cyclic monomers seems to be less developed. 

Ketley, A. D., Stereospecific Polymerization of Vinyl Ethers, Chap. 2 in The Stereochemistry of Macromolecules, Vol. 2, A. D. Ketley ed., Marcel Dekker, New York, 1967a. 

The various kinds of growing species differ not only in their propagation but also in their stereochemical preferences. Professor Hogen-Esch will review this subject in his talk on anionic oligomerization of some vinyl monomer, and mechanisms of anionic, stereospecific polymerization of 2-vinyl pyridine will be discussed by Dr. Fontanille. In this context, the interesting paper of Schuerch et al.(12) deserves attention. Their work clearly reveals the effect of cation solvation upon the mode of monomer s approach to the growing centers. 

Living Anionic Stereospecific Polymerization of 2-Vinyl pyridine... 

Natta et al. have shown (1,2) that stereospecific polymerization of 2-vinyl pyridine (2VP) can be achieved by using de-solvated Grlgnard reagents as Initiators, In solution In hydrocarbon solvents. Although the essential characteristics were not revealed In this work, there Is no doubt that an anionic mechanism Is operative In such a polymerization. 

They are able to polymerize a large variety of vinyl monomers. The polymer microstructure can be controlled by the symmetry of the catalyst precursor. Prochiral alkenes such as propylene can be polymerized to give stereospecific polymers,554 572-574 allowing production of polyolefin elastomers. They can give polyolefins with regularly distributed short- and long-chain branches which are new materials for new applications. 

Another important use of BC13 is as a Friedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cydophosphazenes to polymers (81,82) polymerization of olefins such as ethylene (75,83—88) graft polymerization of vinyl chloride and isobutylene (89) stereospecific polymerization of propylene (90) copolymerization of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerization of norbomene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

Organo lithium catalysts have been used successfully for stereospecific polymerization of a variety of vinyl monomers and diolefins. They have been covered thoroughly in recent reviews (191,192,195,234) and will not be discussed in detail here except to illustrate some of the evidence which supports a cationic attack by lithium on monomer. 

Polymerization activity was obtained with a variety of catalyst compositions. The best stereospecific catalyst was the split pretreated type (357) in which one mole of VC14 was reduced by a stoichiometric amount of an alkyl metal (0.34 mole AlEt3) in heptane at room temperature and heated 16 hours at 90° C. to obtain the purple crystalline VC13-1/3 A1C13. This reduced transition metal component was then treated with two moles of (i-Bu)3Al tetrahydrofuran complex for 20 hours at room temperature to obtain a chocolate-brown catalyst consisting predominantly of divalent vanadium with 0.21 Al/V and 1.4 i-Bu/Al. Polymerizations at 30° C. gave crystalline polymers from methyl, ethyl, isopropyl, isobutyl, tert.-butyl, and neopentyl vinyl ethers. 

Since coordination of the ether oxygen is involved in the stereoregulating step, any factor which weakens this will decrease stereospecificity. This explains why the more hindered, higher alkyl vinyl ethers give less stereoregular polymerization than vinyl methyl ether. 

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

When the configurations at the centers are more or less random, the polymer is not stereoregular and is said to be atactic. Polymerizations which yield tactic polymers are called stereospecific. Some of the more important stereospecilic polymerizations of vinyl polymers are described briefly in Chapter 9. 

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