Vinyl formate, polymerization

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

In 1965 Dunn and Taylor confirmed the theory for vinyl acetate polymerization (15), and proposed, in the light of the presumed importance of rapid coagulation during the earliest stages of reaction, that the "DLVO" theory for colloid stability (16) be applied. Fitch proposed a kinetic basis for a quantitative theory and observed that for observation of particle formation kinetics, "fast" reaction techniques must be used because "particle formation occurs in a matter of seconds or even less (17)". 

Poly(vinyl alcohol) can be derived from the hydrolysis of a variety of poly(vinyl esters), such as poly(vinyl acetate), poly(vinyl formate), and poly(vinyl benzoate), and of poly(vinyl ethers). However, all commercially produced poly(vinyl alcohol) is manufactured by the hydrolysis of poly(vinyl acetate). The manufacturing process can be viewed as one segment that deals with the polymerization of vinyl acetate and another that handles the hydrolysis of poly(vinyl acetate) to poly(vinyl alcohol). 

Bamford et al. [157] were the first to observe the acceleration of vinyl monomer polymerization caused by inorganic salt addition. They polymerized acrylonitrile dissolved in dimethyl-formamide with 2,2 -azobisisobutyronitrile. The reaction rate was increased by the addition of LiCl. The observed effect was ascribed to the increase of the rate constant and interpreted by complex formation between lithium chloride and the nitrile group of the radical. 

The energies of activation of vinyl ether polymerizations are much larger isobutyl and isopropyl vinyl ether E = 21 kJmol-1 ethyl vinyl ether Ea 54 kJ mol This indicates that carbenium ions of vinyl ethers are less reactive, probably due to an equilibrium with dormant oxonium ions formed by an intramolecular cyclization [Eq. (67)]. The overall activation energies should also increase to more positive values if formation of the active carbenium ions is endothermic. 

A similar effect was observed for trityl derivatives. Initiation of vinyl ether polymerization with trityl salts is very slow and often incomplete [257]. This precludes preparation of well-defined polymers with predetermined molecular weights and narrow MWDs. However, polymerization of vinyl ethers initiated by trityl salts in the presence of tetrahydrothiophene leads to controlled polymers [135]. The equilibrium constant for the formation of sulfonium ions is much smaller for trityl salts than for the growing species (K, < Kp), which increases the ratio of the apparent initiation to the propagation rate constants a thousand times (Scheme 14) ... 

Incidentally, bulk polymerization is also the chief method used for commercial polycondensations. Polycondensations are not as exothermic as free radical catalyzed vinyl-type polymerizations, so thermal control is less of a problem. Bulk polycondensation also favors formation of linear polymer rather than the cyclic products that are favored by dilute solution polymerization, particularly if AB-type monomers are being used. Finally, since a high degree of polymerization (i.e., high DP, or high molecular weight) is only... 

Our search for a more active system led us to bis(pentafluorophenyl)nickel complexes originally reported by Klabunde and co-workers in the 1980s [56]. One of the more interesting complexes reported was (// -toluene)Ni(C6F5)2 (Fig. 4.26). Toluene can be readily replaced by a number of neutral electron donors including xylene, mesitylene, THF, PEtj, and norbornadiene. In fact, Klabunde noted that formation of (norbornadiene)Ni(C6F5)2 was accompanied by intractable polymer. Klabunde speculated that vinyl addition polymerization occurred with possible crosslinking. Unfortunately, the insolubility of the norbornadiene polymer prevented further analysis. 

Lithium alkyls initiate polymerization of polar monomers like methyl-methacrylate, vinyl pyridine, acrylo-nitrile, etc. However, these reactions are more complex. The desired addition to the C=C bond is accompanied by other processes, e.g., attack on the -COOEt group with the formation of ketones and lithium methoxide, or in vinyl pyridine polymerization by the metalation of the pyridine moiety. 

Once the above observations have been made, questions naturally arise concerning the infiuence of the aromatic nuclei of such initiators as benzoyl peroxide on the polymerization process. Indeed chain transfer between benzoyl peroxide and poly(vinyl acetate) fiee radicals has been observed [41]. The copolymerization of benzoyloxy radicals with vinyl formate, vinyl propionate, vinyl butyrate, vinyl benzoate, and vinyl phenylacetate has been studied in considerable detail [42,43]. 

When we combine this observation with the autoaccelerating tendencies of the system, the chain-transfer reactions to both the monomer and the polymer on one of the several positions which leads to branched-chain formation, and the possible reactivation of dead polymer molecules by hydrogen abstraction with monomeric free radicals [78], the complexity of the kinetics of vinyl acetate polymerization may be appreciated. Similar factors may be involved not only in the polymerization of other vinyl esters, but also in the fiee-radical polymerization of other types of monomers. 

The polymerization of vinyl formate is of considerable interest since its polymer is particularly readily hydrolyzed to poly(vinyl alcohol). For example, according to one patent, a solution of 70% vinyl formate in methyl formate containing 0.2% of 2.2 -azobisisobutyronitrile was polymerized in a sealed tube under nitrogen for 10 hr at 30°C. The reaction product was hydrolyzed to give a poly(vinyl alcohol) with a degree of polymerization of 2110 [107]. 

Thus, functionality is not an absolute property of a group, but always has to be considered in relation to the reaction partner. The chemical structure of the resulting macromolecules, moreover, will be decided not only by the functionality of the groups capable of polymerization, but also by the functionality of the molecules. The carbon-carbon double bond in vinyl chloride is bifunctional with respect to free radical polymerization initiators. However, the radicals can also attack already formed polymer, where, for example, a chlorine atom is abstracted with termination of a growing end and formation of a new polymer radical. The new polymer radical can in turn initiate vinyl chloride polymerization again ... 

Several papers deal with the mechanism of stereoregular control in vinyl ether polymerizations. The formation of isotactic poly(allyl vinyl ether) was ascribed to... 

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