Vinyl chloride polymerization chain transfer

Lim and Kolinsky (203) estimated the chain-transfer coefficients of 2,4-dichloropentane and 2,4,6-trichloroheptane (dimer and trimer of vinyl chloride, plus H and CH3 end-groups) as 5 x 10 4 for both compounds at 50° C, though a higher value would be expected for the latter, which has one more —CH2 CHCl-group, than for the former they suggest that this value is appropriate for Ctp for vinyl chloride polymerization. However, the poly-... 

Bengough and Norrish observed this behaviour during vinyl chloride polymerization. They explained it by transfer to polymer chains on which immobile, long-lived and propagating radicals are formed. These centres decay by transfer to monomer or by termination with untrapped radicals from the liquid phase [47], According to these two authors, the acceleration is proportional to the surface area of the solid particles. A similar acceleration of polymerization was observed by Bamford et al. [18] with acrylonitrile... 

Cs value is influenced by the nature of the bonds which are broken and formed and the relative stabilities of both radicals M and A in reaction (6.135). In general, a given transfer agent (XA) is more reactive (Cs is greater) for a reactive radical (M ) like those in ethylene or vinyl chloride polymerizations than for a resonance-stabilized radical like that of styrene. Similarly, when a given monomer is being polymerized, aliphatic compounds that yield tertiary radicals are more effective transfer agents than those that produce secondary radicals, and chain transfer activity is also enhanced by the possibility for resonance stabilization of radical A". 

The rapid chain transfer to monomer is a most important and characteristic feature of vinyl chloride polymerization, ous mechanisms fiar this reaction have been advanced [23]. A polymer chain does not transfer its radical activity to monomer directly by abstraction of a chlorine or a hydrogen atcnn. The chain transfer mechanism starts widi a head-to-head addition of monomer to the growing radical [24], The resulting radical, species (1), is very unstable and stabilizes itself by rearrangement ... 

Aside from conventional chain-transfer agents for vinyl chloride polymerizations, such as chlorinated hydrocarbons or mercaptans, the molecular weight distribution may also be reduced with 2-iodopropane [81] and aldehydes such as propionaldehyde [82]. 

The well-known fact that the concentration of initiator has a very small influence on the PVC molecular weight may be ascribed to the dominating event - the chain transfer to monomer. Hjertberg and Sorvik used a plot of 1/X against [I] to estimate Cm = 1.3x10 [114]. This value is in a good agreement with that for conventional vinyl chloride polymerization [111]. 

As illustrated above, some side reactions occur in vinyl chloride polymerization due to rearrangement effects in the polymer chains, unsaturated structures in the polymer chains (1.5-3.0 double bonds per 1000 monomer units), and the tertiary chlorine structures formed by chain transfer to polymer. The chlorobutyl groups (2-3 per 1000 monomer units) formed by backbiting reactions contribute to the thermal instability of the polymer through the tertiary chlorine on the backbone [63]. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Liquid trichloroethylene has been polymerized by irradiation with Co y-rays or 20-keV x-rays (9). Trichloroethylene has a chain-transfer constant of <1 when copolymerized with vinyl chloride (10) and is used extensively to control the molecular weight of poly(vinyl chloride) polymer. 

Branching occurs especially when free radical initiators are used due to chain transfer reactions (see following section, Free Radical Polymerizations ). For a substituted olefin (such as vinyl chloride), the addition primarily produces the most stable intermediate (I). Intermediate (II) does not form to any appreciable extent ... 

Chain transfer occurs frequently during the polymerization of vinyl chloride. The ratio of propagation events to chain transfer events in a given time period determines the average molecular weight of the final polymer. This finding can be summarized by the empirical formula ... 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Anomolous results have been observed in some emulsion polymerizations—inverse dependencies of N, Rp, and Xn on surfactant concentration. Some surfactants act as inhibitors or retarders of polymerization, especially of the more highly reactive radicals from vinyl acetate and vinyl chloride [Okamura and Motoyama, 1962 Stryker et al., 1967]. This is most apparent with surfactants possessing unsaturation (e.g., certain fatty acid soaps). Degradative chain transfer through allyl hydrogens is probably quite extensive. 

An interesting synthesis of block copolymers by cationic polymerization of vinyl compounds was described by Kennedy and Melby [277] who used 2-chloro-6-bromo-2,6-dimethylheptane as coinitiator. Br- is eliminated by triethylaluminium, and styrene can be polymerized, without transfer, on the generated carbocation. After all the styrene has reacted, diethylaluminium chloride is added to eliminate Cl- from the coinitiator and thus produce new carbocations on the polymer chain. In the presence of 2-methylpropene, the two-block copolymer poly(styrene)-6/ock-poly(2-methylpropene) is formed. 

Poly(vinyl chloride) made by suspension polymerization (Chapter 8) is a polymer in which molecular weight control is effectively by chain transfer—to monomer in this case. The ratio is slightly higher than the expected value... 

Poly(styrene)s containing acylperoxide groups are thus obtained by selective photolysis of the azo moieties at 350 or 371 nm. These prepolymers are successively used as macronitiators for the free radical polymerization of vinyl chloride at 70 °C. Styrene/vinyl chloride block copolymers are thus produced [55] by the above two-step route, although relevant amounts (50-60%) of poly(styrene) and poly(vinyl chloride), due to both low peroxide content ( 0.6 groups per macromolecule of polystyrene) and chain transfer with solvent and monomer, are also pre t. 

Friis and Hamielec (1975) have used GPC to study the MWD development in vinyl acetate and vinyl chloride emulsion polymerizations. For these monomers, the main chain-stopping mechanism is thought to ha transfer, and so the compartmentalize nature of the system is relatively unimportant. These workers found that the MWDs produced at early times, where branching reactions are unimportant, have a P value dose to 2, as expected for transfer-dominated reactions. 

Wang (1962) was the first to report studies on the y-radiation initiated polymerization of vinyl chloride in emulsion at room temperature. Rapid rates to high conversions were obtained after rather long induction periods of 1 to 3 hr. The degrees of polymerization were constant within experimental error at about 2000, in keeping with termination being dominated by chain transfer to monomer. Little or no dependence of the rale on the emulsifier concentration or the monomer concentration was observed. However, tbe rates were proportional to the 1.22 power of the dose rate. 

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