Vinyl chloride, polymerization

Selection of organic peroxides for various commercial appHcations has been reviewed (21,33,48), particularly for vinyl chloride polymerizations (30). 

It seems likely that other polymerizations will be found to depart from Bemoullian statistics as the precision of tacticity measurements improves. One study12 indicated that vinyl chloride polymerizations are also more appropriately described by first order Markov statistics. However, there has been some reassignment of signals since that time. 4 25... 

Polymerization of vinyl chloride occurs through a radical chain addition mechanism, which can be achieved through bulk, suspension, or emulsion polymerization processes. Radical initiators used in vinyl chloride polymerization fall into two classes water-soluble or monomer-soluble. The water-soluble initiators, such as hydrogen peroxide and alkali metal persulfates, are used in emulsion polymerization processes where polymerization begins in the aqueous phase. Monomer-soluble initiators include peroxides, such as dilauryl and benzoyl peroxide, and azo species, such as 1,1 -azobisisobutyrate, which are shown in Fig. 22.2. These initiators are used in emulsion and bulk polymerization processes. 

Vinyl chloride polymerization occurs via an exothermic radical reaction. In fact, the reaction is approximately 25% more exothermic than polyethylene polymerization. The highly exothermic nature of the reaction and the strong molecular weight dependence on temperature make heat transfer, and its control, critical to the manufacture of polyvinyl chloride. 

Two vinyl chloride polymerization reactors were being operated by the same team of operators. Reactor 3 was in the cool down and dump phase of the process, and reactor 4 was nearly full of monomer and in the polymerization phase. The foreman and three employees set to work... 

Falk H, Creech JL Jr, Heath CW Jr, et al Hepatic disease among workers at a vinyl chloride polymerization plant. JAMA 230 59-63, 1974... 

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-... 

Acyl organosulfonyl peroxides (19) such as acetyl cyclohexanesulfonyl peroxide are efficient radical initiators for vinyl chloride polymerization. 

A substantially new and critical feature in bottom-entering agitators is the sealing of the shaft double mechanical seals are used here. On modem designs, they are provided with additional standstill seals for emergencies. The standstill seal allows the mechanical seal to be replaced while the reactor is full and under pressure. In a number of polymerizations, particularly in vinyl-chloride polymerization, the rotating elements must be protected by additional water flushing (4) other-... 

Figu re 13.4 Two-phase model for vinyl-chloride polymerization in suspension. 

If one hydrogen is replaced with chlorine in the ethylene molecule, vinyl chloride is formed. If vinyl chloride polymerizes polyvinyl chloride, known as PVC, is formed. PVC is lightweight, long lasting, and waterproof. In its rigid form, PVC is water-resistant and can be drawn out into pipes, house siding and drainpipes. It is also used in compact discs and computer casings. 

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... 

For vinyl chloride polymerization in bulk, the rate equation... 

An example of this type of polymerization is demonstrated in Fig. 2. It represents the linear dependence between [M] 1,2 and time in vinyl chloride polymerization [5],... 

EXAMPLE KINETIC ANALYSIS OF VINYL CHLORIDE POLYMERIZATION... 

The mixed peroxides, t-butylperoxy 2-ethylhexyl, (I), and isobutanoyl-lauroyl peroxide, (II), were prepared by Overkamp [2] and Tammer [3], respectively, using t-butyl peroxide and isobutanoyl chloride with 30% hydrogen peroxide in the presence of sodium hydroxide in a two-phase system. Both agents were used in vinyl chloride polymerization. 

The mechanism and kinetics of vinyl chloride polymerization in emulsion have been studied extensively, mainly involving the use of chemical initiators, and has been summarized by Talamini and Peg-gion and by Ugelstad et alia (4). The mechanism shows a gradual change as the number of particles (N) is increased. 

At higher values of m, specifically, and where m is increased to 0.10 and tt = 0.02 radical distribution calculations show that 2% of occupied particles contain 2 radicals while. 02% contain 3 radicals or more. This is outside the range of validity of Smith and Ewart s Case I model but is well described by Ugelstad s model for vinyl chloride polymerization wherein the Smith-Ewart recursion formula is solved by considering only latex particles Nq, Ni, and N2. 

Fabini, M., Bobala, S., Rusina, M., Macho, V., Preparation of poly(vinyl alcohol) as the dispersant for suspension vinyl chloride polymerizations, Polymer 35 (1994) 2201. 

Organotin derivatives of maleic acid may have an additional stabilizer function with the Diels-Alder reaction (Figure 3.3.3) by scavenging function. Their performance is good in all types of vinyl chloride polymerization, i.e. suspension, emulsion, and bulk. Optimum results are obtained when they are combined with small amounts of phenolic antioxidants particularly in plasticized PVC, impact-modified PVC, and PVC copolymers. 

---