Vinyl chloride monomer impurities

Other products act insidiously, so that it is harder to establish their effects on human and animal health and more generally on the environment. Indeed, progress in understanding the safe dosage of minute quantities of impurities has enabled governments to fix with greater care the maximum allowed content of vinyl chloride monomer, formaldehyde, and benzene beyond which these products could become dangerous for workers to handle. 

Presence of impurities in excipients can have a dramatic influence on the safety, efficacy or stability of the drug product. Monomers or metal catalysts used during a polymerization process are toxic and can also destabilize the drug product if present in trace amounts. Due to safety concerns, the limit of vinyl chloride (monomer) in polyvinyl pyrrolidone is nmt 10 ppm, and for hydrazine (a side product of polymerization reaction) nmt 1 ppm. Monomeric ethylene oxide is highly toxic and can be present in ethoxylated excipients such as PEGs, ethoxylated fatty acids, etc. 

Wide-bore fused silica column capillary GC has been used to determine impurities in vinyl chloride monomer used in the manufacture of polyvinyl chloride (PVC) [4]. The analysis was performed using a Perkin Elmer model 8500 GC using a 25 m x 0.53 mm fused silica column coated with 5 pm immobilised methyl silicone phase, the column being run at -10 °C for 4 minutes, programmed at 10 °C/min to 10 °C for 1 minute, and programmed at 30 °C/min to 170 °C for 2 minutes. A FID was employed. The three principal impurities present in the monomer, and, consequently, expected to occur in the PVC, were methyl chloride, monovinyl acetylene, and ethyl chloride. 

Table 6. Typical Impurity Levels in Monomer Grade Vinyl Chloride ...

The polymer may be prepared readily in bulk, emulsion and suspension, the latter technique apparently being preferred on an industrial scale. The monomer must be free from oxygen and metallic impurities. Peroxide such as benzoyl peroxide are used in suspension polymerisations which may be carried out at room temperature or at slightly elevated temperatures. Persulphate initiators and the conventional emulsifying soaps may be used in emulsion polymerisation. The polymerisation rate for vinylidene chloride-vinyl chloride copolymers is markedly less than for either monomer polymerised alone. 

Transfer to monomer is an important process in the radical polymerization of vinyl chloride so that the number of initiator fragment in a polymer chain is less than unity. For example, poly(vinyl chloride)s prepared with AIBN in 1,2-dichloroethane at 40°C were found to contain 0.17 0.34 AIBN fragments per chain from the 13C NMR analysis of the polymer reduced with Bu3SnH. The polymers also contained a CN group in the chain (0.025 0.12 per chain), suggesting the copolymerization of methacrylonitrile which was formed in situ or admitted as an impurity in the starting AIBN.79... 

Safety concerns for synthetic polymers are sometimes due to contamination with monomeric impurity. Monomers are sometimes highly toxic (e.g., acrylamide [94]) or even carcinogenic (e.g., ethyleneimine [95] or vinyl chloride [96]) or highly irritating to skin (e.g., acrylamide [94] or acrylic acid [97]).Therefore, control of unreacted monomer level can sometimes be critical to the safety performance of synthetic polymers. 

Polymers contain various elements, metallic and nonmetallic. Some elements are a constituent part of the monomers, such as nitrogen in acrylonitrile or chlorine in poly(vinyl chloride), while other elements occur as impurities or are part of some additives (e.g., zinc stearate). Their concentrations range from a (tg per kg level to several percent. Analysis of the element content is especially important for manufacturing process control. Elements can be determined after chemical or physical destruction of polymer, or directly by nondestructive methods. 

Some peculiarities of the thermal and thermooxidative decomposition of polyvinyl chloride depend on the conditions of its production. Thus, it is known that samples of the polymer produced by initiating the polymerization of vinyl chloride with ultraviolet irradiation possess higher stability in comparison with samples produced in polymerization under the action of chemical agents [26, 27]. Reversibility of the process of dehydrochlorination in the decomposition of samples of polyvinyl chloride produced by the latex method is noted, while in the process of decomposition of suspension polymer, the phenomenon of reversibilily is not observed [21]. It has been shown that the rate of dehydrochlorination of the latex polymer is significantly higher than that of the suspension polymer under the same conditions [21]. It has been established that the polymerization of vinyl chloride in the presence of oxygen leads to the formation of unstable peroxide groups, which can initiate decomposition of the polymer [28, 29]. It is noted that an extremely substantial influence on the stability of polyvinyl chloride is exerted by the purity of the monomer, as well as the presence of impurities of metals of variable valence [28]. 

Copolymersof vinyl chloride with vinylidine chloride, produced by the suspension method, are more stable than the latex copolymers. The rates of thermal decomposition of polyvinyl chloride and the copolymer of vinyl chloride with vinylidine chloride, produced by the latex method, are practically the same for the latex copolymer, just as for the latex homopolymer, reversibility of the process of dehydrochlorination is observed [21]. In an investigation of the stability of copolymers of vinyl chloride with vinyl acetate, vinylidine chloride, and with vinylisobutyl ether in nucleophilic substitution reactions, it was found that the copolymer with vinyl acetate is the least stable to the action of alcoholic alkali the copolymers with vinylidine chloride and vinylisobutyl ether proved more stable [56]. The stability of the copolymer of vinyl chloride witii methyl acrylate is substantially increased when the degree of homogeneity of the copolymer with respect to composition is increased, and when monomers with a smaller content of impurities are used, as well as when the copolymerization is conducted in the presence of chain carriers [57, 58]. 

Some of the applications of headspace analysis include the determination of vinyl chloride and other impurities in PVC, styrene monomer in PS, methyl methacrylate monomer in polyacrylates, ethylene in polyethylene, acrylonitrile monomer in ABS terpolymers, epichlorohydrin in epoxy resins, and residual solvents in polymers (see next). 

Polymerization depends primarily on the chemical and physical properties of monomer. First, the monomer must be sufficiently reactive for radical polymerization. Vinyl acetate failed to polymerize during mastication as a result of the apparent low reactivity of alkyl radicals toward this monomer [86, 88], whereas isoprene, vinyl chloride, and butadiene also had only a low reactivity [88, 95], It has been claimed that results with vinyl acetate result from traces of impurities including oxygen [98]. Gelation occurs on mastication with monomers, which give radicals sufficiently active to react with low-activity groups in natural rubber. The effect of monomer composition... 

Likewise, p-pinene with its exocyclic vinyl group is readily polymerized by cationic techniques however, the polymers obtained have rather low MWs of<3.4 kg/mol [80, 84-86]. High molecular weight poly(p-pinene) with MW up to 40 kg/mol (PDI 2.2) can be obtained with the H20 /EtAlCl2 system ( H2O indicates adventitious moisture impurities). The polymerizations are carried out in mixtures of methyl chloride/ methylcyclohexane (preferred composition 50 50) at —80°C. Quantitative monomer conversions are reached within 20 min or less. The repeat unit of the poly(p-pinene) is found to consist of a cyclohexene unit in the main chain (Scheme 6), which reflects isomerization polymerization [87]. With AICI3 etherates, e.g., AlCl30Ph2, the polymerization can be performed even at room temperature and low catalyst concentration (2.S-5.5 mM, [p-pinene]o = 0.55 M) to yield polymers with = 9-14 kg/mol... 

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