Vinyl chloride monomer proces

The polymerization of vinyl chloride monomer, in common with other vinyl monomers, proceeds by a free-radical mechanism involving the usual steps of initiation, propagation, and termination. Poly(vinyl chloride) is formed in a regular head-to-tail manner Eq. (1) [3-6]. 

Vinyl chloride monomer (VCM) was originally produced by the reaction of hydrochloric acid and acetylene in the presence of HgC catalyst. The reaction is straightforward and proceeds with high conversion (96% on acetylene) ... 

The polymerization of vinyl chloride evidently proceeds in the monomer phase and in monomer-polymer particles. Crosato-Arnoldi et al. [17] and... 

As the polymerization proceeds vinyl chloride monomer is consumed and the pressure drops. At about 0.05 MPa gauge pressure (pressure above normal atmospheric) sufficient monomer has been converted to polymer. Unreacted monomer is vented and the residual vinyl chloride is removed by blowing... 

By far the greatest part of PVC production across the world is now made by the suspension process. Vinyl chloride monomer (derived from a reaction between ethylene (derived from oil) and chlorine (derived from common salt) is dispersed in deionised water with the help of small quantities of chemical dispersants and polymerisation initiators (typically peroxide compounds). At moderately raised temperature (50 C) and pressure (0.7 MPa) polymerisation proceeds and the polymer can be removed from the resulting slurry by de-watering and steam stripping the unconverted vinyl chloride monomer. 

In the case of vinyl chloride, transfer proceeds via chlorine abstraction a monomer molecule adds head-to-head to a propagating radical. The resulting radical is highly unstable and reacts with another monomer molecule in a chlorine abstraction step. This special reaction pathway is the reason for Cm... 

However, if increasing amounts of free radical acceptors are added to the system, the block and graft polymerization gradually decreases, as saturation of free macroradicals proceeds faster than the addition of monomers. In Table 2, the influence of benzoquinone on the graft polymerization of vinyl chloride on poly(methyl methacrylate) is shown (20). 

The selective oxidation of cellulose to dialdehyde by sodium periodate is well known. It has been postulated by Criegee (74) and by Waters (73) that this reaction proceeds by a free radical mechanism. Toda (76) and Morimoto, Okada, Okada, and Nakagawa (77) have concluded that sodium periodate oxidation should initiate graft polymerization. They succeeded in grafting methyl methacrylate and acrylonitrile onto cellulose substrates, such as rayon and paper. A similar procedure is recommended in a patent of Chemische Werke Huels (78) to graft vinyl monomers onto cotton, polyethylene oxide, copolymers of vinyl chloride-vinyl acetate, and others. 

The polymerization reaction in aqueous suspension of vinyl chloride in the presence of an ethylene-propylene saturated elastomer occurs with the formation of poly (vinyl chloride) homopolymer and rubber-poly (vinyl chloride) grafted copolymers. The first grafting reaction proceeds as far as diffusion of the monomer inside the particles in suspension is possible afterwards, some chain branching of grafted PVC is possible. Under our experimental conditions the amount of grafted rubber does not exceed 60% of the initial rubber and is little influenced by the type of initiator used. 

Polyfvinyl chloride) (PVC) is produced by mass, suspension, and emulsion processes. Mass polymerization is an exatiiple of a heterogeneous bulk system. PVC is virtually insoluble in vinyl chloride because the polymer is about 35% more dense than the monomer under normal polymerization conditions. Vinyl chloride, however, is quite soluble in polymer. The two phases in PVC polymerizations are pure monomer and monomer-swollen polymer. Polymerization proceeds in both phases, but it is very much faster in the polymer-rich phase because the mobility of macro radicals and mutual termination reactions are. severely restricted (cf. Section 6.13.2). 

The major process for poly(vinyl chloride) production is the suspension system. Typical reaction temperatures are 50-65 C. As the reaction proceeds, a conversion ( 76%) is reached at which the only monomer left in the system is that absorbed in the polymer particles. This occurs when the monomer concentration is about 30 wt % in the particles. The occurrence of this phenomenon is signaled by a drop in the reactor pressure. Normal pressures in the autoclaves are initially about 150 psig (pounds per square inch, gauge), and it is usual to carry out polymerizations until the pressure drops to about 20-70 psig, depending on the reaction temperature. Water may be injected into the reaction vessel as the polymerization proceeds, to compensate for the volumetric contraction between monomer and polymer. This also helps prevent the reaction mixture from becoming too viscous. As well, the water addition enhances the cooling capacity of the reactor because it increases the heat transfer area on the walls. 

While chain transfer to monomer is negligible for most monomers, it may, however, be significant for some monomers, such as vinyl acetate, vinyl chloride, and a-methyl substituted vinyl monomers, e.g., propylene and methyl methacrylate (MM A). For MM A the chain transfer proceeds by the reaction ... 

The polymerization of ethylene is an example of chain propagation polymerizations. The polymerization of propylene, vinyl chloride, styrene, acrylates, methacrylates, and many other monomers proceeds in the same way. 

In Table VII, it will be noted that in a bulk polymerization, at a conversion of 10-20%, secondary particles form. That is, swollen polymer particles collide to form larger particles. As the process proceeds, virtually no free liquid monomer is present. Using a special autoclave, in which polymer lumps could be broken up, bulk polymerization of vinyl chloride could be carried out beyond this low conversion range [58]. This concept was improved upon in the basic patent for the Pechiney-Saint Gobain process [59]. Evidently Produits Chimique Pechiney-Saint Gobain was a... 

Time-lapse photography has been used to follow the course of the suspension polymerization of vinyl chloride. This study showed that the spherical droplets of monomer burst under the action of the shearing force of the agitator to produce deformed and irregular resin particles. Without agitation, on the other hand, opaque spots appear in the spherical monomer droplets that increase in size as the polymerization proceeds [91]. 

It was believed for a long time that head-to-head radical addition to monomers is a major route for formation of labile structures. Kinetic studies, in association with NMR measurements, reveal that formation of internal allylic and tertiary chlorine structures actually proceeds through an intramolecular or intermolecular chain-transfer reaction to polymer [Eqs. (31), (32) VC = vinyl chloride]. 

Polyvinyl chloride (PVC) is a homopolymer of vinyl chloride. Eighty percent of commercial PVC in packaging is produced by chain-reaction polymerization using a suspension method. Other methods are emulsion and solution polymerization. Chain-reaction polymerization requires initiators to produce free radicals, then the reaction proceeds until the chain is terminated. The predominant configuration of the monomer in the polymer chain follows a head-to-tail alignment to yield a syndiotactic polymer. 

In a precipitation polymerization, the system initially is in a homogeneous phase, because monomer and initiator are completely soluble in the initial reaction medium, but, upon initiation, the formed polymer precipitates as soon as it forms. After precipitation, the polymerization proceeds by absorption of monomer and initiator into the polymer particles. Bulk polymerization of vinyl chloride and solution polymerization of acrylonitrile in water are examples of precipitation polymerization. Precipitation polymerizations are often referred to as powder or granular polymerizations because of the forms in which the final polymer products are obtained. 

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