Vinyl bulk polymerization

Often a chain-transfer agent is added to vinyl acetate polymerizations, whether emulsion, suspension, solution, or bulk, to control the polymer molecular weight. Aldehydes, thiols, carbon tetrachloride, etc, have been added. Some emulsion procedures call for the recipe to include a quantity of preformed PVAc emulsion and sometimes antifoamers must be added (see Foams). 

Bulk Polymerizations. In the bulk polymerization of vinyl acetate the viscosity increases significantly as the polymer forms making it difficult to remove heat from the process. Low molecular weight polymers have been made in this fashion. Continuous processes are known to be used for bulk polymerizations (68). 

Suspension Polymerization. This method (10) might be considered as a number of bulk polymerizations carried out simultaneously in the monomer droplets with water acting as a heat-transfer medium. A monomer-soluble initiator, eg, a peroxide or azo compound, and a protective coUoid like poly(vinyl alcohol) or bentonite, are requited. After completion of the polymerization, the excess of monomer(s) is steam stripped, and the beads of polymer are collected and washed on a centrifiige or filter and dried on a vibrating screen or by means of an expeUer—extmder. 

Suspension polymerization produces beads of plastic for styrene, methyl methacrviaie. viny l chloride, and vinyl acetate production. The monomer, in which the catalyst must be soluble, is maintained in droplet fonn suspended in water by agitation in the presence of a stabilizer such as gelatin each droplet of monomer undergoes bulk polymerization. In emulsion polymerization, ihe monomer is dispersed in water by means of a surfactant to form tiny particles held in suspension I micellcsK The monomer enters the hydrocarbon part of the micelles for polymerization by a... 

Perhaps the only process where such correlations have been published is the bulk polymerization of vinyl chloride as reported by Ray, Jain and Salovey (14). 

Finally, similar autoacceleration in the polymerization rate was reported by Crosato-Arnaldi, Gasparini and Talamini (18) for the bulk polymerization of vinyl chloride. 

The literature on the modeling and design of precipitation polymerization reactors is limited primarily to reactor for the bulk polymerization of vinyl chloride (31-38), although other systems have been discussed, particularly in the patent literature (39,40,41). 

Under conditions of low radical initiation, Graessely (21) has shown that the following set of equations describes the molecular weight and branching development in the bulk polymerization of vinyl acetate ... 

SOLUTION OF KINETIC EQUATIONS FOR LONG CHAIN BRANCHING IN BULK VINYL ACETATE POLYMERIZATION ... 

The in situ bulk polymerization of vinyl monomers in PET and the graft polymerization of vinyl monomers to PET are potential useful tools for the chemical modification of this polymer. The distinction between in situ polymerization and graft polymerization is a relatively minor one, and from a practical point of view may be of no significance. In graft polymerization, the newly formed polymer is covalently bonded to a site on the host polymer (PET), while the in situ bulk polymerization of a vinyl monomer results in a polymer that is physically entraped in the PET. The vinyl polymerization in the PET is usually carried out in the presence of the swelling solvent, thereby maintaining the swollen PET structure during polymerization. The swollen structure allows the monomer to diffuse in sufficient quantities to react at the active centers that have been produced by chemical initiation (with AIBM) before termination takes place. 

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. 

The data in Table I are not directly comparable, since the viscosity of the 3-isomer was determined in benzene while the others were measured in DMSO. In addition, the first two polymers were prepared in bulk polymerizations, while the polymerization of methyl 3-vinylsalicylate was carried out with the monomer diluted 1 1 with benzene. Thus no certain conclusion can be drawn the data are, however, an indication of possible difficulty in radical polymerization of substituted styrenes bearing a phenol ortho to the vinyl group. 

Detailed studies on the lipase-catalyzed polymerization of divinyl adipate and 1,4-butanediol were performed [41-44]. Bulk polymerization increased the reaction rate and molecular weight of the polymer however, the hydrolysis of the terminal vinyl ester significantly limited the formation of the polyester with high molecular weight. A mathematical model describing the kinetics of this polymerization was proposed, which effectively predicts the composition (terminal structure) of the polyester. 

In EP of bifunctional vinyl monomers, the reaction rate increases with the emulsifier concentration because the number of particles increases. However, in the crosslinking EP of divinyl monomers, the reaction rate is inversely proportional to the emulsifier concentration. This unusual behavior is due to nucleation taking place in both micelles and monomer droplets. In monomer droplets, the kinetics resembles that of bulk polymerization and therefore the reaction rates... 

Under free radical conditions, we found that the bulk polymerization of 4-allyloxystyrene gave an insoluble crosslinked polymer with AIBN. Similar results were previously reported for the polymerization with benzoyl peroxide (8). The cationic polymerization of /7-alkoxystyrene monomers have been shown to proceed at rates that are comparable to vinyl ethers (12,13). As expected from these studies, we found that alkenyloxystyrene monomers also have a high degree of cationic reac-... 

Another possible explanation is that singlet O2 somehow leads to crosslinking. The reactions of O2 have been extensively studied (34), and do not appear relevant to these copolymers. The only functionality that could conceivably react with singlet O2 is a vinyl chain termination, which could produce a hydroperoxide that might then participate in crosslinking. However, in a study of free radical polymerized PMMA (35), the maximum fraction of polymer chains with vinyl ends was found to be 0.36, for bulk polymerized material in benzene solution the fraction was 0-3%. This result, plus the fact that the insolubilization occurs immediately during photolysis at room temperature, makes it very unlikely that such hydroperoxides are involved. 

It is clear, however, that a simultaneous increase in polymerization rate and molecular weight could either follow from a reduction in the rate of termination or from an increase in the rate of propagation. This last possibility has seldom been considered, except in some of the very early studies such as in the work of BENGOUGH and NORRISH (2J on the bulk polymerization of vinyl chloride where a "catalytic" action was attributed to the precipitated polymer. 

This was derived assuming uniform concentration because good mixing is important for this relationship to hold. It also assumes a constant temperature. Both these assumptions are only approached in most batch systems. Further, stirring becomes more difficult as conversion increases so that both control of localized temperature and concentration become more difficult. In reality, this relationship holds for only a few percentage points of conversion. Overall, temperature is a major concern for vinyl polymerizations because they are relatively quite exothermic. This is particularly important for bulk polymerizations. This, coupled with the general rapid increase in viscosity, leads to the Trommsdorff-like effects. 

Polymerization of a monomer in a solvent overcomes many of the disadvantages of the bulk process. The solvent acts as diluent and aids in the transfer of the heat of polymerization. The solvent also allows easier stirring, since the viscosity of the reaction mixture is decreased. Thermal control is much easier in solution polymerization compared to bulk polymerization. On the other hand, the presence of solvent may present new difficulties. Unless the solvent is chosen with appropriate consideration, chain transfer to solvent can become a problem. Further, the purity of the polymer may be affected if there are difficulties in removal of the solvent. Vinyl acetate, acrylonitrile, and esters of acrylic acid are polymerized in solution. 

For the copolymerization of ethene and vinyl acetate, solution polymerization, suspension polymerization, emulsion polymerization and bulk polymerization may be used, but solution polymerization is preferred (1). A method of either continuous type or batch type may be employed. Methanol is generally used as the solvent. 

The simplest procedure for grafting copolymerization, in terms of number of components in the reaction medium, is a bulk polymerization of the monomer in mixture with the molten polyamide. This has been claimed in an earlier patent (2), related to improvements in dyeability and hydrophylic properties of the resulting yam, obtained by melt spinning of the product of reaction with monomers such as 2,5-dichloro styrene, lauryl methacrylate, N-vinyl pyrrolidone, and N-vinyl carbazole. 

The thermal stability of poly(vinyl chloride) is improved greatly by the in situ polymerization of butadiene or by reaction with preformed cis-1,4-polybutadiene using a diethyl-aluminum chloride-cobalt compound catalyst system. The improved thermal stability at 3-10% add-on is manifested by greatly reduced discoloration when the modified poly-(vinyl chloride) is compression molded at 200°C in air in the absence of a stabilizer, hydrogen chloride evolution at 180°C is retarded, and the temperature for the onset of HCl evolution and the peak decomposition temperature (DTA) increase, i.e. 260°-280°C and 290°-325° C, respectively, compared with 240°-260°C and 260°-280°C for the unmodified homopolymer, in the absence of stabilizer. The grafting reaction may be carried out on suspension, emulsion, or bulk polymerized poly(vinyl chloride) with little or no change in the glass transition temperature. 

Viscometry The specific viscosity of each polymer from the bulk polymerization was measured in acetone at 30°C using an Ubbelohde dilution viscometer. Five concentrations in the range of 1.120 to 0.242 g/d poly(vinyl acetate) and polyvinyl trideuteroacetate) and 0.385 to 0.084 g/dl (poly(trideu-terovinyl acetate)) were run. Intrinsic viscosity was calculated by extrapolation of the Tlsp/c versus c plot to zero concentration. Number average molecular weights were calculated using the equation(20) [q] =1,0 x 10 1 [Mn] 0 72 which is in the mid range of the equations listed. 

Bulk Polymerization The conversion versus time plot for the bulk polymerizations of vinyl acetate and its deuterated analogues is shown in Figure 1. Vinyl tridueteroacetate has a conversion rate of 9.9 x 10 3/min which is identical with that of vinyl acetate (9.5 x 10 3/min) within the experimental error. However, trideuterovinyl acetate has a much higher conversion rate (1.69 x 10 2/min). The ratio of the rate of polymerization of tridueterovinyl acetate to the average of the other two monomers is 1.74 -. 03. 

Model and Nature of Termination Step for Bulk Polymerization of Vinyl Acetate The following is a reasonable kinetic model for the bulk polymerization of vinyl acetate. 

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