Conversion dependency, vinyl polymerization

However, the kinetics of PVC emulsion does not foUow the above theory. The rate shows the same increasing behavior with conversion as mass polymerization (94,95). [N depends on [3], but the relationship varies with the emulsifier type (96,97). However, the rate is nearly independent of [N (95). The average number of radicals per particle is low, 0.0005 to 0.1 (95). The high solubiUty of vinyl chloride in water, 0.6 wt %, accounts for a strong deviation from tme emulsion behavior. Also, PVC s insolubiUty in its own monomer accounts for such behavior as a rate dependence on conversion. 

Fig. 5-2 Dependence of M and MwfMn on conversion for the polymerization of isobutyl vinyl ether by HI/I2 in CH2C12 at - 15°C. [M] = 0.38 M at beginning of each batch [HI] = 0.01 M [I2] = 0.02 M (A), 0.001 M ( ), 0.005 M (o). After Sawamoto and Higashimura [1986] (by permission of Huthig and Wepf Verlag, Basel and Wiley-VCH, Weinheim).

For a detailed analysis of monomer reactivity and of the sequence-distribution of mers in the copolymer, it is necessary to make some mechanistic assumptions. The usual assumptions are those of binary, copolymerization theory their limitations were discussed in Section III,2. There are a number of mathematical transformations of the equation used to calculate the reactivity ratios and r2 from the experimental results. One of the earliest and most widely used transformations, due to Fineman and Ross,114 converts equation (I) into a linear relationship between rx and r2. Kelen and Tudos115 have since developed a method in which the Fineman-Ross equation is used with redefined variables. By means of this new equation, data from a number of cationic, vinyl polymerizations have been evaluated, and the questionable nature of the data has been demonstrated in a number of them.116 (A critique of the significance of this analysis has appeared.117) Both of these methods depend on the use of the derivative form of,the copolymer-composition equation and are, therefore, appropriate only for low-conversion copolymerizations. The integrated... 

Initiation with Triphenylmethyl Cation. When tetrahydrofuran (THF) is used to dissolve triphenylmethyl hexachlorantimonate at room temperature, there is almost immediate decomposition of the triphenylmethyl cation (6). On the other hand, solutions of the trityl salt in THF can be prepared and stored as deep yellow solutions if maintained at temperatures around — 80°C. At room temperature the initial decoloration of the catalyst is followed rapidly by polymerization of the monomer to poly(tetramethylene oxide), and the actual percentage conversion depends markedly on the temperature. This behavior is typical of systems exhibiting monomer-polymer equilibria (28), and Table III shows values for the equilibrium conversion of monomeric THF to polymeric THF obtained with a variety of catalysts. As for vinyl ether polymerization, it is most convenient to use the trityl hexachlorantimonate salt however, recourse to Table III shows clearly that above room temperature this anion yields less than the expected equilibrium conversion monomer... 

From the conversion dependence of the insolubilization process, it was concluded that both inter- and intramolecular propagation reactions occur during the polymerization of the epoxy ring. Blends of epoxidized polyisoprene and difunctional vinyl ether or aciylate monomers were shown to undergo a fast and extensive cross-linking polymerization, with formation of interpenetrating polymer networks. 

Fig. 5-2 Dependence of "M and on conversion for the polymerization of isobutyl vinyl ether by...

Because of the polymerization and monomer transport, the polymer particles grow in size, and after some time the monomer droplets disappear. This marks the end of Interval II. The monomer conversion at which Interval II ends depends on the capability of the polymer particle to be swollen by monomer. The higher the maximum swelling, the earlier the monomer droplets disappear. In general, the more water-soluble the monomer, the higher the maximum swelling, and hence the lower the monomer conversion at the end of Interval II. Thus, the transition from Interval II to Interval III occurs at about 40% conversion for styrene and at about 15% conversion for vinyl acetate. This means that most monomer polymerizes in Interval III (Figure 6.2(d)). In this interval, monomer concentration in the polymer particles decreases continuously. The final product is a waterborne, concentrated (50-60 wt.% solids) dispersion of tiny (80-500 nm in diameter) polymer particles called latex. 

Deviations in the emulsion polymerization of VC from the expected behavior were discussed by Chedron [104] in terms of the aqueous polymerization and its competition with the micellar polymerization. The monomer-saturated aqueous phase and precipitation of growing radicals should support the importance of the aqueous-phase polymerization. The molecular weights of poly(vinyl chloride) slightly decreased with emulsifier concentration and increase with conversion. The trend of the molecular weights vs conversion dependence was similar to that in Ref. [102]. 

The emulsion polymerization of vinyl acetate (to homopolymers and copolymers) is industrially most important for the production of latex paints, adhesives, paper coatings, and textile finishes. It has been known that the emulsion polymerization kinetics of vinyl acetate differs from those of styrene or other less water-soluble monomers largely due to the greater water solubility of vinyl acetate (2.85% at 60°C versus 0.054% for styrene). For example, the emulsion polymerization of vinyl acetate does not follow the well-known Smith-Ewart kinetics and the polymerization exhibits a constant reaction rate even after the separate monomer phase disappears. The following observations have been reported for vinyl acetate emulsion polymerization [78] (a) The polymerization rate is approximately zero order with respect to monomer concentration at least from 20% to 85% Conversion (b) the polymerization rate depends on the particle concentration to about 0.2 power (c) the polymerization rate depends on the emulsifier concentration with a maximum of 0.25 power (d) the molecular weights are independent of all variables and mainly depend on the chain transfer to the monomer (e) in unseeded polymerization, the number of polymer particles is roughly independent of conversion after 30% conversion. 

This paper is about a reinterpretation of the cationic polymerizations of hydrocarbons (HC) and of alkyl vinyl ethers (VE) by ionizing radiations in bulk and in solution. It is shown first that for both classes of monomer, M, in bulk ([M] = niB) the propagation is unimolecular and not bimolecular as was believed previously. This view is in accord with the fact that for many systems the conversion, Y, depends rectilinearly on the reaction time up to high Y. The growth reaction is an isomerization of a 7t-complex, P +M, between the growing cation PB+ and the double bond of M. Therefore the polymerizations are of zero order with respect to m, with first-order rate constant k p]. The previously reported second-order rate constants kp+ are related to these by the equation... 

Carbon blacks have been reported to be capable of initiating the cationic polymerization of vinyl monomers such as vinyl ethers, indene, and acenaphthylene. The grafting sites of the polymer were based on carboxyl groups present on the surface [88]. The polymerization was inhibited by treatment of the carbon blacks with NaHCOs, CH2N2, pyridine, and DMF. Also, the degree of conversion was found to be dependent on temperature and time of polymerization [89]. 

Three arm amphiphilic star block copolymers of IBVE and 2-hydroxyethyl vinyl ether (HOVE) were prepared using the trifunctional initiator 8 with sequential cationic polymerization of two hydrophobic monomers, IBVE and AcOVE. Subsequent hydrolysis of the acetates led to the hydrophilic poly(HOVE) segments [38]. Two types of stars were prepared depending on which monomer was polymerized first three arm star poly(IBVE-h-HOVE), with the hydrophobic part inside and three arm star poly(HOVE-h-IBVE), with the hydrophobic part outside. When IBVE was polymerized first, the experimental conditions were the same as described in Sect. 2.2.1. After reaching quantitative monomer conversion, AcOVE was added and temperature was raised from 0 to 40 °C to accelerate the reaction since this monomer is less reactive than IBVE. When starting with AcOVE as a first block, both polymerizations were carried out at 40 °C. SEC analysis showed that MWDs were narrow for the two steps whatever the se-... 

The color change and the sensitivity of conversion to order of addition of monomers and peroxide indicate that in order to obtain an AFR polymer the polar monomers must first be complexed or allowed to react with the active or living end of the anionic polymer chain, or otherwise solvate it before the polymer chain is attacked by the peroxide. Success or failure of the subsequent free radical block polymerization depends on the nature of the complex or reaction product formed. The resultant species are no longer active for propylene polymerization. The necessity of complex formation has also been observed by Milovskaya and coworkers (4). They have shown that vinyl chloride, a weak complexing agent, can be polymerized effectively with triethylaluminum peroxide only when it is present with a more active complexing compound such as an ester or an ether. 

In some systems it is necessary to add a large amount of salts to obtain polymers with low polydispersities. This happens when salts participate in ligand/anion exchange (special salt effect) and when they enhance ionization of covalent compounds through the increase of ionic strength. The special salt effect may either reduce or enhance ionization. Strong rate increases observed in the polymerization of isobutyl vinyl ether initiated by an alkyl iodide in the presence of tetrabutylammonium perchlorate or triflate can be explained by the special salt effect [109]. The reduction in polymerization rate of cyclohexyl vinyl ether initiated by its HI adduct in the presence of ammonium bromide and chloride can be also ascribed to the special salt effect [33]. The breadth of MWD depends on the relative rate of conversion of ion pairs to covalent species and is affected by the structure of the counterions. 

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. 

In the photopolymerization of methacrylamide by benzoin methyl ether, chain-transfer to monomer has been found to be important, and benzalde-hyde is reported to be an inefficient photoinitiator of methyl methacrylate polymerization unless benzophenone and triethylamine are present. Acetophenone has been found to sensitize the cycloaddition of maleic anhydride to 7-oxabicyclo[2.2.1]heptan-5-one-2,3-dicarboxylic anhydride, , a-hydroxy-acetophenone derivatives have been found to be non-yellowing initiators, and h.p.l.c. has been used to determine residual carbonyl photoinitiators in u.v.-cured resins. In the emulsion-polymerization of methyl methacrylate using an aromatic ketone and a continuous or intermittent laser, the former conditions were found to be similar to those under continuous u.v. irradiation. The dependence of the polymerization rate and average chain-length on the absorbance of the initiator used in the photoinitiated polymerization of vinyl monomers has been studied. Interestingly, irrespective of all conditions, maximum conversion is observed when initiator absorbance is 2.51. "... 

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