Acrylic acid polymerization solvent effects

Influence of temperature on the process of polymerization of acrylic acid in dioxane and toluene was examined. It was found that in dioxane an increase in temperature destroys the oligomeric auto-associations of acrylic acid and gives rise to monomer-solvent association, making matrix effect less pronounced. In toluene, an increase in temperature converts the cyclodimeric autoassociations of the monomer into linear oligomers and the matrix effect appears. 

It was reported that, in the contrast to acrylic acid, methacrylic acid does not exhibit any template effect under conditions described. However, template effect appears if a solvent such as water or methanol is added, and also at higher temperatures of polymerization. 

Acrylamide copolymeiizes witli many vinyl comonomers readily. Tlie copolymerization parameters in the Alfrey-Price scheme are Q = 0.23 and e = 0.54 (74). The effect of temperature on reactivity ratios is small (75). Solvents can produce apparent reactivity ratio differences in copolymerizations of acrylamide with polar monomers (76). Copolymers obtained from acrylamide and weak acids such as acrylic acid have compositions that are sensitive to polymerization pH. Reactivity ratios for acrylamide and many comonomers can be found in reference 77. Reactivity ratios of acrylamide with commercially important cationic monomers are given in Table 3. 

The rate of polymerization increases in this series of metals Mg(II) < Sr(II) < Ba(II) < Ca(II), The nature of the cation is likely to have a significant effect on the kinetics of the polymerization of salts of unsaturated acids in ionizing environments [68-70]. These differences are attributed to a different charge density at the macroradical anion, which influences the rate of interaction in the propagating macroradical-monomeric anion system. In comparable conditions the rate of radical polymerization of transition metal acrylates is lower than that of acrylic acid (AA) and decreases in the series (Fig. 4-7) [71] AA > Co(II) > Ni(II) > Fe(III) > Cu(II) (see Experiment 4-1, Section 4.6). The resulting metallopolymers are insoluble in any organic solvent, which indicates... 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

The PLP-SEC investigations into kp of free-radical polymerization in aqueous phase suggest that kp varies strongly with monomer concentration. For MAA, NIPAm ) and AAm a strong decrease in kp was found upon increasing monomer concentration. The same trend is seen for AA from monomer concentrations of 3 wt.-% on, whereas at very low AA contents kp increases with acrylic acid concentration. Attempts to assign the strong solvent effects to associated struc-tures, ) to dimerization,f l or to local monomer concentrations at the radical site... 

Polymerization Processes. A variety of processes are used commercially to homopolymerize and copolymerize acrylic acid and methacrylic acid. On the basis of economics and environmental considerations, water is generally the preferred industrial solvent or polymerization medium. However, the choice of process is usually dictated by the requirements of the polymer to be produced. As already indicated, pH influences the rate of polymerization. Comonomers and molecular weight of the polymer to be produced also have a profound effect on the tsqje of polymerization process that can be used and on the type of product obtained. The contents of Table 2 indicate the change from water-soluble to alkab-soluble emulsions and ultimately emulsion polsrmers is dependent on the comonomers in copolymers of acryUc and methacryUc acids. This transition from water-soluble polymer to emulsion polymer as the acidic monomer is decreased depends on the hydrophobicity of the comonomer. Introduction of divinyl monomers causes transition to gel materials in all compositions. The gels may vary from highly swollen to tightly bound copolymers, depending on the cross-linker level. 

Effect of the Reaction Medium. Since the transition state in propagation is relatively nonpolar and the propagation reaction is chemically controlled (up to high monomer conversions of 80%), there is weak solvent influence on the propagation rate coefficients. Extensive studies have been carried out, which mainly confirm this small influence of the solvent on the propagation rate coefficient (141-143). Larger effects in solvents have only been observed for specific monomers, eg, EHMA (144), vinyl acetate (145), vinyl benzoate (146), or specific solvents like supercritical CO2 (147-151). Furthermore, the values for the polymerizations of methacrylic acid and acrylic acid in water are significantly affected by the monomer concentrations (152-154). In the case of the solvent effects on EHMA, where hr, falls between 580 L mol s... 

Polar solvents even at temperatures as high as 76 C usually lead to syndiotactic polymers, as does polymerization of the liquid monomer. In this regard, using acetic acid as a solvent is less effective in the production of syndiotactic poly(acrylic acid) than nonacidic polar solvents. This may be caused by acetic acid entering into the multimolecular aggregates and thus interfering with the orientation favorable for such conformation. Carefully dried monomer exhibits a reduced rate of polymerization upon irradiation and forms an atactic polymer. Some spontaneous polymerization does, however, take place when dry acrylic acid is degassed. 

An extensive study has been made of the graft polymerization of vinyl monomers (e.g. methyl methacrylate, sodium vinyl sulphonate, 4-vinylpyridine, acrylamide, and acrylic acid) onto dissolving pulp and groundwood initiated by acetic acid and hydrogen peroxide. It is possible to predict accurately the extent of grafting and the properties of the graft copolymers. The effects of solvents on the radiation-induced grafting of vinylpyridines onto cellulose have been examined. ... 

The structure of a polymeric network Is ultimately determined by the method of synthesis. The monomer and crosslinker concentrations, the initiator type and concentrations, the relative reactivities of the monomers, the specific solvent and reaction temperature are all significant. Commercially, the rate of the polymerization reaction Is also important, since it directly affects the volumetric efficiency of the production equipment. Many of the important structural parameters are determined by the polymerization kinetics and by the various stoichiometries of the reaction. For acrylic acid and sodium acrylate, several studies of the polymerization kinetics reveal that these monomers behave consistently with the standard treatment of polymerization kinetics, with polymerization rate generally first order in monomer concentration except when specific initiator effects occur. 

In contrast to these relatively weak influences of many solvents, water has a pronounced impact on aqueous radical polymerization. It is frequently found that the polymerization rates of water-soluble monomers in aqueous solutions are higher than in organic solvents." PLP-SEC studies into different systems (methacrylic acid (MAA)" acrylic acid (AA)," A-isopropyl acrylamide," acrylamide, and A-vinyl pyrrolidone revealed a huge solvent effect on kp. Figure 1.2 demonstrates this effect on the example of A-vinyl pyrrolidone. 

Attempts to esterify these directly by the use of acrylic acid or acrylic anhydride were not successful. It was found that a convenient, high-yield synthesis could be carried out in fluorocarbon solvent by the reaction of acryloyl chloride and a tertiary amine acid acceptor. Product purification by distillation was generally not satisfactory because of the temperatures required, particularly for the difunctional compounds, but purification by percolation of the fluorocarbon solvent solutions over activated alumina resulted in colorless products of sufficient purity for effective polymerization. 

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