Divinyl cross-linkers copolymerization

Free radical copolymerization of a monovinyl compound and a divinyl cross-linker is by far the most commonly employed mode of polymerization for the preparation of organic monoliths. 

Gelation in Atom Transfer Radieal Copolymerization with a Divinyl Cross-linker... 

ATRP was applied to the copolymerization of a monovinyl monomer and a divinyl cross-linker to study the experimental gelation behavior. The fundamental features of ATRP, including fast initiation and reversible deactivation reactions, resulted in a retarded gelation and the formation of a more homogeneous network in the ATRP process compared to gel formation in a conventional radical polymerization. The experimental gel point based on the monomer conversion in the ATRP reaction occurred later than the calculated value based on Flory-Stockmayer s mean-field theory, which was mainly ascribed to intramolecular cyclization reactions. The dependence of the experimental gel points on several parameters was systematically studied, including the ratio of cross-linker to initiator, the concentration of reagents, reactivity of vinyl groups, initiation efficiency of initiators, and polydispersity of primaiy chains. 

Atom transfer radical polymerization (ATRP) was selected as an exemplary CRP technique to systematically study the kinetics and gelation behavior during the concurrent copolymerization of monovinyl monomers and divinyl cross-linkers (Scheme 2). The effect of different parameters on the experimental gelation was studied, including the initial molar ratio of cross-linker to initiator, the concentrations of reagents, the reactivity of vinyl groups present in the cross-linker, the efficiency of initiation, and the polydispersity of primary chains. Experimental gel points based on the conversions of monomer and/or cross-linker at the moment of gelation, were determined and compared with each other in order to understand the influence of each parameter on the experimental gel points. 

Scheme 29 Illustration of chain growth and gelation process by copolymerization of monomer (M) and divinyl cross-linker (X) using ATRP with fast initiation. Reprinted from Li, W. Gao, H. Matyjaszewski, K. Macromolecules2099, 42.927-932, " with permission from the ACS.

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. 

For network polymer formation a small amount of an appropriate divinyl monomer can be included in the monomer feed as a cross-linking agent. By analogy with copolymerization, as long as the monomer and cross-linker are of similar reactivity then the degree of cross-linking will be statistical and can be controlled by the molar proportion of monomer to cross-linker. An example of the preparation of acrylic network LCPs is shown in Fig. 

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