Photoinitiated polymerization of di- and tetrafunctional methacrylic monomers in a SBS matrix. Kinetic, mechanistic, and structural aspects

The kinetics and mechanism of the photoinitiated polymerization of tetrafunctional and difunctional methacrylate monomers (di-, hexa-, and decamethylene dimethacrylates; and ethylhexyl and dodecyl methacrylates) in a styrene-butadiene-styrene (SBS) block copolymer matrix have been studied. Reaction diffusion was found to be the only termination mechanism for tetrafunctional monomers when the monomer concentration in the matrix is below 30-40%; for higher monomer concentrations, reaction diffusion controls the termination process only after approximately 10% conversion was reached. The values of both the propagation kinetic constant and the overall double bond conversion for the three tetrafunctional monomers studied showed the following order: deca- > hexa- > dimethylene dimethacrylate. The termination process in the photoinitiated polymerization of difunctional methacrylate monomers is clearly controlled by reaction diffusion right from the beginning of the polymerization reaction only at a very low monomer concentration in the matrix (10-15%); for medium monomer concentrations (20-40%), a combination of both mechanisms, segmental diffusion-controlled (autoaccelerated kinetics) and reaction diffusion, was observed until reaching a double bond conversion of 20%, from which point reaction diffusion predominated; for higher monomer concentrations (60-90%), the termination kinetic constant values at low conversions (<30%) were close to those corresponding with standard polymerizations, observing the Trommsdorff effect (autoacceleration) at higher double bond conversions. The SBS matrix participates appreciably in the polymerization process through the direct addition of the macroradical or the primary radical to the double bond of the polybutadiene moiety and through hydrogen abstraction from the matrix with the formation of benzylic and allylic radicals.