We have studied the polymerization of methyl methacrylate in the presence of polyethylmethacrylate or polybutylmethacrylate or copolymers of methyl methacrylate and butyl methacrylate of different composition dissolved therein. It has been found that at certain depths of methyl methacrylate conversion, the system changes its state from homophase to heterophase one. The compatibility region of polymethacrylates narrows with the increasing content and molecular weight of the "foreign" polymethacrylate and with the growing number of CH2- groups in the radical of the lateral substituent of the injected polymer macromolecule. The phase transition of the system is accompanied by an extreme change in the viscosity of the polymerizing mass.
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