Analytical Sciences

Pulsed NMR Study of Network Formation in the Course of Bulk Polymerization of Methyl Acrylate

Hiroki KIMOTO, Atsuko FUKUDA, Atsushi ASANO, and Takuzo KUROTSU

Department of Applied Chemistry, National Defense Academy, Hashirimizu 1-10-20, Yokosuka, Kanagawa 239-8686, Japan

The proton spin-spin relaxation time (T₂) during the bulk polymerization of methyl acrylate was measured as a function of the reaction time at various temperatures. Three kinds of T₂ (T_2L (long), T_2S (short) and T_2M (intermediate)) were obtained as the polymerization proceeded. The fraction of T_2S (F_S) increased sigmoidally at a certain reaction time, while that of T_2L (F_L) decreased reciprocally. The former corresponded to the amount of a polymer whose molecular weight was sufficiently high enough to cause a tight entanglement that produced a transient network structure; the latter reflected a decrease in the mixture of the monomer and the low molecular weight of the polymer. T_2M is considered to arise from a relatively mobile region of the entanglement. The relationship between the fractions of T_2S + T_2M and the polymer yield was found to be linear, which led us to monitor the polymer yield in real time during the polymerization in a non-distractive manner. ¹³C DD (dipolar decoupling)/MAS (magic angle spinning) NMR spectra were also measured to monitor the polymerization process in terms of the molecular motions between the main chain and the side chain in the formation of a network structure. The ¹³C DD/MAS NMR spectra show that the side chain motion became restricted as well as the main chain when the “Trommosdorff effect” (gel effect) was observed, and a part of the monomers were trapped in the network structure.

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