Application of the Spin Trapping Technique to Radical Polymerization. 21. Vinyl Polymerization with the N-Hydroxysuccinimide/Acetylacetonato Metal Chelate System

Abstract

Some combinations of N-hydroxysuccinimide (NHS) and acetyl-acetonato metal chelates were found to induce radical polymerization. A kinetic study on the polymerization of methyl methacrylate (MMA) was performed by using as initiator the NHS/trisacetyl-acetonatomanganese(III) (Mn(acac)₃) system, which can initiate the polymerization effectively even at room temperature. The overall activation energy of polymerization was very low (9.1 kcal/mol). The polymerization rate (R_p) was expressed by

where the initiator concentration was varied by fixing [NHS]/[Mn(acac)₃] = 2. R_p showed a maximum when [Mn(acac)₃] was changed while keeping [NHS] constant. Further, a spin trapping study revealed that the primary propagating radical was trapped even in the system containing only a small amount of monomer (1.6 × 10⁻² mol/L). These results indicate that a complex from NHS and Mn(acac)³ with a very high stability constant plays an important role in the generation of initiating radicals, and that the monomer participates in the initiation process. This conclusion was also supported by electronic spectroscopic results. An initiation mechanism is proposed and discussed.