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As photocuring of monomer mixtures to produce highly crosslinked films with desirable physical
properties for a wide variety of uses continues to grow and expand, there is a continual need to
develop fast photoinitiator packages. Certainly, new low cost I high efficiency photoinitiators
would find widespread uses in both traditional and new photocunng applications. A summary of
photoinitiators and their detailed photochemistry can be found in excellent reviews of the subject
(1-4). There are two basic classes of photoinitiators that proceed from excited states to give radicals capable of initiating free-radical polymerization processes: cleavage photoinitiators and
abstraction photoinitiators. Cleavage type photoinitiators, exemplified by derivatives of acetophenone, produce initiating radicals through a highly efficient excited state V-cleavage
process. The abstraction type photioinitiators are exemplified by the benzophenone/amine
combination which reacts to give two radicals via photoreduction of the benzophenone triplet
excited state by an electron transfer/proton transfer reaction from the ground state amine. In the
benzophenone/amine system, only the aminyl radical generated on the amine by the proton
transfer from the radical cation is capable of initiating acrylate polymerization. The semipinacol
radical that is produced by the proton transfer from the amine radical cation to the benzophenone radical anion serves primarily as a chain terminating species, and therefore actually reduces the rate of the free-radical polymerization process. For these reasons, though they are typically less expensive, abstraction type photoinitiators have found use only in certain applications where cost is the determining factor and/or it is necessary to take advantage of the oxygen scavenging afforded by such systems. An abstraction type photoinitiator system in which both radicals produced by the abstraction reaction are capable of initiating polymerization would be advantageous and potentially competitive with traditional cleavage type photoinitiators.
Reports (5-8) from our laboratory have shown that N-aliphatic and certain N-aryl substituted
maleimides can act as hydrogen abstraction photoinitiators when excited in the UV region. The
mechanism is believed to occur by absorption of a photon by the maleimide followed by somewhat inefficient intersystem crossing to the excited state triplet. The excited state triplet can
then abstract a labile hydrogen atom from hydrogen atom donors such as ethers and alcohols. In
the case of amine hydrogen atom donors, the substituted maleimide is reduced via an electron
Iproton transfer sequence similar to that which occurs for traditional benzophenone/amine
systems. Interestingly, the hydrogen abstraction reaction of maleimides results in two radicals,
the aminyl radical and the succinimido radical, which can initiate polymerization, as opposed to
the case of benzophenone where only one initiating radical is produced.
It was recently reported (8-11) that the addition of a very low concentration of an N-substituted
maleimide to an acrylate monomer with a traditional benzophenone/tertiary amine photoinitiator
package present gave a marked enhancement in the rate of the polymerization as inferred from
the polymerization exotherms. Very fast curing rates of acrylic resins formulated to provide films
that meet the physical and mechanical requirements demanded in industrial applications were
also obtained (12) by the combination of a sensitizer (benzophenone), amine methyldiethanolamine) and low concentrations of a variety of different N-substituted maleimides,
including N-aliphatic, N-alkyl, N-aryl and twisted N-aryl substituents. In this paper we review
some of our previous data on acrylate polymerization initiated using benzophenone as a
sensitizer in combination with N-substituted maleimides with amines. In addition, we provide
laser flash photolysis data that clearly provides a rationale for the success of benzophenone/maleimide/amine combinations. As will be demonstrated, the successful
sensitization of N-substituted maleimides is dependent on several kinetic constants. We also
present initial data obtained for 4-phenylbenzophenone as a sensitizer for acrylate polymerization and show that it is more efficient than benzophenone in leading to enhanced rates of acrylate polymerization.
