Photoinitiators-Maleimides

Photoinitiator Free Polymerization of Maleimides and Vinyl Ethers... 

Difunctional vinvl ether/difunctional N-maleimide. Up until this point, our results have centered on the reactivity of monofunctional maleimide divinyl ether mixtures. From Kloosterboer s26 work for acrylate polymerization, it is known that the rate of polymerization of a free-radical process is increased dramatically as the functionality of the acrylate is increased. In order to enhance the polymerization rates of maleimide divinyl ether systems, it was decided to synthesize difimctional maleimides for copolymerization with difunctional vinyl ethers. The results in Table V indicate that the photoinitiated TTDBM [bismaleimide made from maleic anhydride and 4,7,10-... 

In order for maleimide/vinyl ether photoinitiator free photopolymerization to be useful, it is important that the cured films have good thermal/UV stability. Since there are no small molecule photoinitiators added to the uncured mixture initially, there is no residual small molecule photo initiator present in the final crosslinked film. This accounts for the enhanced UV stability we have observed for cured maleimide/vinyl ether films. In addition, TGA thermograms of photocured films of the MPBM/CHVE mixture (Figure 8) exhibit excellent thermal stability, with decomposition occurring at higher temperatures than for a simple UV cured HDDA film with 3 weight percent DMAP photoinitiator. (Such thermal stability would be... 

The synthetic methods developed for the formation of tricyclic compounds containing a central cyclobutane ring systems continue to be dominated by photochemical approaches <1996CHEC-II(7)841>. In particular, photodimerization reactions of maleimides (Equation 40) have attracted significant recent attention in the field of copolymeriz-able photoinitiators <1995JOC2353, 2002CEJ4199>. 

When using a suitable type I photoinitiator, e.g., acylphosphine oxides and non-amine-containing acetophenones in this system, the polymerization process is very efficient and the resulting product is a 1 1 alternating copolymer. This system is susceptible to oxygen inhibition, but to a much lesser extent than acrylate polymerization. An important advantage of this system is its low toxicity. Typical components include a variety of vinyl ethers and unsaturated esters, such as maleate, fumarate, citra-conate, imides (maleimides), or N-vinylformamides. The system is also... 

X. Photochemical cycloaddition of maleimide to certain benzene derivatives. Zhournal Organicheskoi Khimii, 14, 1522-1529 (b) Zhubanov, B.A., Almabekov, O.A., and Ismailova, Zh.M. (1981) Photoinitiated addition of maleic anhydride to monoalkylbenzenes. Zhournal Organicheskoi Khimii, 17, 996-999. 

Photocondensation Polymerization.—This term is used in the sense that the polymerization requires the absorption of a photon for every propagation step, as distinct from the photoinitiation of the free-radical chain addition polymerization discussed above. The photodimerization reaction (14) of maleimides in... 

Figure U Photoinitiated copolymerization of a 1/1 molar mixture of a N-substituted bis-maleimide and a divinylether...

The use of N-substituted maleimides as photoinitiators for radical polymerization, has gained attention as an alternative material to ketone based photoinitiators.Sensitization of maleimides to the triplet state markedly increases polymerization efficiencies. The maleimide amine system produces two radicals, one centered on the amine and the other on the maleimide. Both... 

In addition, when used to photoinitiate the polymerization of acrylic monomers, low concentrations of N-substituted maleimide coupled with isopropylthioxanthone and a tertiary amine system result in markedly increased rates of polymerization, faster than the traditional isopropyl-thioxanthone/amine system alone (see the next section). Maleimides can also be used in combinations with a diarylketones and amines to initiate polymerization. ... 

There are definite attractions for monomers that can be used without the aid of initiators. Such monomers are maleimides. The monomers based on vinyl acrylate are also capable of self initiation. The vinyl ester itself, however, is too volatile for practical use and its initiation of polymerization is slower than obtained with the traditional photoinitiators. When the acrylate group is replaced by crotonate, cinnamate, fumarate, or maleate chromophores, these monomers copolymerize readily with thiol and vinyl ether monomers and initiate free-radical polymerization upon direct excitation in the absence of any added photoinitiator. 

In some cases, a monomer may function as a photoinitiator and become incorporated into a copolymer chain. This has been shown for styrene and other conjugated monomers when exposed to deep-UV light however, the initiation efficiency in these systems is substantially less than when a photoinitiator is present (p. 223 of Ref 33, and Ref 34). A better polymerizable initiator scheme is illustrated by the acceptor-donor chemistry of maleimide-donor systems (35-37). Maleimide acts as both photoinitiator and comonomer in the presence of hydrogen donors such as vinyl ethers or vinyl esters (38,39) an example of this copolymerization is shown in equation 5. It shows the molecular structure of the acceptor er -butylmaleimide (left), the donor 4-hydroxy-butyl vinyl ether (right), and their corresponding copolymer repeat unit. 

Vinyl ethers (VE) are amongst the most reactive monomers that polymerise cationically, and they are increasingly used in photocurable resins, either neat [63] or in association with epoxides [64,65], acrylates [66], maleimides [67] or unsaturated polyesters [68]. Vinyl ether end-capped polyurethane, polyether, polysiloxane and polyesters were shown to undergo fast and extensive polymerisation when exposed to UV-radiation in the presence of an aryliodonium salt, with the formation of tridimensional polymer networks. The photoinitiated catioiuc polymerisation of a divinyl ether can be formally written as follows ... 

Decker, C., Morel F., Jonsson, S., Clark, S. and Hoyle, C.E., Light-induced polymerization of photoinitiator-liee vinyl-ether-maleimide systems, Macrotnol. Chem. Phys., 1999, 200, 1005-1013. 

The electron-transfer reaction as an initiation mechanism of the copolymerization of maleimide and vinyl ethers was investigated by time-resolved optical spectroscopy and FT EPR. Maleimide vinyl ether resins polymerize upon UV irradiation without the addition of a photoinitiator. The hrst step of initiation is an electron transfer from the ground-state vinyl ether molecule to the triplet maleimide. The maleimide radical-anion was detected by FT EPR in systems with hydroxybutyl vinyl ether and thiocyanate as electron donors whereas the maleimide hydrogen-adduct radical was found with 2-propanol as triplet quencher. 

Clearcoats based on combinations of various maleimides (MI) and vinyl ethers (VE) or VE blends were tested with respect to their UV-photocuring in air [175]. It was found that MI/VE binders and reactive thinners could be cured without additional photoinitiators. Moreover, the equimolar MI/VE blends could be used as photoinitiators for acrylate systems, although their reactivity compared with commercial Norrish type I initiators proved to be poor. The observed limitations, e.g. changes in the properties of cured UPRs, limited reactivity in the air and irritation effects, could be ehminated using aliphatic MI types with further increased reactivity, higher functionality and/or molecular weight and lower irritation. 

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