Background of the Invention Ethylenically unsaturated compound, such as acrylate derivatives, can be polymerized by exposure to radiation, typically ultraviolet light, in the presence of a photoinitiating system. Typically, the photoinitiating system inclues (1) a compound capable of initiating polymerization of the ethylenically msaturated compound upon exposure to radiation (a"photoinitiator") and optionally (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor.

The coinitiators or synergists are typically alcools, tertiary amines, amides, or ethers which have labile hydrogens attache to a carbon adjacent to a heteroatom. Currently commercially available photoinitiators include benzophenones and derivatives thereof, such as thioxanthone derivatives.

Maleimides, and in particular N-aliphatic and ortho-substituted (i. e.

"twisted") N-aromatic maleimides have been investigated as comonomer- photoinitiators of UV-curable acrylic systems. These maleimides have been observe to require the presence of a hydrogen atom-donor synergist (such as an amine, ether, thiol, or the like) to obtain reasonable rates of initiation and polymerization.

J. Put and F. C. De Schryver,"Photochemistry of Nonconjugated Bichromophoric Systems,"Journal of the American Chemical Society. 95,1,137-45 (1973);"Photocyloaddition Polymerization. I. Preparation and Characterization of Poly-N, N'-polymethylenebisdichloromaleimides," Journal of Polymer Science: Part A-1.8,1939-48 (1970); and N. Boens et al., "Solid-State Ultraviolet Irradiation of Some Maleimides and Bismaleimides,"Journal of Polymer Science: Polymer Chemistry Edition 13,210-13 (1975) report studies of the sensitization of maleimides with benzophenone and thioxanthones for the formation of maleimide 2 + 2 cycloadducts. F. C. De Schryver, et al,"Photochemistry of Nonconjugated

Bichromophoric Systems. Photopolymerization of N, N-Alkylenebis (dimethylmaleimides),"Journal of the American Chemical Society 96,20,6463-69 (1974) also report the formation polymer networks by the sensitized step-wise 2 + 2 cycloaddition of maleimides.

Summum of the Invention The present invention is directe to photopolymerizable compositions and methods for radiation curing of the same. The compositions of the invention include at least one radiation curable compound. The compositions preferably include ethylenically unsaturated compound, and in particular, acrylate derivatives.

The compositions further include at least one maleimide which is capable of initiating the photopolymerization of a radiation curable compound. The maleimide can be an alkyl maleimide, functionalized aliphatic maleimide, aromatic maleimide, maleimide, maleic anhydride, or mixtures thereof. The maleimide component can be substantially completely consume during initiation and photopolymerization (incorporated into the polymer structure).

The composition further inclues additional photoactive compound as sensitizers. Specifically, the composition inclues at least one benzophenone compound which is capable of initiating polymerization of the ethylenically unsaturated compound upon exposure to radiation. As used herein, the term "benzophenone compound"includes benzophenone and derivatives thereof, including thioxanthone and derivatives of thioxanthone. In addition, the composition optionally inclues at least one coinitiator or synergist as a hydrogen atom donor or an electron donor. The hydrogen atom donor can be present as a separate compound or can be included as a molecular component of the photopolymerizable compound. The synergistic effect described herein which occurs when a sensitizer is used in conjunction with a maleimide in the presence of a hydrogen atom donor has been observe to dramatically increase rates of polymerization of typical acrylic photo- curable systems, as compare to rates obtained using the same acrylic photo-curable system with either maleimide with a hydrogen-atom donor or the sensitizer with a hydrogen-atom donor as the initiating system.

The present invention also provides methods for radiation curing of photopolymerizable compositions of the invention.

Brief Description of the Drawings Some of the features and avantages of the invention having been described, others will become apparent from the detailed description which follows, and from the accompanying drawings, in which: Figure 1 is a graph illustrating photo-differential scanning calorimetry (DSC) exotherm rates for the photopolymerization of 1,6-hexanediol diacrylate (HDDA) with photoinitiator pacages of 3% benzophenone (BZPN)/1 % methyl diethanol amine (NMDEA) and 0.31% N-methyl maleimide (MMI)/l % NMDEA; Figure 2 is a graph illustrating photo-DSC exotherm rates for the photopolymerization of polyethylene glycol diacrylate (PEG 400 DA) with photoinitiator pacages of 3 % BZPN/1 % NMDEA and 0. 31% MMI/1% NMDEA; Figure 3 is a graph illustrating photo-DSC exotherm rates for the photopolymerization of HDDA with photoinitiator pacages of 3% BZPN/1% NMDEA, 0. 31% MMI/1% NMDEA, and 3% BZPN/0. 31% MMI/1% NMDEA; Figure 4 is a graph illustrating photo-DSC exotherm rates for the photopolymerization of HDDA with photoinitiator pacages of 3% BZPN/0. 31% n- phenyl maleimide (N-PHMI)/1% NMDEA, 3% BZPN/0.31% N-hexyl maleimide (N- HMI)/1% NMDEA, 3% BZPN/0.31% N-cyclohexyl maleimide (N-CHMI)/1% NMDEA, 3% BZPN/0. 31% maleic anhydride/1% NMDEA, and 3% BZPN/3% Irgacure 651 (a, a-dimethoxy deoxybenzoin (BDK)); and Figure 5 is a graph illustrating photo-DSC exotherm rates for the photopolymerization of HDDA with photoinitiator pacages of 3% BZPN/0. 31% maleimide (MI)/1% NMDEA, 3% BZPN/0.31% N-2-n-tert-butyl-phenyl maleimide/1% NMDEA, 3% BZPN/0.31% DEFMR (diethylfumarate)/1% NMDEA, 3% BZPN/0. 31% HMI/1% NMDEA, and 3% BZPN/0.31% maleic anhydride/1% NMDEA.

Detailed Description of the Invention Maleimides useful in the compositions of the invention include maleimides of the formula

and maleic anhydrides of the formula wherein: each Rl and R2 is independently selected from the group consisting of hydrogen, C 1 to C 10 alkyl, cycloalkyl, aryl, alkoxy, and halogen, or Rl and R2 together form a fused saturated or unsaturated five or six membered cyclic hydrocarbon or heterocyclic ring system containing one or two O, N or S atoms, optionally substituted with alkyl, aryl, halogen, arylalkyl, alkylaryl, cycloalkyl, alkoxy, heteroatoms, silicon, and the like. Such compound are commercially available and/or can be prepared using commercially available starting materials using techniques known in the art.

Alkyl maleimides useful in the invention include compound having at least one maleimide functional group substituted with a linear, branche or cyclic C 1- C 10 alkyl radical at the nitrogen atom. Exemplary alkyl maleimide compound can have the formula below:

wherein: each Rl and R2 is independently selected from the group consisting of hydrogen, C I to C 10 alkyl, cycloalkyl, aryl, alkoxy, and halogen, or Rl and R2 together form a fused saturated or unsaturated five or six membered cyclic hydrocarbon or heterocyclic ring system containing one or two O, N or S atoms, optionally substituted with alkyl, aryl, halogen, arylalkyl, alkylaryl, cycloalkyl, alkoxy, heteroatoms, silicon, and the like; and R is straight chain, branche or cyclic C1-C10 alkyl, optionally substituted with one or more C I-C4 alkyl, and preferably is C I-C4 alkyl or C6 cycloalkyl.

Exemplary alkyl maleimides include without limitation methyl maleimide, hexyl maleimide, cyclohexyl maleimide, and the like. Such compound are known in the art and can be prepared using techniques known in the art. See, for example, Z. Y. Wang, Synthetic Coma. 20 (11) 1607-1610 (1990); P. O. Tawney et al., J. Org. Chem. 26,15 (1961); and U. S. Patent No. 2,542,145.

Aliphatic maleimide compound useful in the invention include compound having at least one maleimide unit substituted with a functionalized aliphatic substituent at the nitrogen atom. The aliphatic substituent preferably is a linear or branche C 1 to C 10 alkyl, and more preferably methyl or ethyl. The alkyl is optionally substituted with C 1 to C4 alkyl, C 1 to C4 alkoxy, halogen, and the like as described below.

Aliphatic maleimides useful in the invention can be monofunctional (have one maleimide functional group), or can be di-or multi-functional (have two or more maleimide functional groups). For example, two or more aliphatic maleimide units can be connecte or coupled via a spacer group (s), such as, but not limited to, linear or branche C I to C 10 alkyl, C3 to C6 cycloalkyl optionally substituted with C 1 to C4 alkyl, C I to C 10 oxyalkyl, which can include one or more oxygen atoms,

such as that derived from ethylene glycol, carbonate, aryl, alkylaryl, arylalkyl, and the like. Still further, maleimide compound useful in the invention include maleimide units connecte to polymeric or oligomeric compound (typically having a molecular weight of at least about 1000). For example, unsaturated polyester resins with varying percent maleimide functionality incorporated therein as known in the art can be used.

Exemplary aliphatic maleimide compound can have the formula below: wherein: (a) each Rl and R2 is independently selected from the group consisting of hydrogen, C 1 to C 10 alkyl, cycloalkyl, aryl, alkoxy, and halogen, or Rl and R2 together form a fused saturated or unsaturated five or six membered cyclic hydrocarbon or heterocyclic ring system containing one or two O, N or S atoms, optionally substituted with alkyl, aryl, halogen, arylalkyl, alkylaryl, cycloalkyl, alkoxy, heteroatoms, silicon, and the like; (b) R4 is linear or branche C I to C 10 alkyl, heteroatom, or silicon- SiH2- ; and (cl) when R4 is C1 to C10 alkyl, FG is a functional group selected from the group consisting of-OR3,-SR3,-SiH2R3,-OC (O) N (R3) 2,- OC (O) C (=CHR3) R3,-OC (O) R3,-C (O) R3,-N (R3) 2,-C (O) OR3,-NCO,-C (O) N (R3) 2,- OC (O) OR3,-CN, halogen,-CH2N-aryl-FG',-CH2N-aryl-R3-FG', sulfonic acid, quaternary ammonium, and salts thereof, in which each R3 is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, arylalkyl, and alkylaryl, and in which FG'is selected from the group consisting of-OR3,-SR3,-SiH2R3,-OC (O) N (R3) 2,- OC (O) C (=CHR3) R3,-OC (O) R3,-C (O) R3,-N (R3) 2,-C (O) OR3,-NCO,-C (O) N (R3) 2,- OC (O) OR3,-CN, halogen, sulfonic acid, and quaternary ammonium, or

(c2) when R4 is a heteroatom or silicon-SiH2-, FG is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, alkylaryl, arylalkyl, alkyl-FG", and aryl-FG", wherein FG"is the same as FG'as defined in (cl) above, or (c3) FG is a functional group as defined in (cl) in combination with a spacer group linking said maleimide unit with at least one other maleimide unit to form a di-or multifunctional maleimide compound. Exemplary spacer groups include without limitation linear or branche Cl to C 10 alkyl, C3 to C6 cycloalkyl, optionally substituted with lower C 1 to C4 alkyl, C I to C 10 oxyalkyl, which can include one or more oxygen atoms, such as that derived from ethylene glycol, carbonate, and the like. Aliphatic maleimides useful in the invention are described, for example, in pending U. S. application Serial No. 08/917,024, filed August 22,1997, titled "Polymerization Processes Using Aliphatic Maleimides,"and its corresponding published international application PCT Publication No. WO 98/07759, the entire disclosure of each of which is hereby incorporated by reference.

Exemplary aliphatic maleimides useful in the invention inclue, but are not limited to: hydroxy methylmaleimide (HMMI) hydroxy ethylmaleimide (HEMI) triethylene glycol biscarbonate bisethylmaleimide (TEGBCBEMI)

2-ethylcarbonate ethylmaleimide (2ECEMI)

2-isopropyl urethane ethylmaleimide (2IPUEMI) 2-acryloyl ethylmaleimide (2AEMI)

acetoxy ethyl maleimide (AcOEMI) isophorone bisurethane bisethylmaleimide (IPBUBEMI) bisethylmaleimide carbonate (BEMIC)

4,9-dioxa-1,12 dodecane bismaleimide (4,9-DO-1, 12-DDBMI) bispropyl maleimide (BPMI) dodecane N, N'-bismaleimide

and the like.

Generally, aliphatic maleimides which include at least one maleimide unit as described above can be prepared using techniques known in the art. See, for example, Z. Y. Wang, Synthetic Comm. 20 (11) 1607-1610 (1990); P. O. Tawney et al., J. Org. Chem. 26,15 (1961); and U. S. Patent No. 2,542,145.

Aromatic maleimides useful in invention include compound according to formula below: wherein: each of R5, R6, R7, R8, and Rg is independently selected from the group consisting of H, CX3, COOR12, COR12, OR12, CN, SR12, N (Rz2) 2, R13, X, and MI; Rlo and RI, each is independently selected from the group consisting of H, C 1 to C 10 alkyl, preferably C 1-C4 alkyl, more preferably CH3, cycloalkyl, aryl, alkoxy, and halogen, preferably chloride, or Rl and R2 together form a fused saturated or unsaturated five or six membered cyclic hydrocarbon or heterocyclic ring system containing one or two O, N or S atoms, optionally substituted with alkyl, aryl, halogen, arylalkyl, alkylaryl, cycloalkyl, alkoxy, heteroatoms, silicon, and the like; X is halide, preferably F, Cl, Br, or I ;

R12 is selected from the group consisting of H, lower alkyl, cycloalkyl, and aryl; R13 is selected from the group consisting of lower alkyl, cycloalkyl, and aryl, or R13 is a spacer group connecting at least two compound of the above formula to form a di-or multi-functional maleimide; and MI is

wherein RIO and RI, are as defined above. For example, two or more aromatic maleimide units can be connecte or coupled via a spacer group (s), such as, but not limited to, linear or branche C I to C 10 alkyl, C3 to C6 cycloalkyl optionally substituted with C 1 to C4 alkyl, C 1 to C 10 oxyalkyl, which can include one or more oxygen atoms, such as that derived from ethylene glycol, carbonate, aryl, alkylaryl, arylalkyl, and the like. The spacer group can be, for example:

and

wherein Y and Z are each independently selected from C2 to C 10 alkylene, m is an integer from 1 to 10, and n is an integer from 1 to 10; Exemplary aromatic maleimides useful in the invention inclue, but are not limited to: phenyl maleimide (PMI)

N-(2-CF3-phenyl)maleimide(2CF3PMI) N- (2-t-butylphenyl) maleimide (NtBPMI) N-(2-CF3-phenyl)methylmaleimide(2CF3PCI) N-(2, 4,6-isopropyl-3-maloimide phenyl) maleimide

N-(2-iodophenyl)maleimide

N- (2-bromo-3,5-CF3-phenyl) maleimide di (4-maleimidophenyl) methane

N-(2-chlorophenyl)maleimide

N-(2-bromophenyl)maleimide N- (2-fluorophenyl)maleimide N- (4-CF3-phenyl)maleimide

di (3,5-diethyl-4-maleimidophenyl) methane and the like.

Generally, aromatic maleimides can be prepared using techniques known in the art, with slight modifications as noted herein. The compound, for example, can be synthesized using a two step method, which begins with the rection of a suitably substituted aromatic amine with maleic anhydride (or a substituted maleic anhydride, such as citraconic anhydride) in a polar solvent, such as diethylether, to produce the amic acid in near quantitative yields. The amic acid is then recovered from the solvent and residual solvent and water can be removed from the recovered product.

The second step is acid catalyzed ring closure to form the imide. This rection is performed by dissolving the amic acid in a suitable solvent, such as an organic hydrocarbon solvent such as toluene, optionally with a small amount of cosolvent, such as dimethylsulfoxide (DMSO), adding a catalytic amount of concentrated sulfuric acid, heating the mixture, preferably to reflux, and removing water through the water/solvent azeotrope. Excess solvent can then be removed, and

the residual concentrated solution of the imide in solvent precipitated. The imide is then collecte and dried to remove water and residual solvent (s). The second step can also give near quantitative yields. Aromatic maleimides useful in the invention are described, for example, in pending provisional application Serial No. 60/047,729, filed May 27,1997, titled"Aromatic Maleimides,"and it s corresponding published international application PCT Publication No. WO 98/54134, the entire disclosure of each of which is hereby incorporated by reference.

As used herein, the term alkyl refers to linear or branche C 1 to C 10 alkyl, such as but not limited to methyl, ethyl, propyl, butyl, isopropyl, and the like, optionally substituted with halogen, aryl, arylalkyl, alkylaryl, cycloalkyl, alkoxy, heteroatoms, silicon, and the like. The term alkoxy refers to linear or branche C 1 to C 10 alkoxy. The term cycloalkyl refers to C3 to C6 cycloalkyl, such as but not limited to cyclopentyl and cyclohexyl, also optionally substituted with halogen, aryl, alkyl, arylalkyl, alkylaryl, alkoxy, heteroatoms, silicon and the like. The term aryl refers to C3 to C 10 cyclic aromatic groups such as but not limited to phenyl, naphthyl, and the like, optionally substituted with halogen, alkyl, arylalkyl, alkylaryl, cycloalkyl, alkoxy, heteroatoms, silicon, and the like. The term heteroatom refers to oxygen, sulfur, and nitrogen.

The photopolymerizable compositions include at least one maleimide as defined above as a component thereof, for example, as a photoinitiator, a comonomer, and the like. As used herein, and as will be appreciated by the skilled artisan, the term photopolymerizable composition refers to compositions which harden or cure upon exposure to radiation.

Generally the compositions of the invention include ethylenically unsaturated compound, including monomers and oligomers derived from acrylic and methacrylic acid, optionally disperse or dissolve in a suitable solvent that is copolymerizable therewith, and mixtures thereof, which are photopolymerizable when exposed to a source of radiation (ultraviolet or UV radiation, or radiation outside the UV spectrum), particularly free radical polymerizable systems. As will be appreciated by the skilled artisan, the photopolymerizable compound can be monofunctional, or can include two or more polymerizable ethylenically unsaturated groupings per molecule.

Exemplary photopolymerizable compound or precursors inclue, but are not limited to, reactive vinyl monomers, including acrylic monomers, such as acrylic and methacrylic acids, and their amides, esters, salts and corresponding nitriles. Suitable vinyl monomers inclue, but are not limited to, methyl acrylate, ethyl acrylate, n-or tert-butylacrylate, isooctyl acrylate, methyl methacrylate, ethylmethacrylate, 2-ethylhexyl methacrylate, butylacrylate, isobutyl methacrylate, the corresponding hydroxy acrylates, i. e., hydroxy ethylacrylate, hydroxy propylacrylate, hydroxy ethylhexyl methacrylate, glycol acrylates, i. e., ethylene glycol dimethacrylate, hexamethylene glycol dimethacrylate, the allyl acrylates, i. e., allyl methacrylate, diallyl methacrylate, the epoxy acrylates, i. e., glycidyl methacrylate, and the aminoplast acrylates, i. e., melamine acrylate. Others such as vinyl acetate, vinyl and vinylidene halides and amides, i. e., methacrylamide, acrylamide, diacetone acrylamide, vinyl and vinylidene esters, vinyl and vinylidene ethers, vinyl and vinylidene ketones, butadiene, vinyl aromatics, i. e., styrene, alkyl styrenes, halostyrenes, alkoxystyrenes, divinyl benzenes, vinyl toluene, and the like are also included. Prepolymers include acrylated epoxides, polyesters and polyurethanes, and are typically combine with a suitable monomer for viscosity control.

The photopolymerizable compound may be polymerized to form homopolymers or copolymerized with various other monomers.

The photopolymerizable compound can be present in the compositions of the invention in amounts from about 0 to about 99.8, preferably about 80 to about 99.8, percent by weight, based on the total weight of the composition.

The maleimides can be used singly or as mixtures thereof, and are useful as photopolymerization initiators. In this aspect of the invention, the maleimide compound can be present in the photopolymerizable composition in an amount sufficient to initiate polymerization thereof upon exposure to radiation. The composition can include about 0.01 to about 100, preferably about 0.01 to about 20, more preferably about 0.01 to about 10, percent by weight maleimide compound, based on the total weight of the photopolymerizable compound. The inventors have found that only relatively low concentrations of the maleimide are required for desirable results. For example, the maleimide can be present in amounts of about 0.01 mole percent to about 2 mole percent, and even less than about 0.1 mole percent.

The benzophenone compound can be benzophenone or any derivative thereof, including thioxanthone and derivatives thereof, and mixtures of such compound, having photoactive activity. Such compound are known in the art and include compound of the following structure:

wherein: B is (H, H),-CH2-,-S-,-0-,-CO-,-NR15-, or a bond bridging the two aromatic rings; each R14 is independently selected from the group consisting of hydrogen, alkyl, cycloalkyl, alkoxy, aryl, alkylaryl, arylalkyl, halogen, trihaloalkyl,- CN,CN,-NO2, -OR15,-N(R15)2,-OC(O)CR15=CHR15,R16,--C(O)R15, OR16,-R17-OC (O) CR15=CHR15, polymerizable moities, and oligomeric and polymeric moities; R15 is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, arylalkyl, and alkylaryl; R16 is one or more saturated or unsaturated five or six membered hydrocarbon or heterocyclic ring system, optionally substituted with one or more alkyl, cycloalkyl, or halogen; and R17 is selected from the group consisting of alkyl, aryl, cycloalkyl, arylalkyl, and alkylaryl.

The term polymerizable moiety refers to ethylenically unsaturated moities known in the art which are capable of rection with another compound (for example, by a free radical mechanism), such as but not limited to, acrylate and methacrylate moities. For example, Rl4 can have the structure- (CH2)"-OC (O)- CRH=CH2 wherein n is an integer from 1 to 10 and R is H or lower alkyl. Another exemplary polymerizable entity is a maleimide moiety as described above.

The terms oligomeric and polymeric moities refer to a moiety including two or more monomer units (dimer, trimer, etc.), typically having a molecular weight of at least about 500, and higher. Thus the benzophenone compound can be an oligomer or polymer having one or more benzophenone functionalities incorporated therein (along the backbone and/or side chain).

Exemplary oligomeric and polymeric moities include but are not limited to a C2-C20 alkylene or polyalkylene polyol, wherein hydroxy groups of the polyol moiety are optionally alkylated, and preferably an alkylene or polyalkylene polyol derived from ethylene glycol. Other suitable oligomer or polymer moities include C2-C20 alkylene or polyalkylene moities end capped with trihaloalkyl, and optionally substituted with one or more halogen atoms along the chain, preferably fluorine.

Other exemplary oligomeric or polymer moities include C2-C20 alkylene or polyalkylene moities including carbonate groups and end capped with lower alkyl.

Other exemplary oligomeric or polymeric moities include polyether ketones and polyacrylate and polymethacrylate based compound. Such oligomeric and polymeric benzophenones are known in the art and are commercially available.

Exemplary benzophenone compound useful in the invention include without limitation benzophenone, thioxanthone, isopropylthioxanthone, chlorothioxanthone, methyl o-benzoylbenzoate, 4-morpholinobenzophenone, 4,4'- diphenoxybenzophenone, 1-methyl-2- (2-ethylhexyloxy) thioxanthone, 4,4'-di- (4- isopropylphenoxy) benzophenone, acrylic acid 4-benzoylphenyl ester, 4,4'- diphenylbenzophenone, 4-phenylbenzophenone, and the like, as well as mixtures thereof. These and other benzophenone compound are in known in the art and are commercially available or can be produced using commercially available starting materials. The benzophenone photoinitiator can be present in an amount ranging from about 0.001 to about 5 percent by weight, based on the total weight of the composition.

The sensitizer package also inclues at least one coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor or an electron donor. Coinitiators or synergists are known in the art, and are typically alcools, tertiary amines or ethers which have available hydrogens attache to a carbon adjacent to a heteroatom. The hydrogen atom donor can be present as a separate compound or can be included as a molecular component of the photopolymerizable

compound (for example, polyethylene glycol diacrylate). Such coinitiators when a separate component are typically present in an amount between about 0.1 and about 5 percent by weight based on the total weight of the composition. When present as a molecular component of a photopolymerizable compound, the hydrogen atom donor component can be up to about 85 weight percent, and higher, of the photopolymerizable compound. Suitable compound inclue, but are not limited to, triethanolamine, methyl-diethanolamine, ethyldiethanolamine and esters of dimethylamino benzoic acid. Other known coinitiators or accelerators can also be used.

The photopolymerizable compositions of the invention may also contain other conventional agents, such as polymerization inhibitors, filles, ultraviolet absorbers, organic peroxides, dyes, pigments, and the like.

The photopolymerizable compositions can be applied or deposited on a surface of a substrate using conventional techniques and apparats. The composition can be applied as a substantially continuous film. Alternatively, the composition can be applied in a discontinuous pattern. The thickness of the deposited composition can vary, depending upon the desired thickness of the resultant cured product.

Typically, the substrate is coated with the uncured photopolymerizable composition and passed under a commercially available UV or excimer lamp on a conveyer moving at predetermined speeds. The substrate to be coated can be, for example, metal, wood, mineral, glass, paper, plastic, fabric, ceramic, and the like.

The active energy beams used in accordance with the present invention may be visible light or ultraviolet light or may contain in their spectra both visible and ultraviolet light. The polymerization may be activated by irradiating the composition with ultraviolet light using any of the techniques known in the art for providing ultraviolet radiation, i. e., in the range of 200 nm and 450 nm ultraviolet radiation, and especially with the 308 mu emission from xenon chloride exciter lamps, commercially available from Fusion Systems, or by irradiating the composition with radiation outside of the ultraviolet spectrum. The radiation may be natural or artificial, monochromatic or polychromatic, incoherent or coherent and should be sufficiently intense to activate the photoinitiators of the invention and thus the polymerization.

Conventional radiation sources include fluorescent lamps, excimer lamps, mercury, metal additive and arc lamps. Coherent light sources include pulsed nitrogen, xenon,

argon ion-and ionized neon lasers whose emissions fall within or overlap the ultraviolet or visible absorption bands of the compound of the invention.

The compositions are useful in any of the types of applications known in the art for photopolymerizations, including as a binder for solids to yield a cured product in the nature of a paint, varnish, enamel, laquer, stain or ink. The compositions can also be useful in the production of photopolymerizable surface coatings in printing processes, such as lithographic printing, screen printing, and the like. The compositions can also be useful in applications in which the compositions are applied to articles which are to be exposed to the environment, such as signage.

Radiation cured coatings produced using conventional photoinitiators typically degrade over time (as evidenced by yellowing, increasing brittleness, and the like), which degradation is exacerbated by direct exposure to sunlight. In contrast, radiation cured coatings prepared using the maleimide compound can exhibit minimal degradation over time, even when exposed to direct sunlight. The maleimides can also be water soluble.

The present invention will be further illustrated by the following non- limiting examples.

EXAMPLE 1 Synthesis of HvdroxY Methylmaleimide (HMMI ! Maleimide (10 g, 0.103 mol) was added to 10 mL of a 37% solution of formaldehyde and 0.31 mL of a 5% solution of NaOH was added. Within 10 minutes all of the maleimide had dissolve and an exothermic rection proceeded. The solution was stirred for 2 hours where white crystals were observe. The solution was placed in a freezer overnight and the resulting crystals filtered and washed with ice cold ethanol and diethyl ether. The white crystals were purifie twice by sublimation.

See P. O. Tawney, R. H. Snyder, R. P. Conger, K. A. Leibbrand, C. H. Stiteler, and A.

R. Williams J. Org. Chem. 26,15 (1961). m. p. 104-106°C (9.77 g, 74. 6%). 1H-NMR (Acetone-db, 8, ppm): 4.96 (2H,-CH2-, s), 5.33 (1H,-OH, s), 6.93 (2H,-CH=CH-, s).

13C-NMR (Acetone-d6,5, ppm): 60.9 (1C, -CH2-), 135. 6 (2C,-CH=CH-), 173.1 (1C,- C=O).

EXAMPLE 2 Synthesis of Hv Ethmaleimide (HEMI Ethanolamine (80.96 g, 1.32 mol) was added to 500 mL of ethanol and cooled to 0°C using an ice bath. 3,6-Endoxo-1,2,3,6-tetrahydrophthalic anhydride (220.21 g, 1.32 mol) was added to the solution and allowed to stir overnight. The yellow tinted crystals were used without purification. The solution was refluxed for four hours with azeotropic removal of water. The solution was cooled to 0°C and the resulting crystals filtered (151.74 g, 54.95 %). Removal of furan was facilitated by refluxing the crystals in xylenes for 4 hours with quantitative yield of hydroxy ethylmaleimide after purification by sublimation to yield white crystals,-CH20-), 134.2 (2C,-CH=CH-), 171.2 (2C,-NC=O m. p. 68°C. 1H-NMR (CDCl3, #, ppm) : 2.62 (1H,-OH, s), 3.82-3.77 (4H,-NCH2CH20-, overlapping), 6.76 (2H,-CH=CH-, s). I3C-NMR (CDCl3, 6, ppm): 40.5 (1C, -NCH2-), 60.5 (1C,).

EXAMPLE 3 Triethylene Glycol Biscarbonate Bisethylmaleimide TEGBCBEMI) HEMI (25.65 g, 0.182 moles) and pyridine (14.38 g, 0.182 moles) were dissolve in THF (130 mL) and the solution was stirred at room temperature.

Triethylene glycol bischloroformate (25.0 g, 0.091 moles) was added dropwise and stirred for 90 minutes. The pyridine salt was filtered off and the solution was combine with 200 mL of a 1N HCl solution. The product was extracted with methylene chloride and washed with a 1N HCl solution followed by water and then dried over magnesium sulfate. The red solution was diluted to a volume of 150 mL and purifie by column chromatography (2.5 cm x 21 cm) using silica gel as the packing and methylene chloride as the mobile phase yielding white crystals, m. p.

65°C (26.75 g, 60. 76%). 1H-NMR (CDCl3), #, ppm): 3.64-3.68 (4H #-OCH2-, t), 3.69-3.74 (4H, #-OCH2-, t), 3.81-3.86 (4H,-NCH2-, t), 4.26-4.3 (8H, -CH2O (C=O) OCH2-, t), 6.75 (4H,-CH=CH-, s). I3C-NMR (CDCl3, 6, ppm): 36.6 (2C,-NCH2-), 64.7 (2C), 67.3 (2C), 68.8 (2C), 70.6 (2C), 134.4 (4C,-CH=CH-), 154.8 (2C, O (C=O)O), 170.4 (4C,-NC=O).

EXAMPLE 4 Svnthesis of 2-Ethvlcarbonate Ethylmaleimide (2ECEMI) Hydroxy ethylmaleimide (29.87 g, 0.212 moles) and pyridine (16.7 g, 0.212 moles) were dissolve in THF (170 mL) and the solution was stirred at room temperature. Ethyl chloroformate (22.97 g, 0.212 moles) was added dropwise and stirred for 90 minutes. The pyridine salt was filtered off and the solution was combine with 200 mL of a 1N HCl solution. The product was extracted with methylene chloride and washed with a 1N HCl solution followed by water and then dried over magnesium sulfate. The red solution was concentrated and the red crystals purifie by sublimation yielding white crystals, m. p. 52°C (34.76 g, 77. 04%). 1H- NMR (CDCl3), 8, ppm): 1.26-1.34 (3H,-CH3, t), 3.81-3.87 (2H-NCH2-, t), 4.14-4.25 (2H, 8-CH20C=O, q), 4.25-4.30 (2H, ß-CH2O-, t), 6.74 (2H,-CH=CH-, s). 13C-NMR (CDC13, 8, ppm): 14.2 (1C, -CH3), 36. 8 (1C, -NCH2-), 64.3 (1C, #-CH2O), 64.5 (1C,/3 -CH2-), 134.4 (2C,-CH=CH-), 154.9 (1C, O (C=O) O), 170.4 (2C,-C=O).

EXAMPLE 5 Svnthesis of 2-Isoprop ; rl Urethane Ethylmaleimide (21PUEM) Hydroxy ethylmaleimide (5 g, 35.4 mmol) was dissolve in 75 mL of methylene chloride 1 drop of dibutyl tin dilaurate catalyst was added. Isopropyl isocyanate (3.01 g, 35.4 mmol) was added dropwise and the solution was stirred for 3 hours. The solution was washed with water dried with magnesium sulfate.

Concentration yielded white crystals which were further purifie sublimation yielding white crystals, m. p. 117°C (6.49 g, 81%). 1H-NMR (CDCl3, #, ppm) : 1.11-1.14 (6H, -C (CH3) 2, d), 3.74-3.79 (2H,-NCH2-, t), 4.28-4.32 (2H,-CH20-, t), 4.44-4.53 (1H, - CH-, p), 6.72 (2H,-OC-CH=CH-CO-, s). NMR 13C-NMR (CDCl3, 8, ppm): 170.5 (2C, C=O, maleimide), 155.1 (1C, C=O, urethane), 134.2 (2C,-CH=CH-), 61.7 1C, ß-CH2, 43.6 (1C,-CH-), 37.4 (1C, a-CH2), 22.9 (2C,-CH3).

EXAMPLE 6 Synthesis of 2-Acryloyl Ethylmaleimide (2AEMI ! 2-Hydroxyethyl maleimide (5 g, 35.4 mmol) and Et3N (4.25g, 43.0 mmol) was dissolve in 75 mL of methylene chloride and cooled to 0°C. Acryloyl chloride

(3.20 g, 35.4 mmol) in 25 mL of methylene chloride was added dropwise over 30 minutes. The solution was stirred at room temperature for 30 minutes followed by refluxing for 1 hour. The triethylamine hydrochloride was removed by filtration and the yellow solution was concentrated. The yellow crystals were purifie by sublimation yielding white crystals, m. p. 77-78°C (5.00 g, 72. 3%). 1H-NMR (CDCl3, 8, ppm): 3.81-3.87 (2H,-NCH2-, t) 4.22-4.33 (2H,-OCH2-, t), 5.81-5.85 (2H, CH2=CH, cis), 6.00-6.13 (1H, CH2=CH-, q), 6.34-6.42 (1H CH2=CH, trans), 6.73 (2H,-OC-CH=CH-CO-, s). 13C-NMR (CDCl3, #, ppm) : 170.4 (2C, C=O, maleimide), 165.8 (1C, C=O, ester), 134.2 (2C,-CH=CH-), 131.45 (1C,-CH=), 127.9 (1C, CH2=), 61.5 (1C, ß-CH2-), 36. 8 (1C, oc-CH2-).

EXAMPLE 7 Svnthesis of Acetoxy Ethyl Maleimide (AcOEMI ! Maleic anhydride (172.32 g, 1.75 mol) was added to ethanolamine (107.34 g, 1.75 mol) and dissolve in 500 mL of acetone while stirring overnight in an ice bath. To the solution, 400 mL of acetic anhydride (4.23 mol) was added with sodium acetate (144 g, 1.75 mol). The solution was heated to 80°C and stirred for lhour. The contents were poured into ice water and the acetic acid neutralized with K2CO3. The product was extracted with methylene chloride and then dried using magnesium sulfate. The product was purifie by sublimation yielding white crystals, m. p. 76°C (44.1 g, 13. 45%). 1H-NMR (CDCl3, #, ppm): 2.02 (3H,-CH3, s), 3.82-3.77 (2H,-NCH2-, t), 4.25-4.20 (2H,-CH20-, t), 6.75 (2H,-CH=CH-, s). 13C-NMR (CDC13, 6, ppm): 20.7 (1C,-CH3), 37.0 (1C, -NCH2-), 61.5 (1C, -CH2O-), 134.3 (2C, -CH=CH-), 170.5 (2C,-NC=O), 170.8 (1C,-OC=O).

EXAMPLE 8 Synthesis of 4,9-Dioxa-1,12 Dodecane Bismaleimide (4*9-DO-1,12-DDBMI) 4,9-Dioxa-1,12-dodecane diamine (25 g, 0.122 mol) was dissolve in acetone and added dropwise to a solution of maleic anhydride (24 g 0.244 mol) in 120 mL of acetone under cooling using an ice bath in a nitrogen atmosphere. The solution was stirred overnight and the contents were then poured into water and the bismaleamic acid filtered and waslled with ethanol and diethylether.

The bismaleamic acid (29.96 g, 74.8 mmol) was dissolve in an acetone 120 mL and triethylamine (41.7 mL, 0.300 mol) solution. The solution was heated to reflux, where acetic anhydride (21.2 mL, 0.224 mol) was added dropwise, the solution was refluxed for 12 hours. The solution was added to ice water and the precipitate filtered and dried. The sample was purifie by column chromatography using silica gel as the adsorbent and methylene chloride as the mobile phase yielding white crystals, m. p. 65°C (5.5 g, 20.2 %). 1H-NMR (CDC13,8, PPM): 1.60-1.54 (2H, E-CH2-), 1.91-1.78 (2H, ß-CH2-, 3.44-3.28 (4H,-CH20CH2-), 3.66-3.59 (2H,- NCH2-, t), 6.70 (2H,-CH=CH-, s). 13C-NMR (CDCl3, #, ppm): 26.4 (2C, g-CH2-), 28.6 (2C, ß-CH2-), 35.6 (2C,-NCH2-), 68.2,70.7 (4C,-CH20CH2-), 134.2 (2C,- CH=CH-), 170.8 (2C,-NC=O).

EXAMPLE 9 Synthesis of Isophorone bisurethane biselhylmaleimide (IPBUBEMI) Hydroxy ethylmaleimide (10 g, 0.141 mol) was dissolve in acetone and purged with nitrogen while stirring in an ice bath. One drop of dibutyl tin dilaurate was added to the solution. The isophorone diisocyanate (30.5 g, 0.141 mol) was added dropwise over 2-3 hours. The solution was allowed to stir overnight and a white precipitate was obtained after solvent removal m. p. 105-112°C (40.5 g, 100%).

1H-NMR (D6-DMSO, 8, ppm): 3.63-3.58 (2H,-NCH2-), 4.08-4.03 (2H,-CH20-, t), 7.03 (2H,-CH=CH-, s). 13C-NMR (D6-DMSO, 8, ppm): 134.5 (2C,-CH=CH-), 154.9 (2C, NHC=O), (170.7 (2C,-NC=O).

EXAMPLE 10 Svnthesis of Bispropyl Maleimide (PBMI) Diaminopropane was dissolve in 100 mL of dimethyl acetamide (DMAC) and added dropwise to a solution of maleic anhydride in DMAC under cooling using an ice bath in a nitrogen atmosphere. The solution was stirred overnight and the contents were then poured into water and the bismaleamic acid filtered and washed with ethanol and diethylether.

The bismaleamic acid (84.54 g, 0.312 mol) was dissolve in 312 mL of an acetone and triethylamine (87 mL, 0.624 mol) solution. The solution was heated to reflux and acetic anhydride (88 mL, 0.61 mol) was added dropwise through a reflux condenser and the solution was refluxed for 12 hours. The solution was added to ice

water and the precipitate filtered and dried. The sample was purifie by column chromatography using silica gel as the adsorbent and methylene chloride as the mobile phase yielding white crystals, m. p. 166°C (19.28 g, 26. 3%). 1H-NMR (CDCl3, 8, ppm): 2.00-1.86 (2H,-CH2-, p), 3.57-3.50 (4H,-NCH2-, t), 6.71 (2H,-CH=CH-, s).

13C-NMR (CDCl3, #, ppm): 27.3 (1C, -CH2-), 35. 3 (2C,-NCH2-), 134.2 (2C,- CH=CH-), 170.6 (2C,-NC=O).

EXAMPLE 11 Synthesis of Bisethylmaleimide Carbonate (BEMIC) Hydroxy ethylmaleimide (10.0 g, 70.1 mmol) was dissolve in a solution of methylene chloride (71 mL) and triethylamine (9.87 mL, 70.8 mmol) and cooled with an ice bath to 0°C. Triphosgene (3.504 g, 12 mmol) was added over a period of 4 hours and the resulting solution filtered. The supernatant was washed with 1N hydrochloric acid, 5% potassium carbonate solution and water. The solution was dried over magnesium sulfate and purifie using activated carbon to yield white crystals (4.0 g, 18. 5%). 1H-NMR (CDCl3, #, ppm): 3.82 (4H,-NCH2-, t), 4.27 (2H,- CH2O-, t), 6.73 (2H,-CH=CH-, s). 13C-NMR (CDCl3, #, ppm): 36.6 (2C,-NCH2-), 64.9 (2C,-CHqO-), 134.2 (2C,-CH=CH-), 154.5 (1C, C=O) 170.8 (2C,-NC=O).

EXAMPLE 12 Synthesis of N- (2-CF-phenYl) maleimide 32.5 g of maleic anhydride is placed in 700 mL diethylether and allowed to dissolve. 41.48 mL of 2-trifluoromethyl aniline is added squirtwise to the stirring solution. The rection is allowed to proceed at room temperature overnight and a pale white precipitate is observe. The stirring solution is then warmed for several hours and filtered, yielding a pale white solid. The mother liquor is then combine with the wash liquor and allowed to stir again, and additional product is filtered off. The solids are then combine and dried in vacuum. Typical yields are 90-98%.

44.8 g of the maleamic acid is then placed in a three necked flask with 100 mL toluene and 25 mL DMSO, and allowed to dissolve. 1.8 mL of concentrated sulfuric acid is then added to the stirring mixture, which is then heated to about 130° C. The rection mixture is allowed to reflux for 4 hours or until an azeotrope is no

longer observe. Excess toluene is then removed via vacuum distillation. The mixture is then added to stirring distille water to precipitate the imide and remove excess DMSO. The aqueous suspension is allowed to stir overnight and is then filtered. The pale white solid is then dried under vacuum to remove water, DMSO and toluene. Yields are typically 90-97%.

Products from both steps are analyzed using IH and 13C NMR in d6- DMSO. The maleamic acid is characterized by a broad peak due to the acid proton near 13-14 ppm, a peak due to the amide proton near 10 ppm, and two doublets due to the ene protons near 6.4 ppm in the proton spectra. The maleamic acid is also characterized by two peaks near 165 ppm due to the two carbonyls in the carbon spectrum. The maleimide is characterized by the absence of the acid and amide proton peaks, and the single ene proton peak shifted to near 7.6 ppm in the proton spectrum. In the carbon spectrum the maleimide shows a single carbonyl peak near 170 ppm.

The melting point of N- (2-CF3-phenyl) maleimide is measured using DSC at a heating rate of 20°C/min and found to be 115.91C (endotherm peak maximum). Heat of fusion is also calculated to be 71.62 J/g.

EXAMPLE 13 Svnthesis of N- (2-t-butphenvl) maleimide N- (2-t-butylphenyl) maleimide is also prepared as described in Example 12, except substituting 2-trifluoromethyl aniline with 2-tert-butyl aniline.

The melting point of N- (2-t-butylphenyl) maleimide is measured using DSC at a heating rate of 20°C/min and is found to be 99.64C (endotherm peak maxima). Heat of fusion is also calculated to be 94.02 J/g.

EXAMPLE 14 Synthesis of N- (2-CF3-phenvl) methvlmaleimide N-(2-CF3-phenyl) methylmaleimide is also prepared as described in Example 12, except substituting citraconic anhydride for maleic anhydride.

EXAMPLE 15 Synthesis of N-(2-iodo-phenyl ! maleimide N- (2-iodo-phenyl) maleimide is prepared as described in Example 12, except substituting 2-trifluoromethyl aniline with 2-iodoaniline.

EXAMPLE 16 Synthesis of N- (2-bromo-3, 5-CF3-phenvl) maleimide N- (2-bromo-3,5-CF3-phenyl) maleimide is prepared as described in Example 12, except substituting 2-trifluoromethyl aniline with 2-bromo-3,5- trifluoromethyl aniline.

EXAMPLE 17 Photopolymerization Using Benzophenone/Amine/Maleimide Formulation Figure 1 is a graph illustrating photo-DSC exotherm rate results for the photopolymerization of a typical UV curable monomer (HDDA is 1,6-hexanediol diacrylate) with photoinitiator pacages of benzophenone (BZPN)/1% methyl diethanol amine (NMDEA) and N-methyl maleimide (MMI)/1% NMDEA. For the results in Figure 1, the concentrations were adjusted to give comparable initiator absorbances. The benzophenone/NMDEA system generates a higher exotherm peak rate and a shorter time is required to attain the peak maximum as compare to the MMI/NMDEA system.

Similar results are illustrated in Figure 2 for the photopolymerization of polyethylene glycol (400) diacrylate (PEG 400 DA) which has abstractable hydrogens between the acrylate functionalities. While it is possible to achieve somewhat faster rates by using maleimides with groups that have abstractable hydrogens (such as heteroatom containing groups), the rates achieved are still quite small compare to conventional photoinitiator systems.

Figure 3 gives photo-DSC exotherm results in nitrogen saturated systems that incorporate three components (benzophenone sensitizer, methyl maleimide, and NMDEA) as the photoinitiator package for the polymerization of HDDA. The exotherm rate maximum increased dramatically when both methyl maleimide and benzophenone are present as compare to the exotherm rates for

systems in which methyl maleimide/NMDEA are present or in which benzophenone/NMDEA are used and maleimide has been excluded. In other words, the synergistic effect on polymerization rate of having the N-alkyl maleimide présent is dramatic, resulting in a substantially faster overall curing process under the conditions employed. The results in Figure 3 were attained for systems in which the samples were exposed to the unfiltered 30 mW output of a medium pressure mercury lamp similar in spectral output (but lower in overall intensity) to the lamps used in the UV curing industry.

To extend the results shown in Figure 3, exotherms were recorde for the photopolymerization of HDDA initiated by exposing benzophenone/N-alkyl maleimide/NMDEA and benzophenone/N-aryl maleimide/NMDEA systems to the output of a filtered (365-nu mercury line filter) source (Figures 4 and 5). When either an N-alkyl or an N-aryl maleimide was used in combination with the benzophenone sensitizer, a dramatic increase in the polymerization rate was observe as compare to results using the benzophenone/NMDEA system alone. Results for maleic anhydride (Figures 4 and 5) and Irgacure-651 (BDK) (Figure 4) are shown for comparison. Figures 4 and 5 indicate that N-alkyl maleimides of widely varying structures (methyl maleimide MMI, hexyl maleimide HMI, and cyclohexyl maleimide CHMI) as well as both ortho-substituted (i. e., twisted) (tert-butyl maleimide NtBMI) N-aryl maleimides and simple substituted or unsubstituted (i. e., planar) (phenyl maleimide N-PMI) N-aryl maleimides (heretofore found to be ineffective upon direct excitation), can be sensitized to have much greater initiation efficiencies than benzophenone/NMDEA systems.

Although not wishing to be bound by any explanation of the invention, it is currently believed that energy transfer occurs from the exited state triplet of the benzophenone to the maleimide, which then abstracts a hydrogen atom and initiates the polymerization process.

EXAMPLE 18 Photopolymerization Using Thioxanthone/Amine/Maleimide Formulations Additional studies examine the use of benzophenone derivatives, and in particular thioxanthone derivatives, to sensitize initiation by maleimides. Three types of experiments were performed using N-methyl maleimide and a variety of thioxanthone photoinitiators. Specific thioxanthones (available from First Chemical Corporation) include isopropyl thioxanthone (ITX), LTX having the formula

4-morpholinobenzophenone (XPI-115) having the formula 4,4'-diphenoxybenzophenone (XPI-113) having the formula

and XPI-133 having the formula First, formulations with various photoinitiators were prepared containing HDDA and NMDEA, with and without MMI. These formulations were then tested by photo-differential scanning calorimetry (photo-DSC), using a Xenon- doped mercury lamp at 25°C using (1) a 313 nm band pass filter, (2) a 365 nm band pass filter, and (3) the full arc of the lamp at different intensities.

313 nm Band Pass Filter (BPFLperiments At 313 nm, both the maleimide and the photoinitiator absorb photons, with the relative absorbance depending of the ratio of the respective molar extinction coefficients. Formulations were prepared to have equal absorbance of different benzophenone derivatives at 313 nm. The experiments were performed in nitrogen, with a filtered light intensity of about 1 mW/cm2.

The data in Table 1 show that when the maleimide is used in conjunction with ITX and XPI-115 at these concentrations and under these conditions, the average peak exotherm maximum (polymerization rate) is enhanced. Also, enhanced relative conversions are obtained when the maleimide is used in conjunction with ITX, XPI-113, XPI-115, and XPI-133.

Table 1

Data from 313 nm BPF Photo-DSC Experiments Using HDDA Formulation Average Average (mol% of Exotherm Exotherm Photoinitiator compound) Peak Maximum Integration 0.080% ITX 0.86% 28.32 554.35 ITX MDEA 0.080% ITX 68.32 579.79 1.2% MDEA 0.66% MMI 0.0113% LTX 21.3 481.93 LTX 1.07% MDEA 0.0113% LTX 12.8 470.1 0.974% MDEA 0.608% MMI 0.013% XPI-113 20.69 469.38 XPI-113 109% MDEA 0.012% XPI-113 15.99 504.39 1.09% MDEA 0.56% MMI 0.0025% XPI-115 24.42 481.9 XPI-115 1.18% MDEA 0.0025% XPI-I 15 29.59 489.51 1.07% MDEA 063% MMI 0.0056% XPI-133 16.17 435.63 XPI-133 1.02% MDEA 0.0055% XPI-133 10.63 470.66 1.03% MDEA 0.607% MMI 365 nm BPF Experiments A second set of photo-DSC experiments were performed which were similar to those reporte above except that a 365 mu band pass filter was used in place of the 313 nm band pass filter. At 365 nm, the principle absorbing species in these formulations should be the photoinitiator. Exotherm data from these experiments are shown below in Table 2.

Table 2 Data from 365 nm BPF Photo-DSC Experiments Using HDDA Formulation Average Average Photoinitiator (mol% Exotherm Exotherm of compound) Peak Maximum Integration (W/g) (J/g) 0.090% ITX 18.13 447.69 ITX 0.942% MDEA 0.090% ITX 63.45 521.39 1.0%MDEA 0.62% MMI 0.013% LTX 23.77 508.91 LTX 1.07% MDEA 0.0113% LTX 42.96 509.14 0.974% MDEA 0.608% MMI

At these concentrations and under these conditions, there is dramatic enhancement of the maximum rates of polymerization when the maleimide is used in conjunction with ITX as well as with LTX. There is also enhancement of the relative conversion when the maleimide is included in the formulations. Absolut comparisons between ITX and LTX should not be made using this data, as the formulations were not prepared with equal photoinitiator absorption at 365 nm.

Full Arc Experiments Two formulations of HDDA with ITX, MDEA, with and without MMI were also tested using the full arc of the lamp at three different intensities, in nitrogen and in air. Peak exotherm maxima are listed below in Table 3. In nitrogen, at low intensity (30 mW/cm2), the formulation with the maleimide is approximately twice as efficient as that without. As the intensity increases to 100 and 300 mW/cm2, the ratio between with maleimide and without decreases, while the absolut peak exotherm values continue to increase, as expected. In air, at 30 mW/cm2, there is little difference between the exotherms of the two formulations. However, as the intensity increases to 100 and 300 mW/cm2, the ratio (with MMI/without MMI) goes from 1.06 to 1.12 to 1.34, and the exotherm peak values get larger.

Table 3 Data from full arc photo-DSC experiments with HDDA Light Exotherm Peak Formulation Intensity Maximum (mol% of (W/g) compound) NitrogenAir 0.090% ITX 30 50 39 0.942% MDEA 100 73 64 300 97 77 0.090% ITX 30-41 1.0% MDEA 0.61% 100 97 80 MMI 300 129 104

Short Duration Irradiation Photo-DSC Experiments To investigate the performance of sensitized maleimide-acrylic formulations in air a third battery of experiments was performed using formulations of HDDA, MDEA, and ITX, and with and without methyl maleimide. These photo-DSC experiments were performed at 25°C in nitrogen and in air with 1 second irradiation times using a shuttered xenon-doped mercury light source.

The experiments were conducted in the absence of a filter, in nitrogen and in air, at light intensities from 30 to 750 mW/cm2. Three formulations were tested containing HDDA, MDEA, and ITX, with and without MMI. Representative exotherm peak maxima and exotherm integration values are shown below in Table 4.

Table 4 Data from 1-Second Exposure Photo-DSC Experiments with HDDA and ITX Formulations (mol% of compound) 0.090% ITX 0.090% ITX 0.0090% ITX 0.942% MDEA 1.0% MDEA MDEA LightLight0.62% MMI 0.0806% MMI Intensity Peak Peak Peak Peak Peak Peak IntegralMaximuIntegralMaximuIntegral(J/(mW/cm2)Maximu m (J/g)mg)m (W/g) (W/g) (W/g) Values in Nitrogen 30 10.25 55.30 31.63 155. 97 58.57 272.24 100 25.61 108.70 77.16 400.42 104.95 477.98 300 51.06 216.73 111.13 531.72 122.04 524.88 500 66.52 317.29 117.97 545.81-- 750 # 93. 68 i 414.60 i 135.31 i 564.38 # -# -# Values in Air 30 9.29 36.49 2. 21# 9.34# 14.54# 55.71 100 23.26 91.58 30.90 132.60 65.20 281.53 300 38.91 170.58 100.60 429.47 111.87 449.55 500 57.56 242.04 115.17 475.92-- 750--124. 72 510.68--

From this data, it can be clearly observe that in nitrogen, at all intensities and at both concentrations of MMI, the formulations containing maleimide perform much better than the HDDA formulation with only ITX and amine.

Another useful comparison is that between the sensitized maleimide/ITX/amine initiating system and a typical conventional alpha-cleavage type photoinitiator. A short set of experiments was performed using one of the sensitized maleimide containing formulations from above and an HDDA formulation which contained 2,2-dimethoxy-2-phenylacetophenone (known commercially as BDK and

Irgacure 651). These photo-DSC runs were conducted at 25°C in nitrogen, at 100 mW/cm2 on-sample light intensity, with a one second exposure time. Photo-DSC exotherm values are given below in Table 5. The sensitized maleimide-containing formulation gave a peak maximum on average about 69% of the peak maximum obtained with the Irgacure 651 formulation. The relative double bond conversion for the sensitized maleimide initiated formulations was about 79% of that obtained when the conventional alpha-cleavage photoinitiator was used. It is interesting to note that there is 0.97 weight percent initiator in the case of the Irgacure 651/HDDA formulation, but only 0.59 weight percent of initiating system in the maleimide/ITX/MDEA/HDDA formulation.

Table 5 Photo-DSC Exotherm Values Comparing Maleimide/ITX/MDEA to IC651 in HDDA Formulation Average Exotherm Peak Average Exotherm Peak (mol% of compound) Maximum (W/g) Integration (J/g) 0.0090% ITX 108.37 489.36 1.01% MDEA 0.0806%MMI 0.86% Irgacure 651 156.70 622. 82 Sensitization of Aromatic Maleimides Experiments were also performed to determine if initiation of acrylic systems via sensitized aromatic maleimides is possible. Formulations were prepared using HDDA, ITX, and MDEA with several aromatic maleimides. The aromatic maleimides used in these experiments are N-phenylmaleimide (PMI), N- (2- trifluoromethylphenyl) maleimide (2CF3PMI), N- (2-t-butylphenyl) maleimide (2tBPMI), and a-methyl-N-(2-trifluoromethyl phenyl) maleimide (2CF3PCI). Photo-DSC experiments were conducted in nitrogen at 25°C using a medium pressure mercury lamp fitted with a 365 nm band pass filter, with an unfiltered on-ample light intensity of 28 mW/cm2. Exotherm peak maxima are listed below in Table 6. The data demonstrates that aromatic maleimides can be"sensitized" (again, though sensitization has not been proven mechanistically) using ITX in acrylic systems. Also very significant is that the aromatic maleimides are not required to be"twisted" (as in the initiation of acrylic systems by direct excitation of the maleimide), as N-phenyl maleimide appears to have the same synergistic effect.

Table 6 Photo-DSC Peak Maxima for ITX/MDEA/Arylmaleimides in HDDA 0.040% 0.040% 0.040% 0.040% 0. 040% Formulation ITX ITX ITX ITX ITX 1.04% (mol% of 1.00% 0.976% 0.996% 1.03% MDEA compound) MDEA MDEA MDEA MDEA 0.325% 0.309% 0.311% 0.313% 2CF3PCI PMI 2CF3PMI 2tBPMI Exotherm Peak 12 34 40 42 46 Maximum )

In summary, the data reporte herein show that a dramatic synergistic effect occurs when N-aliphatic and N-aromatic maleimides are used in conjunction with benzophenone/thioxanthone photoinitiators in the presence of an amine hydrogen atom- donor in hexanedioldiacrylate. Although not wishing to be bound by any explanation of the invention, this effect likely occurs through excitation of the conventional photoinitiator followed by energy transfer to the maleimide, which then hydrogen-atom abstracts and initiates free-radical polymerization. Both aliphatic and aromatic maleimides appear to exhibait the synergism, and the aromatic maleimides are not required to be"twisted" (the phenyl ring out of plane with respect to the maleimide ring).

Specifically, synergism leading to enhanced rates of cure and enhanced conversions has been observe when N-methylmaleimide is used at low concentrations in conjunction with benzophenone, ITX, LTX, and XPI-115. Enhanced conversions are also noted when MMI is used in conjunction with XPI-113 and XPI-133. Similar results have been observe using N-aromatic maleimides including N-phenylmaleimide, N- (2- trifluoromethylphenyl) maleimide, N- (2-t-butyl phenyl) maleimide. and a-methyl-N-(2- trifluoromethyl phenyl) maleimide. Under conditions designed to begin to simulate industrial curing conditions, sensitized maleimide-containing acrylic formulations were observe to have higher rates of cure and higher relative conversions in air than the same formulations without the maleimide in nitrogen.

The foregoing examples are illustrative of the present invention and are not to be construed as limiting thereof. Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appende claims.

Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.