Background

Novel addition-fragmentation monomers (AFMs) as stress-relieving additives in free-radically cured materials have been disclosed. These addition fragmentation monomers are incorporated into a polymer chain by free radical addition, and fragment to relieve stress, then recombine.

Free-radical polymerization is typically accompanied by a reduction in volume as monomers are converted to polymer. The volumetric shrinkage produces stress in the cured composition, leading to microcracks and deformation. Stress transferred to an interface between the cured composition and a substrate can cause failure in adhesion and can affect the durability of the cured composition.

Methacrylate dimer-based addition-fragmentation monomers provide stress relief by including labile crosslinks that can cleave and reform during the polymerization process. Crosslink cleavage provides a mechanism to allow for network reorganization, relieve polymerization stress, and prevent the development of high stress regions.

Methacrylate dimer-based addition-fragmentation monomers may further provide stress relief by delaying the gel point, the point at which the polymerizable composition transitions from a viscous material to an elastic solid. The longer the polymerizable mixture remains viscous, the more time available during which material flow can act to alleviate stress during the polymerization process.

The addition-fragmentation crosslinking agents have application in dental restoratives, thin films, hardcoats, composites, adhesives, and other uses subject to stress reduction. In addition, the addition-fragmentation process of crosslinking results in a chain-transfer event that provides novel polymers that may be further functionalized.

For example U.S. 2012/0208965 and WO 2012/112350 describe addition- fragmentation agents of the formula:

wherein

R 1 , 2 and R 3 ^J are each independently Z _m -Q-, a (hetero)alkyl group or a (hetero)aryl group with the proviso that at least one of R 1 , R2 and R 3 is Z _m -Q-,

Q is a linking group have a valence of m +1 ;

Z is an ethylenically unsaturated polymerizable group,

m is 1 to 6, preferably 1 to 2;

each X ¹ is independently -O- or -NR ⁴ -, where R ⁴ is H or Q-C4 alkyl, and

n is 0 or 1. These addition-fragmentation agents of Formula I may be added to

polymerizable monomer mixtures to reduce the polymerization-induced stresses

Such addition-fragmentation monomers are prepared from methacrylate dimers and oligomers using cobalt-based catalytic chain-transfer agents and thermal radical initiators. U.S. 4,526,945 (Carlson), discloses a process comprising polymerizing methacrylate monomers, in the presence of an azo initiator and between 0.0001 and 0.01 of Cobalt(II) dimethylglyoxime pyridine or similar Cobalt(II) complexes to produce low molecular weight polymer or copolymer.

Summary

The present disclosure provides a process for the oligomerization of methacrylate esters, by photoinitiated free radical addition in the presence of a cobalt chelate chain transfer agent. The prior art use of thermally initiated oligomerization has safety concerns and the potential for undesired and potentially uncontrolled thermal polymerization. The process overcomes problems in the art of thermally-initiated free radical oligomerization as the photoinitiated oligomerizations are potentially safer and there is greater control over the radical initiation event. The photoinitiated oligomerization disclosed herein has potential safety advantages as it is easier to control or exclude photons than thermal energy. Also, the photoinitiated reaction may provide a continuous-flow process for the oligomerization of methacrylate monomers.

Detailed Description

The methacrylate esters useful in the oligomerization method include methacrylic acid and any functional and nonfunctional methacrylate esters. The methacrylate esters are of the formula:

CH ₂ =C(CH ₃ )-CO-0-R ¹ -Z,

where R ¹ is a covalent bond, an alkylene, arylene or combination thereof; and

Z is H or a functional group, with the proviso that when R ¹ is a covalent bond, then Z is H.

Nonfunctional esters

Nonfunctional alkyl methacrylate ester monomers useful in the invention include straight-chain, cyclic, and branched-chain isomers of alkyl esters containing C - C ₃ o alkyl groups. Useful specific examples of alkyl methacrylate esters include: methyl

methacrylate, ethyl methacrylate, n-propyl methacrylate, 2-butyl methacrylate, iso-amyl methacrylate, n-hexyl methacrylate, n-heptyl methacrylate, isobornyl methacrylate, n-octyl methacrylate, iso-octyl methacrylate, 2-ethylhexyl methacrylate, iso-nonyl methacrylate, decyl methacrylate, undecyl methacrylate, dodecyl methacrylate, tridecyl methacrylate, and tetradecyl methacrylate.

Methacylate monomers may have any functional group Z that does not interfere with the photoinitiated free-radical oligomerization. Such functional groups may include hydroxyl, amino, acetylacetonate, isocyanate, acid, ester, epoxy, aziridinyl, acyl halide, poly( alkylene oxide), silyl, and cyclic anhydride groups.

Useful hydroxyl functional monomers include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 2-hydroxy-2-phenoxypropyl (meth)acrylate, and

hydroxybutyl (meth)acrylate.

Useful silane monomers include, for example, 3-(methacryloyloxy)

propyltrimethoxysilane, 3-(methacryloyloxy)propyltriethoxysilane, 3- (methacryloyloxy)propylmethyldimethoxysilane, 3- (methacryloyloxy)propyldimethylethoxysilane, and 3-(methacryloyloxy)

propyldiethylethoxysilane. Representative epoxy monomers include glycidyl methacrylate, thioglycidyl methacrylate, 3-(2,3- epoxypropoxy)phenyl methacrylate, 2-[4-(2,3- epoxypropoxy)phenyl]-2-(4- methacryloyloxy-phenyl)propane, 4-(2,3- epoxypropoxy)cyclohexyl methacrylate, 2,3-epoxycyclohexyl methacrylate, and 3,4- epoxycyclohexyl methacrylate.

Exemplary amino methacrylates include Ν,Ν-dialkylaminoalkyl methacrylates such as, for exampleN,N-dimethylaminoethylmethacrylateN,N- diethylaminoethylmethacrylate, Ν,Ν-dimethylaminopropylmethacrylate, N-tert- butylaminopropylmethacrylate, N-tert-butylaminopropylacrylate and the like.

Poly(alkylene oxide) monomers having a methacryloyl group and a non- polymerizable terminus may be used. Such monomers may be of the formula:

CH ₂ =C(CH ₃ )-C(0)-0-(CH(R ¹¹ )-CH ₂ -0) _n -R ¹¹ ,

wherein each R ¹¹ is independently H or Q-C4 alkyl and n is 2 to 100.

Anhydride monomers may be cyclic or non-cyclic. Non-cyclic anhydrides include those anhydrides derived from methacrylic acid and an aliphatic,

cycloaliphatic or aromatic carboxylic acid, or functional equivalent thereof such as acyl halides.

Useful aziridine-containing monomer have one or more aziridine groups and at least one methacrylate group, including those described in US 8349962 (Erdogen et al.), US 8263711 (Kavanagh et al.), US 8263711 (Krepski et al.) and US 8329851

(Zhang et al.); incorporated herein by reference. Corresponding monomers having an oxazolinyl group or an oxazolonyl group instead of an aziridine groups, may also be used.

Co catalyst

Cobalt chain transfer catalysts for use in the practice of the present invention include cobalt (II) and cobalt (ΠΙ) chelates as disclosed in US 6388153 (Gridnev), incorporated herein by reference. Examples of such cobalt compounds and their structure are disclosed in Davis et al., J. M. S. -Rev. Macromol. Chem. Phys., C34(l), 243-324 (1994). Additional examples of such cobalt chain transfer catalysts are disclosed in U.S. Pat. No. 4,680,352 (Ittel et al.), U.S. 4,694,054 (Ittel et al.), U.S. 5,324,879 (Hawthorne et al.), WO 87/03605 (Hawthorne et al.), U.S. 5,362,826 (Antonelli et al.), and U.S. 5,264,530 (Antonelli et al.). Useful cobalt compounds include cobalt complexes of porphyrins, phthalocyanines, tetraazoporphyrins, and cobaloximes. Examples of cobalt (II) and cobalt (ΙΠ) chain transfer catalysts include, but are not limited to, those represented by the followin structures:

wherein R ⁴ is selected from the group consisting of hydrogen and B(R ⁵ ) ₂ where each R ⁵ isindependently selected from the group consisting of unsubstituted and substituted aryl, unsubstituted and substituted CrC ₁₂ alkyl, unsubstituted and substituted CrC ₁₂ alkoxy, unsubstituted and substituted aryloxy, and a halogen; R 2 and R 3 are each independently selected from the group consisting of phenyl, substituted phenyl, methyl, ethyl, and - (CH ₂ ) ₄ -; and Lig is selected from the group consisting of water, an amine, a pyridine, an ammonia, a phosphine and combinations thereof, and R ⁶ is an organic radical, preferably an alkyl group. It will be appreciated that the R ⁶ group may derived from the methacrylate monomer as shown in Scheme 1 below. In some embodiments R ⁴ is BF ₂ . In other preferred embodiments, R ⁴ =H, in which each N-0 group has H.

The cobalt chelates of structures I and II can be prepared by reacting a Co(II) compound with the desired ligand compound. As will be apparent to one skilled in the art, these chelates also can be prepared in situ by adding the cobalt salt and the ligand, as separate components, to a mixture of optional solvent, monomer and photoinitiator prior to irradiation. Alternatively, the complex can be prepared and stored as a standard solution for subsequent addition to the mixture to be polymerized. For such standard solutions the cobalt(II) salt can be in the form of the nitrate, chloride, bromide or iodide, either as hydrated or anhydrous, or as an alkanoate, the lower (C ₂ -C ₃ ) alkanoates being soluble in methanol or ethanol, the higher (C ₄ -C ₈ ) alkanoates providing a means of preparing the standard solutions in hydrocarbon solvents. The analogous cobalt(III) complexes can be used in the process of this invention if the cobalt(ni) can easily be reduced to cobalt(II) by reaction with the monomers or free radicals produced by the initiator. This permits the in situ production of the cobalt(II).

Useful cobalt Π complexes include those derived from diphenylglyoxime

(Co ⁿ (DPG-BF ₂ ) ₂ , dimethylglyoxime (Co ⁿ (DMG-BF ₂ ) ₂ , ethylmethylglyoxime (Co ⁿ (EMG- BF ₂ ) ₂ , diethylglyoxime (Co ⁿ (DEG-BF ₂ ) ₂ , cyclohexylglyoxime (Co ⁿ (CHG-BF ₂ ) ₂ , where R2+R3 = -(CH ₂ ) ₄ -. Hydrogen may be substituted for -BF ₂ -.

The reaction mixture further comprises one or more photoinitiators. The term "photoinitiator" as used above and below comprises free-radical polymerization initiators which can be activated by some kind of actinic radiation such as for example, light sources, especially UV-light sources. Free-radical radiation polymerization initiators which can be activated by light, are often referred to as free-radical photoinitiators.

Radiation-curable precursors which include one or more photoinitiators are preferred. The free-radical photoinitiators which are suitable preferably include both type I and type II photoinitiators.

Type I photoinitiators are defined to essentially undergo a unimolecular bond cleavage reaction upon irradiation thereby yielding free-radicals. Suitable type I photoinitiators are selected from a group consisting of benzoin ethers, benzil ketals, a- dialkoxyacetophenones, a -hydroxyalkylphenones and acylphosphine oxides. Suitable type I photoinitiators are commercially available, for example, as Esacure™ KIP 100 from

Lamberti Spa, Gallarate, Italy, or as Irgacure™ 651 from Ciba-Geigy, Lautertal, Germany.

Type II photoinitiators are defined to essentially undergo a bimolecular reaction where the photoinitiators interact in an excited state with a second compound acting as co- initiator, to generate free-radicals. Suitable type Π photoinitiators (hydrogen abstracting) are selected from a group comprising benzophenones, thioxanthones and titanocenes. Included among those hydrogen abstracting photoinitiators are benzophenone, 4-(3- sulfopropyloxy)benzophenone sodium salt, Michler's ketone, benzil, anthraquinone, 5,12- naphthacenequinone, aceanthracenequinone, benz(A)anthracene-7,12-dione, 1,4- chrysenequinone, 6,13-pentacenequinone, 5,7,12,14-pentacenetetrone, 9-fluorenone, anthrone, xanthone, thioxanthone, 2-(3-sulfopropyloxy)thioxanthen-9-one, acridone, dibenzosuberone, acetophenone, and chromone. Suitable type II photoinitiators are commercially available, for example, as Esacure ™ TZT from Lamberti Spa., Gallarate, Italy, or as 2- or 3-methylbenzophenone from Aldrich Co., Milwaukee, Wis. Suitable amine co-initiators are commercially available, for example, as GENOMER™ 5275 from Rahn AG, Zurich, Switzerland.

Useful hotoinitiators include those of the formula: wherein R is

wherein R ¹ is H or a Q to C ₄ alkyl group,

R ⁸ , R ⁹ and R ¹⁰ are independently a hydroxyl group, a phenyl group, a Cj to Cg alkyl group, or a Ci to C alkoxy group.

The mixture is oligomerized by subjecting it to actinic irradiation and preferably to UV-irradiation. Actinic radiation from any source and of any type can be used for the curing of the composition whereby light sources are preferred over e-beam sources. The light can be in the form of parallel rays or divergent beams. Since many photoinitiators generating free-radicals exhibit their absorption maximum in the ultraviolet (UV) range, the light source is preferably selected to emit an effective amount of such radiation.

Suitable light sources include carbon arc lamps, mercury vapor lamps, fluorescent lamps comprising ultraviolet light-emitting phosphors, ultraviolet light-emitting diodes, argon glow lamps and photographic flood lamps. Preferred are high-intensity light sources having a lamp power density of at least 80 mW/cm and more preferably of at least 120 mW/cm . The composition may be irradiated with activating UV radiation to polymerize the monomer component(s). UV light sources can be of two types: 1) relatively low light intensity sources such as Blacklights which provide generally 10 mW/cm or less (as measured in accordance with procedures approved by the United States National Institute of Standards and Technology as, for example, with a UVIMAP UM 365 L-S radiometer manufactured by Electronic Instrumentation & Technology, Inc., in Sterling, VA) over a wavelength range of 280 to 400 nanometers; and 2) relatively high light intensity sources such as medium pressure mercury lamps which provide intensities generally greater than 10 mW/cm ² , preferably 15 to 450 mW/cm ² .

The reaction mechanism for the cobalt-catalyzed chain-transfer reaction is shown in Scheme 1, as proposed by Gridnev, A. A.; and Ittel, S. D. Chem. Rev. 2001, 101, 3611- 3659. The key step in the proposed mechanism is the abstraction of a hydrogen atom from a growing radical chain by a cobalt(II) complex The hydrogen abstraction step generates an α,β-unsaturated ester-terminated oligomer and a cobalt(III)-hydride. The cobalt(III)- hydride can reinitiate polymerization by transferring a hydrogen atom to monomer to generate monomeric radical. In addition to productive chain-transfer chemistry, the cobalt(II) complex can also react with alkyl radicals to produce cobalt(III)-alkyl species that lie off the catalytic cycle. The generation of cobalt(III)-alkyl complexes, as well as the chain-transfer process itself, serves to slow the polymerization reaction. The available mechanistic data is also consistent with a traditional organometallic mechanism involving a β-hydride elimination as a key step.

Scheme 1. Cobalt-Catalyzed Chain-Transfer Mechanism

Analysis of the Gridnev cobalt-catalyzed chain-transfer mechanism reveals that reaction rate should decrease with increasing cobalt concentration whereas selectivity for dimer should increase with increasing cobalt concentration. In other words, oligomer molecular weight will decrease with increasing cobalt concentration. The concentration of free radicals in this reaction should have the opposite effects; an increase in free radical concentration should increase rate and decrease dimer selectivity. A primary reason for reduced selectivity in the presence of high radical concentrations is that high radical concentrations push the cobalt(II)/cobalt(III)-alkyl equilibrium towards cobalt(III)-alkyl complexes. The more cobalt that is tied up outside the catalytic cycle as a cobalt(III)-alkyl species, the less active cobalt(II) catalyst available to act as a chain-transfer catalyst.

Lower relative levels of active cobalt(II) complex leads to higher molecular weight oligomers and lower selectivity for dimer; the chain-transfer reaction is first order in cobalt whereas propagation is zero order in cobalt. Essentially, at sufficiently high radical concentrations the catalytic chain-transfer cycle is saturated with free radicals and a further increase in free radical concentration will not increase the rate of catalytic chain transfer but will increase the rate of propagation. Therefore, in this situation an increase in free radical concentration will lead to lower selectivity for dimer and the production of higher molecular weight material. Reaction rate decreases with increasing cobalt concentration because the production of cobalt(III)-alkyl complexes sequesters a portion of the free radicals, thereby slowing the polymerization rate. Also, the chain-transfer process and reinitiation by transfer of hydrogen from a cobalt(III)-hydride to monomer also likely slows the overall reaction rate.

Given the decrease in selectivity with increasing radical concentration it is desirable to control the rate of radical generation; there are potential advantages in generating radicals slowly over time rather than generating a high concentration of radicals quickly. The rate of radical production can be controlled by temperature and concentration using thermal initiators. The thermally initiated reaction of the prior art to produce methyl methacrylate dimer involves the slow addition of a solution of MMA and Vazo™ 67 to a separate solution of Vazo™ 67 in MMA in the heated reaction pot. Presumably, the slow introduction of additional Vazo™ 67 to the reaction pot reduces the maximum radical concentration relative to a reaction in which all the reagents were combined at the beginning of the reaction.

However, photochemical radical generation may provide superior control over the production of free radicals. Also, there may be safety advantages to using photoinitiators instead of thermal initiators. In general, cobalt-catalyzed methacrylate oligomerizations are run using relatively high concentrations of thermal initiator, and often these reactions are run in neat monomer. The presence of highly initiated solutions of neat monomer may pose potential safety concerns. Substituting photoinitiators for thermal initiators may be beneficial from a safety perspective in that it is easier to control photons than thermal energy. Also, the use of photoinitiators decouples the radical generation process from the thermal energy required to drive the chain-transfer cycle, a potential advantage in reaction optimization. Cobalt(III)-alkyl complexes are known to be photochemically active; the cobalt(in)-alkyl bond can be photolyzed to generate an alkyl radical and a cobalt(II) species. Therefore, irradiation may influence the cobalt(III)-alkyl / cobalt(II) equilibrium.

Simply, the methacrylate ester, the cobalt complex and the photoinitiator are combined and irradiated as described supra. The monomers may be oligomerized neat, but a non-reactive solvent may be employed.

Useful solvents include aromatic hydrocarbons, such as benzene, toluene and the xylenes; ethers, such as tetrahydrofuran, diethyl ether and the commonly available ethylene glycol and polyethylene glycol monoalkyl and dialkyl ethers, including the Cellosolves and Carbitols ; alkyl esters of acetic, propionic and butyric acids; and mixed ester-ethers, such as monoalkyl ether-monoalkanoate esters of ethylene glycol; ketones, such as acetone, butanone, pentanone and hexanone; and alcohols, such as methanol, ethanol, propanol and butanol. In some instances, it may be advantageous to use mixtures of two or more solvents.

The oligomerization is carried out in the range 0-150°C. The preferred range is 50-

100°C.

To ensure maximum catalyst activity the oligomerization should be carried out in the substantial absence of oxygen under an inert atmosphere, such as nitrogen, argon or other non-oxidizing gas.

In free radical polymerizations, the degree of conversion of monomer to oligomer will depend on several factors, including, the monomer/initiator molar ratio (M/I); the reaction temperature; the inherent chain transfer activity of the solvent (if any); and the relative rates of initiation and propagation; the relative activity of the catalyst and the catalyst/ initiator molar ratio. The degree of conversion may be monitored by typical analytical techniques including 1H NMR, GC, and IR.

The process of the invention generally is carried out as a batch process in accordance with techniques which are well known to one skilled in the art. Such techniques are demonstrated in the examples. As a further demonstration of batch process techniques, the reactor can be charged with optional solvent and methacrylate monomer with the mixture being stirred under an inert atmosphere (such as nitrogen, argon or helium), for the substantial removal of oxygen. To the mixture can be then added the requisite amount of photoinitiator, typically such that methacylate monomenphotoinitiator molar ratio ( M/PI) is 50 to 1500, preferably 150 to 750. When the photoinitiator has dissolved, the cobalt catalyst can be added in or, alternatively, the catalyst can be formed in situ by adding the components thereof, ligand and the appropriate cobalt(II) compound. In some cases the catalyst can be added in solid form if the chelate has previously been isolated as such. In typical examples, it is added in amount such that the catalyst/initiator molar ratio (C/I) is in the range 0.01-0.3, preferably 0.02 to 0.25. After all additions have been completed, the mixture is irradiated.

In some embodiments the oligomerizable mixture comprises: a) at least one methacrylate ester;

b) 0.005 to 0.1wt.% of a cobalt chelate chain transfer agent, preferably 0.01 to 0.04 wt.%, based on wt.% of the initial cobalt salt, not the final cobalt complex formed in-situ;

c) 0.05 to 3 wt.%., preferably 0.1 to 2.5 wt.%, and more preferably 0.3 to 1.5 wt.% of a photoinitiator.

The mixture is irradiated to the desired degree of conversion (i.e. percent of monomer that has been oligomerized), and may be monitored by standard analytical technique. Lower degrees of conversion provide relatively greater amounts of the desired dimer, relative to higher oligomers.

The reaction product is of the formula:

wherein R is an alkylene, arylene or combination thereof;

Z is H or a functional group, and n is zero or greater. It will be appreciated that n=0 describes the desired dimers.

The separation of the product mixture, including unreacted methacrylate monomer, dimer, trimer, higher oligomers, cobalt catalyst, photoinitiator byproducts and optional solvent may be performed as described in Moad, C. L.; Moad, G.; Rizzardo, E.; and Thang, S. H. Macromolecules, 1996, 29, 7717-7726. Unreacted monomer, solvent and catalyst may be recycled.

Examples

Materials - Commercial reagents were used as received.

• Cobalt(II) acetate tetrahydrate Alfa Aesar, Ward Hill, MA

• Dimethyl glyoxime, Alfa Aesar, Ward Hill, MA

• Irg 2959 - Irgacure™ 2959 type I photoinitiator, BASF Corp., Florham Park, NJ • Irg 651 - Irgacure™ 651 type I photoinitiator, BASF Corp., Florham Park, NJ

• Methyl methacrylate, Alfa Aesar, Ward Hill, MA

• Pyridine, Alfa Aesar, Ward Hill, MA

• Vazo™ 67 - thermal initiator, DuPont, Wilmington, DE

Instrumentation - Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a 500 MHz spectrometer.

Examples 1 - 6

A three-neck, 50 mL round-bottomed flask, equipped with a magnetic stir bar, glass stopper, rubber septum, and a reflux condenser with a gas inlet adapter that was used to prepare each composition in Examples 1 - 6. All glassware was oven dried and the apparatus was allowed to cool to room temperature under nitrogen. Methyl methacrylate (14.04 g) was added to the pot and stirred while sparged with nitrogen for 30 minutes, after which the reaction was maintained under a positive pressure of nitrogen. Cobalt(II) acetate tetrahydrate (0.0059 g, 0.017 mol%), dimethyl glyoxime (0.0089 g, 0.055 mol %), pyridine (0.0143 g, 0.13 mol %), and Irg 2959 photoinitiator in the amounts shown in Table 1 were added to the pot.

The flask was exposed to 350 nanometer black light positioned approximately 2.54 cm from the exterior surface of the flask (153 mJ/cm /min) for 4 hours before sampling.

The reaction in Example 1 was carried out at 21°C yielded approximately 10% conversion.

The flask was heated in an oil bath at increased temperature in Examples 2 - 6, and the amounts of photoinitiator were decreased in Examples 3 - 6 as shown in Table 1.

Increasing the reaction temperature to 75 °C in Example 2 produced a significant increase in reaction rate; after 4 hours of irradiation at 75 °C the oligomerization had proceeded to

59% conversion with 71% selectivity for methyl methacrylate dimer.

Conversion was calculated using 1H NMR by comparing the integration of the singlet for the methyl ester of methyl methacrylate monomer to the sum of the integrations of the resonances for all other methyl esters of the oligomer products. Selectivity was calculated by doubling the integration of the methyl ester singlet at 3.73 ppm (the resonance for one of the methyl esters of the desired dimer) to the sum of the integration for all other methyl ester protons for the higher molecular weight oligomers. The integration for the singlet at 3.73 ppm was doubled because the other dimer methyl ester singlet at approximately 3.64 ppm overlapped with resonances from higher molecular weight oligomers and could not be reliably integrated. The photochemical reaction provided similar selectivity and conversion to oligomers prepared using a thermal initiator in Example CI.

Comparative Example CI

A three-neck, 500 mL round-bottomed flask was equipped with a magnetic stir bar, gas inlet adapter, rubber septum, and a 250 mL pressure equalizing addition funnel with a rubber septum. All glassware was oven dried and the apparatus was allowed to cool to room temperature under nitrogen. Methyl methacrylate (107 mL) and Vaso ^tm 67 initiator (0.500 g) were added to the pot and the mixture was stirred. The addition funnel was charged with methyl methacrylate (200 mL) and Vazo™ 67 (1.00 g). The two solutions of Vazo™ 67 in methyl methacrylate were sparged with nitrogen for 30 minutes after which the reaction was maintained under a positive pressure of nitrogen. Cobalt(II) acetate tetrahydrate (0.1206 g), dimethyl glyoxime (0.183 g), and pyridine (0.30 mL) were added to the pot. The reaction was then heated to 92 °C in an oil bath with stirring. The solution of Vazo™ 67 in methyl methacrylate was then added to the pot dropwise over a period of approximately 1.5 hours. After an additional hour, an additional portion of Vazo™ 67 (0.038 g) was added and the reaction mixture was stirred at 92 °C for another hour. The reaction was then removed from the oil bath and allowed to cool to room temperature. A sample was taken and 1H NMR analysis showed that the reaction had progressed to approximately 79% conversion with 56% selectivity for methyl methacrylate dimer.

Methyl methacrylate starting material was removed from the reaction mixture under reduced pressure. Then, methyl methacrylate dimer was distilled under reduced pressure (bp ~ 48 °C at 0.14 mm Hg) to provide the desired dimer as a clear, colorless liquid (119.51 g, 42 %). Table 1

a 1H NMR Analysis (integration of methyl esters, no internal standard) after 4 hours. ^b Calculated by doubling the integration at 3.73 ppm to obtain the relative amount of dimer.

c The reaction was sampled after 7 hours.

d Isolated yield in parenthesis

e. Vazo 67 used in place of Irgacure 651

Examples 7 - 9

Methacrylate oligomers were prepared according to the procedure for Examples 1 -

6 except the reaction mixtures were heated to the temperatures shown in Table 2 with corresponding amounts of photoinitiator. The photochemical reaction provided similar selectivity and conversion to oligomers prepared using a thermal initiator in Example C2. Example C2

A methyl methacrylate dimer was prepared as described in Example CI except as that the initial amount of Vazo™ 67 added to the pot was increased to 1.0 gram; the amount of Vazo™ 67 added to the addition funnel was increased to 2.0 g; and the additional amount of Vazo™ 67 added later was increased to 0.075 g. After the reaction was removed from the oil bath and allowed to cool to room temperature, a sample was taken and 1H NMR analysis showed that the reaction had progressed to approximately 85% conversion with 53% selectivity for methyl methacrylate dimer. Methyl methacrylate starting material was removed from the reaction mixture under reduced pressure. Then, methyl methacrylate dimer was distilled under reduced pressure (bp ~ 48 °C at 0.14 mm Hg) to provide the desired dimer as a clear, colorless liquid (116.14 g, 40.4 %). Example 10

A three-neck, 500 mL round-bottomed flask was equipped with a magnetic stir bar, glass stopper, rubber septum, and a reflux condenser with a gas inlet adapter. All glassware was oven dried and the apparatus was allowed to cool to room temperature under nitrogen. Methyl methacrylate (300 mL, 280.8 g) was added to the pot. With stirring, the methyl methacrylate was sparged with nitrogen for 30 minutes after which the reaction was maintained under a positive pressure of nitrogen. Cobalt(II) acetate tetrahydrate (0.118 g), dimethyl glyoxime (0.179 g), pyridine (0.29 mL), and Irg 651 (4.025 g) were added to the pot. The reaction was then heated to 75°C in an oil bath. The reaction vessel was irradiated using a 350 nm black light positioned approximately 1" from the exterior surface of the reaction flask (153 mJ/cm /min). After 5 hours, the irradiation was stopped and the reaction was removed from the oil bath and allowed to cool to room temperature. A sample was taken and 1H NMR analysis showed that the reaction had progressed to approximately 92% conversion with 47% selectivity for methyl methacrylate dimer. The remaining methyl methacrylate starting material was removed from the reaction mixture under reduced pressure. Methyl methacrylate dimer was then distilled under reduced pressure (bp ~ 48 °C at 0.14 mm Hg) to provide the desired dimer as a clear, colorless liquid (121.82 g, 43.4 %).

Table 2

a 1H NMR Analysis (integration of methyl esters, no internal std.).

b Calculated by doubling the integration at 3.73 ppm to obtain the relative amount of dimer.

c The reaction was sampled after 5 hours and was run using 300 mL of methyl

methacrylate.

d Isolated yield in parenthesis. This specification provides the following illustrative embodiments:

1. A method of preparing methacrylate oligomers comprising irradiating a mixture of a) at least one methacrylate ester;

b) 0.005 to 0.1 wt.% of a cobalt chelate chain transfer agent,

c) 0.05 to 3 wt.%. of a photoinitiator.

2. The method of embodiment 1 wherein the cobalt chelate chain transfer agent comprises a Co(II) chelate, a Co(III) chelate, or mixture thereof.

3. The method of any of the previous embodiments wherein the monomenphotoinitiator molar ratio ( M/PI) is 50 to 1500.

4. The method of any of the previous embodiments wherein the catalyst/initiator molar ratio (C/I) is in the range 0.01-0.3.

5. The method of any of the previous embodiments wherein the methacrylate esters are of the formula:

CH ₂ =C(CH ₃ )-CO-0-R ¹ -Z,

where R ¹ is a covalent bond, an alkylene, arylene or combination thereof; and

Z is H or a functional group.

6. The method of embodiment 5 wherein Z is selected from hydroxyl, amino, acetylacetonate, acid, ester, epoxy, isocyanate, aziridinyl, acyl halide, poly(alkylene oxide), silyl, and cyclic anhydride groups.

7. The method of any of the previous embodiments wherein the cobalt chelate chain transfer agent is of the formula:

wherein R ⁴ is selected from the group consisting of hydrogen and B(R ⁵ ) ₂ where each R ⁵ is independently selected from the group consisting of unsubstituted and substituted aryl, unsubstituted and substituted CrC ₁₂ alkyl, unsubstituted and substituted CrC ₁₂ alkoxy, unsubstituted and substituted aryloxy, and a halogen; R 2 and R 3 are each independently selected from the group consisting of phenyl, substituted phenyl, methyl, ethyl, and - (CH ₂ ) ₄ -; and Lig is selected from the group consisting of water, an amine, a pyridine, ammonia, a phosphine and combinations thereof, and R ⁶ is an organic radical. 8. The method of embodiment 7 wherein R ⁴ is BF ₂ .

9. The method of embodiment 7 wherein R ⁴ is H.

10. The method of any of the previous embodiments wherein the cobalt chelate chain transfer agent is a Co ¹¹ porphyrin.

11. The method of any of the previous embodiments wherein the photoinitiator is of the formula:

wherein R ¹ is H or a Ci to C ₄ alkyl group,

R ⁸ , R ⁹ aanndd RR ¹⁰ aarree iinnddeeppeennddeennttllyy a hydroxyl group, a phenyl group, a Ci to Cg alkyl group, or a Ci to Cg alkoxy group.

The method of any of the previous embodiments in the absence of solvent.

13. A methacrylate oligomer prepared by the method of of any of the previous embodiments of the formula:

where R is a covalent bond, an alkylene, arylene or combination thereof; and

Z is H or a functional group.