MULTIDENTATE LIGANDS

FIELD OF INVENTION

[001] A method for preparing a boron complex to be used as an activation initiator is provided, as well as a method for addition reaction polymerization using the boron complex obtained by said method.

BACKGROUND

[002] Boranes can be used as polymerization initiators, but suffer from pyrophoric characteristics and sometimes too-powerful reaction promotion that makes them unsuitable for compositions that need to remain stable until a desired time. Certain ligands are known to stabilize boranes, but the resulting borane complexes, such as triethylborane THF complex, are still too reactive for materials such as (meth)acryl monomers that would polymerize in seconds. Boron complexes with oxygen-containing multidentate ligands are known, and offer a possible solution to this stability issue. For example, M.F. Hawthorne & M. Reintjes, J. Org. Chem., 1965, 30, 3851-3853, discloses such compounds. However, synthesis of such boron complexes with oxygen-containing multidentate ligands, such as disclosed in this reference, requires drastic reaction conditions.

SUMMARY OF INVENTION

[003] This invention relates to a new method for preparing boron complexes comprising oxygen-containing multidentate ligands, such method carried out under a mild condition.

[004] The method of present invention comprises contacting and allowing to react at a temperature within a range of from 0 to l00°C:

(a) an alkoxyborane of formula (I)

wherein R ¹ and R ² and independently are hydrogen, C1-C30 hydrocarbyl or C5-C30 heterocyclic; and R ³ is C ₁ -C ₃₀ hydrocarbyl; and

(b) either (i) a compound comprising an a-diketo structure, wherein the compound has from 4 to 30 carbon atoms, or (ii) a compound comprising a b-diketo structure, wherein the compound has from 5 to 40 carbon atoms.

[005] The present invention further provides a method for addition polymerization, said method comprising contacting:

(a) at least one boron complex of formula (IV) or formula (V)

wherein R ¹ and R ² and independently are hydrogen, C1-C30 hydrocarbyl or C5-C30 heterocyclic; and R ⁴ , R ⁵ , R ⁶ , R ⁷ and R ⁸ independently are hydrogen, C1-C30 hydrocarbyl or C5-C30 heterocyclic; wherein R ⁷ and R ⁸ or any two of R ⁴ , R ⁵ and R ⁶ may be joined to form a ring; and wherein the total number of carbon atoms among R ⁴ , R ⁵ , R ⁶ is at least 1 and no more than 27, and the total number of carbon atoms among R ⁷ and R ⁸ is at least 1 and no more than 38;

(b) at least one compound having at least one carbon-carbon double bond or carbon- carbon triple bond.

DETAILED DESCRIPTION OF THE INVENTION

General definitions

[006] Percentages are weight percentages (wt%) and temperatures are in °C, unless specified otherwise. Experimental work is carried out at room temperature (20-25°C), unless otherwise specified.

[007] A“hydrocarbyl” group is a substituent derived from an aliphatic or aromatic hydrocarbon, which may be linear, branched or cyclic; and which may have one or more substituents selected from halo (preferably fluoro), hydroxyl, alkoxy, alkanoyl, aroyl (aryl attached to carbonyl), aryl, heterocyclic, and substituted or unsubstituted amino (-NRR’, where R, R’ may be hydrogen, hydrocarbyl as defined herein, alkoxy, or heterocyclic).

Preferably, hydrocarbyl groups are unsubstituted. A hydrocarbyl group may be an“alkyl,” “alkenyl,” or“aryl.”

[008] An“alkyl” group is a substituted or unsubstituted saturated hydrocarbyl group having a linear, branched or cyclic structure. Alkyl groups may have one or more substituents described above for the hydrocarbyl group in general. Preferably, alkyl groups are unsubstituted. Preferably, alkyl groups are linear or branched, i.e., acyclic. Preferably, each alkyl substituent is not a mixture of different alkyl groups, i.e., it comprises at least 98% of one particular alkyl group.

[009] An“alkenyl” group is a substituted or unsubstituted hydrocarbyl group having a linear, branched or cyclic arrangement and having at least one carbon-carbon double bond. Preferably, alkenyl groups have no more than three carbon-carbon double bonds, preferably no more than two, preferably one. Alkenyl groups may have one or more substituents described above for the hydrocarbyl group in general. Preferably, alkenyl groups are unsubstituted. Preferably, alkenyl groups are linear or branched, i.e., acyclic.

[0010] An“aryl” group is a substituent derived from an aromatic hydrocarbon which may contain an aliphatic structure as well as aromatic structure. An aryl group may be bonded to the rest of the molecular structure via an aromatic ring carbon atom or via an aliphatic carbon atom. Aryl groups may have one or more substituents described above for the hydrocarbyl group in general. Preferably, aryl groups are unsubstituted.

[0011] A“heterocylic” group is a substituent derived from a heterocyclic compound, which may be aromatic or aliphatic, and which may comprise more than one ring. Heterocyclic groups may have one or more substituents selected from hydroxyl, alkoxy, alkanoyl, aroyl and aryl, and substituted or unsubstituted amino (-NRR’, where R,

R’ may be hydrogen, hydrocarbyl as defined herein, alkoxy, or heterocyclic). Preferably, heterocyclic groups are unsubstituted.

[0012] A“hydrocarbyloxy,”“alkoxy,”“alkenyloxy” or“aryloxy” group is a substituent formed by adding an oxygen atom at the point of attachment of a hydrocarbyl, alkyl, alkenyl or aryl group, respectively (e.g., between an alkyl group and a carbon atom).

A“hydrocarbylcarbonyl,”“hydrocarbyloxycarbonyl,”� ��alkylcarbonyl,”“alkoxycarbonyl,” “alkenylcarbonyl,”“alkenyloxycarbonyl,”“arylcarbon yl,” or“aryloxycarbonyl” group is a substituent formed by adding an oxygen atom at the point of attachment of a hydrocarbyl, hydrocarbyloxy, alkyl, alkoxy, alkenyl, alkenyloxy, aryl or aryloxy group, respectively (e.g., between an alkyl group and a carbon atom).

[0013] An expression Ca-Cb means a substituent selected from a group of structure having as few as“a” number of carbon atoms and as many as“b” number of carbon atoms. Ca-Cb hydrocarbyl, for example, means a hydrocarbyl group having as few as“a” number of carbon atoms and as many as“b” number of carbon atoms. The number of carbon atoms in such a substituent includes any carbon atoms which may be in substituents thereof. [0014] “(Meth)acrylic,”“(meth)acrylate” and“(meth)acrylamide” mean acrylic or methacrylic or mixtures thereof; acrylate or methacrylate or mixture thereof; and acrylamide or methacrylamide or mixture thereof, respectively.“Acac” means acetylacetate, acetylacetone, or a mixture thereof, as permitted by constraints due to chemical valence and electron-distribution structure.

Borane complex preparation

[0015] The method of the present invention utilizes (a) an alkoxyborane as the source of boron, which is complexed with (b) a diketo compound, where (a) and (b) are brought in contact with each other and allowed to react at a temperature within the range of from 0 to l00°C.

[0016] The alkoxyborane useful for this method has a formula (I)

wherein R ¹ and R ² independently are hydrogen, C1-C30 hydrocarbyl or C5-C30 heterocyclic; and R ³ is C1-C30 hydrocarbyl. Preferably, the C1-C30 hydrocarbyl is methyl, ethyl, any of the isomers of propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, or dodecyl, phenyl, benzyl, or phenylalkyl. More preferably, the hydrocarbyl is methyl, ethyl, propyl, n-butyl, phenyl, or benzyl. Thus, preferably, R ¹ and R ² independently are hydrogen, hydrocarbyl group having at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, carbon atoms and at the same time having no more than 12, 14, 15, 16, 18, 20, 24, 30 carbon atoms. More preferably, R ¹ and R ² are independently selected from hydrogen, methyl, and ethyl, and R ³ is selected from methyl and ethyl. Preferably, R ¹ , R ² and R ³ are unsubstituted.

[0017] The compound useful in the inventive method as component (b) is either (i) a compound comprising an a-diketo structure with at least 4 carbon atoms and any number up to 30 carbon atoms, or (ii) a compound comprising a b-diketo structure with at least 5 carbon atoms and any number up to 40 carbon atoms. Preferably, the a-diketo compound has from 4 to 20 carbon atoms, preferably from 4 to 15 carbon atoms. Preferably, the b- diketo compound has from 5 to 20 carbon atoms, preferably from 5 to 15 carbon atoms. Preferably, the compound useful as component (b) has a molecular weight no greater than 600, preferably no greater than 400. Preferably, the a-diketo compound is of formula (III) and the b-diketo compound is of formula (II)

wherein R ⁴ , R ⁵ , R ⁶ , R ⁷ and R ⁸ independently are hydrogen, hydrocarbyl, hydrocarbyloxy or heterocyclic; wherein the total number of carbon atoms among R ⁴ , R ⁵ , R ⁶ is at least 1, 2, 3, 5, or 10 and at the same time no more than 10, 12, 15, 20, 25, or 27, and the total number of carbon atoms of R ⁷ and R ⁸ is at least 3, 4, 5, 6, 10, or 12 and at the same time no more than 15, 18, 20, 24, 28, 32, 36, or 38. Any two of R ⁴ , R ⁵ and R ⁶ or R ⁷ and R ⁸ may be joined to form a ring, in which case such a ring is formed by at least 5 and up to 30 atoms and may include a hetero atom. The ring may have a substituent or a hydrocarbyl branch. Preferably, none of R ⁴ , R ⁵ , R ⁶ , R ⁷ and R ⁸ is part of a ring. Preferably, R ⁴ , R ⁵ , R ⁶ , R ⁷ and R ⁸ is alkenyl or alkoxy.

[0018] The a-diketo and b-diketo compound may comprise one or more additional ketone groups or aldehyde groups. Preferably compounds have only two carbonyl groups.

[0019] Although formulae (II) and (III) are shown with two ketones represented, an a-diketo or b-diketo compound is in an equilibrium of its keto form and enol form, in a state called keto-enol tautomerism. The enol form is a stronger nucleophile than the keto form and is believed to be responsible for the formation of the boron-O, O-bidenate chelates. The percentage of equilibrium enol varies. For example, 87% of 2,4-pentanedione exists in enol form. For tropolone, a l,2-diketo compound, the enol form is predominant; the keto form is not detectable.

[0020] Preferred a-diketo compounds include butane-2, 3-dione, 2,3-pentanedione, acetaldehyde, and tropolone. Preferred b-diketo compounds include 2,4-pentanedione, 3,5-heptanedione, 2,3-heptanedione, 2,4-octanedione, 1, 1,1, 2, 2, 3, 3, 7, 7, 8, 8, 9,9,9- tetradecafluoro-4,6-nonanedione, 1,3-cyclohexanedione, 1,3-cycloheptanedione, 1-benzyl- 4,4,4-trifluoro-l,3-butanedione, dibenzyl- 1,3-propanedione, 2-acetylcyclohexanone, 2-[(2- methylacryloyl)oxy]ethyl-3-oxobutanoate, ethyl acetylacetonate, 2,4,6-heptanetrione and ethyl diacetylacetate.

[0021] Reactions of component (a), a compound of formula (I), with component (b), a compound of formula (II) or compound of formula (III), are as follows:

[0022] Preferably, the mole ratio of alkoxyborane to diketo compound is in the range from 0.95:1 to 10:1, preferably 0.95:1 to 5:1, preferably 0.95:1 to 2:1, preferably 1:1 to 1.2:1.

[0023] Preferably, the reaction temperature is no greater than 80°C, preferably no greater than 70°C, preferably no greater than 60°C, preferably no greater than 50°C, preferably no greater than 40°C; while at the same time preferably, at least 5°C, preferably at least l0°C, preferably at least l5°C. In one embodiment of the invention, the reactants are combined at a temperature below the stated lower limit for reaction temperature and then warmed to a temperature within the stated range where most of the reaction occurs. Typically, the reaction is carried out at room temperature, i.e. 20-25 °C. After the reaction, any volatiles are removed under vacuum. The reacted mixture may be used as is (i.e.

without purifying) or purified by vacuum factional distillation to yield the complex. There is no limitation regarding the level of vacuum, but typically, the vacuum is at less than 30- mmHg and can be as low as O.OlmmHg.

[0024] Optionally, at least one organic solvent is present in the reaction mixture where the alkoxyborane and the diketo compound are in contact. Preferably, the solvent(s) are not reactive towards the alkoxyborane or diketo compound; preferably, the solvent is non-protic and free of amines and keto groups, preferably with boiling point below 120 °C. Preferred solvents include pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, l,4-dioxane, chloroform, dichloromethane, tetrahydrofuran, diethyl ether, and acetonitrile. The amount of the optional organic solvent is not particularly limited, but not in the amount that would dilute the alkoxyborane and diketo compound so much that there is no significant reaction. In a typical reaction mixture, the concentrations of component (a) and component (b) are no less than 0.1 mole/liter each.

[0025] The boron complex obtained by the above described inventive method has a formula (IV) or formula (V)

wherein R ¹ and R ² independently are hydrogen, C1-C30 hydrocarbyl or C5-C30 heterocyclic; and R ⁴ , R ⁵ , R ⁶ , R ⁷ and R ⁸ independently are hydrogen, C ₁ -C ₃₀ hydrocarbyl or C ₅ -C ₃₀ heterocyclic; wherein R ⁷ and R ⁸ or any two of R ⁴ , R ⁵ and R ⁶ may be joined to form a ring; and wherein the total number of carbon atoms among R ⁴ , R ⁵ , R ⁶ is at least 1 and no more than 27, and the total number of carbon atoms among R ⁷ and R ⁸ is at least 1 and no more than 38. The boron complex of formula (IV) is obtained when using b-diketo compound as component (b), and the boron complex of formula (V) is obtained when using a-diketo compound as component (a).

Addition polymerization method

[0026] These boron complexes are useful in a method for addition polymerization as initiators. Preferably, in the method for addition polymerization, the polymerization reaction mixture comprises from 0.1 to 5.0 wt% of at least one compound of formula (IV) or formula (V), preferably from 0.5 to 3 wt%, preferably from 0.8 to 2.2 wt%. A boron complex may be purified from the preparation reaction mixture or the preparation reaction mixture may be used without purification after the preparation reaction.

[0027] The unsaturated compound, i.e., the compound having at least one carbon- carbon double bond or triple bond, which is polymerized by addition reaction in the method of the present invention preferably has from three to thirty carbon atoms. Preferably, the compound is an acrylic monomer, a diene (e.g., butadiene), styrenic monomer (e.g., styrene and methyl-substituted styrenes), C4-C22 vinyl ester (e.g., vinyl acetate), vinyl halide (e.g., vinyl chloride) or a vinyl heterocycle (e.g., N-vinylpyrrolidone, N-vinylimidazole). The term“acrylic monomer” includes (meth)acrylic acids, their salts and their C1-C22 alkyl or hydroxyalkyl esters; crotonic acid, itaconic acid, fumaric acid, maleic acid, maleic anhydride, (meth)acrylamides, (meth) acrylonitrile and C1-C22 alkyl or hydroxyalkyl esters of crotonic acid, itaconic acid, fumaric acid or maleic acid. Preferably, the unsaturated compound is an acrylic monomer having from three to thirty carbon atoms, preferably three to twenty.

EXAMPLES

Test methods.

Structure determination

[0028] The molecular structure of a reaction product was determined using nuclear magnetic resonance (NMR) analysis of ¹ H and ⁿ B under standard conditions.

Addition reaction polymerization test

[0029] To 97.0 parts of a (meth)acrylic monomer was added 3.0 parts of initiators.

The mixture was thoroughly mixed and was thermally cured in an oven (140 °C). The time for the liquid monomer mixture to cure into a white solid (as visually confirmed) was measured.

Shelf-life study

[0030] To 97.0 parts of a (meth)acrylic monomer was added 3.0 parts of boron-acac initiator. The mixture was thoroughly mixed and stored at room temperature. The samples were visually inspected periodically for the emergence of gels.

Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC).

[0031] The thermal properties of the boron initiators were examined by DSC using a

DuPont 900 series DSC with Universal Analysis software (TA Instruments) at a scanning speed of 10 °C/min in both He and air. Flow rate of both gases was 25 mL/min. The complexes were run neat in crimped Al pans with sample masses ranging from 3 to 6 mg.

[0032] TGA (heating ramp rate of 10 °C/min under He) was used to determine the thermal stability of the boron initiators. A 5% mass loss is defined as the thermolysis threshold.

Preparation Method Comparative Examples. Preparation of boron-acetylacetate complexes from triethylborane (TEB).

General procedure [0033] To neat acetylacetate (1 eq) was added triethylborane (TEB, 1.2 eq, neat) under argon. The mixture was heated to 60 °C and kept at the temperature for one hour before it was raised to 120 °C for one more hour. An orange to red liquid was obtained. The volatiles were removed under vacuum, and the residue was purified by vacuum factional distillation to yield the complex. There is no limitation regarding the level of vacuum, but typically, the vacuum was at less than 30 mmHg and can be as low as O.OlmmHg.

Comparative Example 1. Ethyl acetylacetate-TEB chelate.

[0034] Ethyl acetylacetate-TEB chelate: Ή NMR (CDCh, 300 MHz) d 4.89 (=C H-

C=0, s, 1H), 4.27 (0=C-0-CH ₂ , q, 2H), 1.97 ( CH3-C=0 , s, 3H), 1.31 (O-CH2CH3, t, 3H), 0.73 (B-CH2CH3, t, 6H), 0.38 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 12.17 ppm. Boiling point: 48-50 °C/0.4 mmHg. Yield: 12.75 g (83%).

Comparative Example 2. Methyl acetylacetate-TEB chelate.

[0035] Methyl acetylacetate-TEB chelate: Ή NMR (CDCh, 300 MHz) d 4.70

(=CH-C=0, s, 1H), 3.41 (-0-CH3, s, 3H), 1.70 (CH ₃ -C=C, s, 3H), 0.46 (B-CH2CH3, t, 6H), 0.09 ( B-C//2CH3, q, 4H) ppm; Yield: no attempt was made to separate this compound. Comparative Example 3. Allyl acetylacetate-triethylborane chelate.

[0036] Allyl acetylacetate-triethylborane chelate: ¹ H NMR (CDCh, 300 MHz) d

5.56-5.66 (0-CH ₂ -CH=CH ₂ , m, 1H), 4.90-5.10 (0-CH ₂ -CH=C/¾, m, 2H), 4.72 (C=CH, s, 1H), 4.37-4.43 (0-CH2-CH=CH ₂ , m, 2H), 1.70 (CH ₃ -C=0, s, 3H), 0.44 (B-CH2CH3, t, 6H),

0.07 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 12.32 ppm. Boiling point: 53-55 °C/0.5 mmHg. Yield: 10.93 g (71%). Comparative Example 4. Preparation and evaluation of boron complexes with O,N-chelating ligand.

[0037] To a suspension of glycine (0.75 g, 10.0 mmol) in dry tetrahydrofuran (THF)

(10 mL) at 0 °C under argon was added triethylborane (neat, 12.0 mmol, 1.7 mL) dropwise under vigorous stirring. The mixture was stirred overnight under argon. Glycine gradually dissolved and a clear solution was obtained. THF was evaporated under vacuum to afford a white solid (1.13 g, 80% based on glycine). ¾ NMR (DMSO-d ₆ ) d 3.62 (m, 2H), 0.68 (t, 6H), 0.22 (q, 4H) ppm.

[0038] The complex is freely soluble in dimethyl sulfoxide (“DMSO”), fairly soluble in THF and acetonitrile, and slightly soluble in water. The complex cannot be decomplexed by glacial acetic acid. Aqueous solution of the complex decomposed overnight as indicated by ¹ H NMR.

Preparation Method Practical Examples. Preparation of boron- complexes from

methoxydiethylborane (MDEB).

General procedure.

[0039] To neat acetylacetate (1 eq) was added MDEB (1.2 eq) in THF (optional) under argon. The mixture was stirred overnight at room temperature to give an orange to red liquid. The volatiles were removed under vacuum, and the residue was purified by vacuum factional distillation to yield the complex.

Example 1. Ethyl Trifluoroacetylacetate-MDEB chelate.

[0040] Ethyl Trifluoroacetylacetate-MDEB chelate: ¹ H NMR (CDCh, 300 MHz) d

5.40 (=CH-C=0, s, 1H), 4.30 ( C=0-0-CH ₂ , q, 2H), 1.33 (O-CH2CH3, t, 3H), 0.71 (B- CH2CH3, t, 6H), 0.42 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 14.67 ppm. Boiling point: 45-47 °C/5 mmHg. Yield: 4.02 g (37%).

Example 2. Hexafluoroacetylacetone-MDEB chelate.

[0041] Hexafluoroacetylacetone-MDEB chelate: ¹ H NMR (CDCh, 300 MHz) d 6.30

(C=CH, s, 1H), 0.83 (B-CH2CH3, m, 10H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 17.39 ppm. The product decomposed during distillation.

Example 3. 2-[(2-methylacryloyl)oxy]ethyl-3-oxobutanoate-MDEB chelate.

[0042] 2-[(2-methylacryloyl)oxy]ethyl-3-oxobutanoate-MDEB chelate: ¹ H NMR

(CDCh, 300 MHz) d 5.72 (0=C-CH=C/¾, m, 1H), 5.22 (0=C-CH=C/¾, m, 1H), 4.66 (HO- CR=CH, s, 1H), 4.00 (O-CH2-CH2-O, t, 4H), 1.91 ( CH3-C=0 , s, 3H), 1.64 (CH3-CH=CH ₂ , m, 3H), 0.36 (B-CH2C//3, t, 6H), 0.0 (B-C// ₂ CH ₃ , q, 4H) ppm; Yield: the compound polymerized before distillation.

Example 4. Acetylacetone-MDEB chelate.

[0043] Acetylacetone-MDEB chelate: Ή NMR (CDCh, 300 MHz) d 5.37 (C=CH, s, 1H), 1.99 (CH ₃ -C=0, s, 6H), 0.69 (B-CH ₂ CH ₃ , t, 6H), 0.33 (B-CH ₂ CH ₃ , q, 4H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 13.08 ppm. Boiling point: 57-59 °C/3 mmHg. Yield: 9.67 g (83%).

Example 5. Ethyl Diacetylacetate-MDEB chelate.

[0044] Ethyl Diacetylacetate-MDEB chelate: Ή NMR (CDCh, 300 MHz) d 4.02

(0-CH2, q, 2H), 2.18 (CH ₃ -C=0, s, 6H), 1.09 (O-CH2CH3, t, 3H), 0.47 (B-CH2CH3, t, 6H), 0.14 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCb, 96.25 MHz) d 10.38 ppm. Boiling point: 70-75 °C/0.22 mmHg. Yield: 3.07 g (83%).

Example 6. Trifluoroacetylacetone-MDEB chelate.

[0045] Trifluoroacetylacetone-MDEB chelate: Ή NMR (CDCh, 300 MHz) d 5.79 (C=C H, s, 1H), 2.19 ( CH ₃ -C=0 , s, 6H), 0.68 (B-CH2CH3, t, 6H), 0.41 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 13.43 ppm. Boiling point: 88-92 °C/28 mmHg. Yield: 6.40 g (74%).

Example 7. 2-Acetylcyclohexanone-MDEB chelate.

[0046] 2-Acetylcyclohexanone-MDEB chelate: Ή NMR (CDCh, 300 MHz) d 2.06

(CH2CH2, m, 4H), 1.83 (CH ₃ -C=0, s, 3H), 1.44 (CH2CH2, m, 4H), 0.48 (B-CH2CH3, t, 6H), 0.10 (B-CH ₂ CH ₃ , q, 4H) ppm; ⁿ B NMR (CDCh, 96.25 MHz) d 9.49 ppm. Boiling point: 75-76 °C/0.25 mmHg. Yield: 3.66 g (82%). Example 8. Benzyl-1, 3-butanedione-MDEB chelate.

[0047] Benzyl- 1,3-butanedione-MDEB chelate: Ή NMR (CDCb, 300 MHz) d 7.84-

7.94 (Ce¾-, m, 2H), 7.49-7.58 (Ce¾-, m, 1H), 7.37-7.47 (Ce¾-, m, 2H), 6.05 (C=CH, s, 1H), 2.08 ( CH ₃ -C=C , s, 3H), 0.83 (B-CH2CH3, t, 6H), 0.53 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCb, 96.25 MHz) d 10.99 ppm. Boiling point: 134-135 °C/0.7 mmHg. Yield: 11.55 g (68%).

Example 9. Dibenzyl- 1,3-propanedione-MDEB chelate.

[0048] Dibenzyl- l,3-propanedione-MDEB chelate: ¹ H NMR (CDCb, 300 MHz) d 7.89-8.06 (C ₆ H5-, m, 4H), 7.47-7.57 (C6H5-, m, 2H), 7.35-7.47 (Ce¾-, m, 4H), 6.79

(C=CH, s, 1H), 6.70 (C=CH, s, 1H), 1.05 (B-CH2CH3, t, 6H), 0.80 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCb, 96.25 MHz) d 10.17 ppm. The product was obtained by recrystallization from ethanol and diethyl ether. Yield: 6.30 g (97%).

Example 10. l-Benzyl-4,4,4-trifluoro-l,3-butanedione-MDEB chelate.

[0049] l-Benzyl-4,4,4-trifluoro-l,3-butanedione-MDEB chelate: ¹ H NMR (CDCb,

300 MHz) d 7.92-8.00 (Ceft-, m, 2H), 7.52-7.60 (Ceft-, m, 1H), 7.40-7.51 (Ce¾-, m, 2H), 6.49 (C=CH, s, 1H), 0.88 (B-CH2CH3, t, 6H), 0.66 (B-CH2CH3, q, 4H) ppm; ⁿ B NMR (CDCb, 96.25 MHz) d 13.95 ppm. Boiling point: 53-55 °C/0.3 mmHg. Yield: 5.35 g (81%).

Example 11. Tropolone-MDEB chelate.

[0050] Tropolone-MDEB chelate: Ή NMR (CDCb, 300 MHz) d 7.42-7.57 (C ₅ H ₅ -, m, 2H), 7.12-7.26 (C5H5-, m, 2H), 6.95-7.09 0.65 (B-CH2CH3, t, 6H), 0.40 (B-C /2CH3, q, 4H) ppm; ⁿ B NMR (CDCb, 96.25 MHz) d 17.89 ppm. No attempt was made to separate this compound.

[0051] When monoketo compounds such as 4-Methyl-4-hydroxy-2-pentanone or

Methyl salicylate, other di-functional compounds such as 2,4-pentanedione dioxime, 1,4- Anhydroerythritol, or catechol were used, there was no reaction observed under the experimental condition.

Summary of Preparation Method

Addition Polymerization

Example 12. Polymerization test using the boron complex initiator.

[0052] The prepared boron complex was tested for its initiator activities. To 97.0 parts of a (meth)acrylic monomer was added 3.0 parts of initiator prepared as in Example 4 (Acetylacetone-MDEB chelate ). The mixture was thoroughly mixed and was thermally cured in an oven (140 °C). Results were summarized in table below. The boron complex was active as an initiator.

Example 13. Shelf-life of mixtures of monomers with borane complex initiator.

[0053] All the boron-acac complexes were assessed for in-air stability by paper smear test. In a typical example, a complex was smeared on a piece of Kimwipes tissue using a metal spatula. If the smeared spot did not turn dark or char in 0.5 hours at 2l±l °C and 50% relative humidity, the compound was deemed air-stable.

[0054] To 97.0 parts of a (meth)acrylic monomer was added 3.0 parts of a boron- complex initiator. The mixture was thoroughly mixed and stored at room temperature. The samples were visually inspected periodically for the cure progress.

-: no cure was observed in 129 days.

+ (time): the time soft or hard gel appeared the test mixture.

Example 14. TGA and DSC results of boron complex initiators.

[0055] TGA and DSC provided two pieces of important information about the boron-acac complexes. They revealed the ambient stability of the initiators. The higher onset temperatures in TGA and DSC mean better storage stability and shelf-life.

Additionally, the DSC helped establish the thermal stability and the initiation temperature of the initiators. The higher the DSC onset temperature, the more thermally stable an initiator. Finally, TGA and DSC results also gave clues about the storage stability of the boron-acac complexes with (meth)acrylic monomers in a one-part formulation. If the initiator is more thermally stable, it is likely to yield longer shelf- life for formulations. The preparation method of invention did not adversely affect the initiator performance, stability or quality.

*: Negative peak INDUSTRIAL APPLICABILITY

[0056] The method to prepare a stable boron complex under a mild condition is suitable for various industrial applications, as the boron complex obtained by this method is useful as activation initiator and not distinguishable from the boron complex prepared under the conventional, more drastic conditions. The addition polymerization reaction is known and widely applicable, and such polymerization facilitated by the boron complex obtained by the presently described method is advantageous because of the long shelf-life even though polymerization occurs quickly once initiated.