Synthesis, Characterisation and Evaluation of Alkyl 2-Bromoacrylates as Adhesives

Eur. Polym. J. Vol. 29, No. 10, pp. 1323-1328, 1993 Printed in Great Britain 0014-3057/93 $6.00+0.00 Pergamon Press Ltd SYNTHESIS, CHARACTERIZATION A N D EVALUATION OF ALKYL 2-BROMOACRYLATES AS ADHESIVES V. VIJAYALAKSHM1,J. N. RUPAVANIand N. KRISHNAMURTI* Organic Coatings and Polymers, Indian Institute of Chemical Technology, Hyderabad 500007, India (Received 20 November 1992) Abstract--Various alkyl 2-bromoacrylates were synthesized in high purity and yield. They were characterized by spectroscopic techniques. The monomers were then formulated into aerobic adhesive compositions and their adhesive bond strengths between various surfaces were evaluated. I. INTRODUCTION The most c o m m o n alkyl 2-bromoacrylates are the methyl, ethyl and butyl compounds [1, 2]. They are useful in the synthesis of graft polymers, by treating them with halosilanes [3]. They are also useful in organic synthesis and in bactericidal and fungicidal compositions [4-6], but no work has yet been reported on their use in adhesive formulations. In this communication, syntheses of some alkyi 2-bromoacrylates are reported along with their spectral characterization. Aerobic adhesive compositions based on these monomers and their bond strengths between various metal surfaces are reported. 2. EXPERIMENTAL PROCEDURES 2.1. Synthesis of 2-bromoacrylates Synthesis of a 2-bromoacrylate consists of the sequence of reactions as shown in Scheme !. 2.1. I. Synthesis of alkyl acrylates (1). Alkyl acrylates were prepared by reacting an alcohol (0.25 mol) with acrylic acid (0.3 tool) in the presence ofp-toluenesulphonic acid (PTSA) (1% of total reactants) and benzene in an amount equal to the total weight of reactants. The water of reaction was removed azcotropically using a Dean-Stark trap. After completion of the reaction, the reaction mixture was washed with water to remove excess acrylic acid and dried with anhydrous Na2SO £ the solvent was removed and the product was distilled under reduced pressure. 2.1.2. Synthesis of alkyl 2,3-dibromopropionates (2). Alkyl acrylate (0.15 mol) was placed in a flask fitted with a stirrer, thermometer and dropping funnel. Chloroform (10 ml) was added, and the flask was immersed in an ice bath. Bromine (0.17 mol) was added slowly to the reaction mixture over a period of 3-4 hr, while the temperature was maintained below 40 °. After completion of the reaction, CHC13 was removed and the product was purified by vacuum distillation. The physical properties of various alkyl 2,3-dibromopropionates are reported in Table 1. 2.1.3. Synthesis of alkyl 2-bromoacrylates (3). Alkyl 2bromoacrylates were prepared by heating alkyl 2,3-dibromopropionate (0.04 mol) with quinoline (0.057 mol) at 100° for 30 min under N v The reaction mixture was then dissolved in CHCI 3 (50ml) and this solution was washed thoroughly with 5% aqueous HC1 to remove unreacted quinoline, and then with water to remove the mineral acid. The organic layer was separated and dried, and then the solvent was removed. The physical properties of various alkyl 2-bromoacrylates are reported in Table 2. 2 2 Synthesis of bis(2-bromoacrylate) Diethyleneglycol bis(2-bromoacrylate) was prepared accordingly as shown in Scheme 2. 22.1. Synthesis of diethylene glycol diacrylate (4). Diethyleneglycol diacrylate was prepared by reacting diethylene glycol (53g, 0.5 mol) with acrylic acid (86.4g, 1.2 mol) in the presence of PTSA (1.0% of total reactants) as catalyst, and benzene (150ml) as solvent. The water formed during the reaction was removed azeotropically p-toluene sulphonic acid COOH + R'--OH D- COOR' ~ 1 Br 2 Br Br Quinoline / ~ 3 COOR' Scheme 1 *To whom all correspondence should be addressed. EPJ 29/10--D 1323 COOR' 1324 V. VIJAYALAKSHMI et "~"~ COOH+HO ~ O ~ o ~-~°~o~°'~ PTSA ~- H aL O 4 O Br Br 2 B r Br ~ o O ~ O ~ O ~ 5 B r O aco .o o Br Br A 0 6 0 Scheme 2 through a Dean-Stark trap. After completion of reaction, the reaction mixture was washed and dried; the solvent was removed and the product was distilled under reduced pressure. 2.2.2. Synthesis of diethyleneglycol bis(2,3-dibromopropionate) (5). To diethyleneglycol diacrylate (18.9 g, 0.093 tool) and chloroform (20ml), bromine (29.7g, 0.186mol) was added slowly. The product was purified by vacuum distillation, as described for the synthesis of alkyl 2,3-dibromopropionates (yield 85%, b.p. 200 ° at 2mmHg, n~ 1.5320, d4 as 1.9100). 2.23. Synthesis of diethyleneglycol bis(2-bromoacrylate) (6). Diethyleneglycol bis(2-bromoacrylate) was prepared by heating diethyleneglycol bis(2,3-dibromopropionate) (13.78g, 0.026mol) and quinoline (6.84g, 0.053mol) in CHCI 3 (20 ml) at 100° for about I hr under N2, and then processed as described for 2-bromoacrylates (yield 75%, n~ 1,5128, d43s 1.6112). 2.3. Characterization of alkyl 2-bromoacrylates by spectroscopy 2.3.1. tH-NMR spectroscopy. In the ~H-NMR spectra of alkyl 2,3-dibromopropionates, the two methylene protons adjacent to 'Br' (Br---CH2---CHBr---COOR) appeared as distorted triplet at 6 3.8~3.95 and double doublet (dd) at 3.60-3.70 and the proton on carbon attached to 'Br' and 'COOR' (Br---CH2----CHBr--COOR) appeared as 'dd' at 6 4.36--4.66ppm. When alkyl 2,3-dibromopropionate was converted to alkyl 2-bromoacrylate, the above three peaks disappeared and two doublets at 6 6.87 and at 6 6.25 appeared for the two protons (CH~----C(Br)---COOR) as shown in Scheme 3. In the ~H-NMR spectra of alkyl 2-cyanoacrylates, the two terminal protons appeared at lower field than for alkyl 2-bromoacrylates, because of the greater electron-withdrawing nature of CN. The chemical shifts of other protons in the alkyl group for all 2-bromo-acrylates prepared are reported in Table 3. The IH-NMR spectrum of 2-(l-methoxy propyl)bromoacrylate is shown in Fig. 1. 2.3.2. ~JC-NMR spectroscopy. The three characteristic peaks for the three carbons at 161.24 (H2C----C(Br)-~OOR), 130.34 (CH~---C(Br)--COOR), and 121.58 (H2C----C(Br)COOR) ppm appeared in the 13C-NMR spectra of 2-bromoacrylates. The chemical shifts of other carbons are reported in Table 4. The 13C-NMR spectra of bromoethyl 2-bromoacrylate, and diethyleneglycol bis(2-bromoacrylate) are shown in Figs 2 and 3, respectively. 2.3.3. Mass spectroscopy. The most important diagnostic peaks for alkyl 2-bromoacrylates are shown in Scheme 4. In alkyl 2-bromoacrylates, the base peak appeared at m/z 135, but in benzyl and 2-(1-methoxy propyl)bromoacrylates the base peak was R + . Other important peaks are given below: Methyl m/z: 164, 166 (M + ), 84 (M + - H B r ) Bromoethyl m/z: 256, 258, 260 (M + ), 69 [(M + - - ( R - n B r ) ] Benzyl re~z: 240, 242 (M + ), 91 (Tropilium ion) Cyclopentyl m/z: 218, 220 (M + ), 138 (M +--HBr), 69 (CsH9+ ) 2-(1-methoxy propyl) m/z+222, 224 (M ++), 45 (CHEOCH 3), [CH(CH 3) CH2OCH3], 142 [M + - H B r ] , 177 (M +--CH2OCH3). Diethyleneglycol bis(2-bromoacrylate) : m/z: 177, 179 (M+---CsH6OBr), + 79, 81 (Br). The mass spectrum of benzyl 2-bromoacrylate is shown in Fig. 4. Table I. Physical properties of the alkyl 2,3-dibromopropionates Surface Density tension Alkyl 2,3Yield Boiling point d~ 8 at 38° dibrornopropionates (%) °C(mmHg) (kg/m3 x 103) (N/m x 10a) Methyl 80 160-165 (25) 1.9167 40.10 Bromoethyl 75 140-141 (8) 2.0730 48.00 Bcnzyl 78 160-161 (7) 1.6527 41.50 Methoxy 2-propyl 85 139-140 (I 5) 1.6200 38.50 2,3-Dibromo propyl 83 160-161 (10) 2.2129 50.50 Cyclopentyl 90 140-142 (7) 1.6518 41.20 Acetoxy ethyl 88 180-181 (20) 1.6758 42.05 Refractive index n~ 1.5082 1.5368 1.5573 1.4880 1.5643 1.5128 1.4839 Alkyl 2-bromoacrylates H Br / H H Table 2. Physical properties of the alkyl 2-bromoacrylates Br \c=c / / \ \ COOR H g 6.25 Surface Alkyl 2-bromoacrylates COOR Methyl Bromoethyl Benzyl Methoxy 2-propyl 2-Bromoallyl Cyclopentyl Acetoxy ethyl c5 6.87 Scheme 3 m/z Fragment 1325 Density tension Yield d~8 at 38° (%) (kg/m 3 x 103) (N/m x 10J 70 65 68 68 60 80 65 1.6614 1.4794 1.7058 1.4592 2.0216 1.5028 1.6102 28.10 28.00 29.15 26.10 35.50 29.20 30.21 Refractive index n~ 1.4356 1.5140 1.5182 1.4740 1.4387 1.4523 1.4268 + H2C ~ Br 105, 107 (Isotopic peak) O C -- 133, 135 (Isotopic peak) Table 3. IH-NMR (6 ppm) values of alkyl 2-bromoacrylates Alkyl (R) ~ ppm values of alkyl protons jBr H2C~-- C ~ H2C= H2C= jBr C~ + COO /Br C~ Methyl Bromethyl Benzyl Methoxy 2-propyl 149, 151 (Isotopic peak) Cyclopentyl Acetoxy ethyl Bis (2-bromoacrylate) 162, 164 (Isotopic peak) + COOCH 2 3.81 (s, 3H) 3.47-3.62 (t, 2H, 6),4.53-4.56(t, 2H, 6.5) 5.21 (s, 2H), 7.37 (s, 5H) 1.18-1.27 (d, 3H, 6.25), 3.35(s, 3H) 3.43 (d, 2H, 7.5),4.93-5.25 (m, IH, 6) 1.82-1.85 (m, 8H, 6.1), 5.18-5.26(m, IH, 6) 2.12 (s, 3H), 3.62-4.37 (m, 4H, 6.2) Diethyleneglycol 3.66-3.78 (t, 4H, 6), 4.25-4.37 (t, 4H, 5.5) The figures in parentheses are type of signal, No. of protons, and J values in Hz. Scheme 4 After complete addition, the reaction mixture was stirred at 80-90 ° for 1 hr. Cone. H2SO4 (0.3 g) was added to neutralize MgCO 3. The reaction mixture was cooled and washed with water until neutral. After adding CH3OH (100 ml), a fine powder of alkyl 2-bromoacrylate polymer was obtained. Suspension stabilizers such as MgCO3 were added to prevent the particles coalescing as the reaction proceeded. 2.4. Suspension polymerization MgCO3 (0.4 g) and distilled water (100 ml) were taken in a flask fitted with magnetic stirrer, thermometer, dropping funnel and condenser and heated to 80 °. Benzoyl peroxide (0.05 wt%) was dissolved in monomer alkyl 2-bromoacrylate (10 g) and added dropwise to the contents of the flask. 3.35 .H\ I / C d H 2 - O - ~H 3 Br C"C--C--O--CH bH / II O c \CH3 f t. 27 kle C 5I 8.0 7.0 I 6.0 I 5.0 I 4.0 I 3.0 I 2.0 .0 6 ppm Fig. 1. ]H-NMR spectrum of 2-(l-methoxy propyl) bromoacrylate. L I 0.0 V. VIJAYALAKSHMIet al. 1326 Table 4. 13C-NMR(dl ppm) values of 2-bromoacrylates Alkyl (R) di ppm values of carbons in alkyl chain Methyl 53.26 Bromethyl 27.95, 65.36 Bcnzyl 68.14, 128.07, 128.40, 128.51,134.99 Methoxy 2-propyl 16.20, 59.00, 71.97, 74.56 Cyelopentyl 31.87, 46.19, 66.91 Bis (2-bromoacrylate) Diethyleneglycol 68.70, 65.45 whereas in 2-bromoacrylates, carbanion is stabilized only by the COOR group, as shown in Scheme 5. This consideration explains the faster polymerization of 2-cyanoacrylates. 2.5. Evaluation of 2-bromoacrylates as aerobic adhesives Aerobic acrylic adhesives are two-part systems, which offer the advantages of rapid cure and high strength along with good shelf life. The term "aerobic" is used generally to refer to a diminished sensitivity to air inhibition of thick layer curing properties and the ability to cure between two surfaces regardless of the presence or absence of air at room temperature, resulting in tough, elastomeric bonds. These adhesives are generally premix and require the use of pre-applied activators to initiate the cure mechanism. This property is distinct from anaerobic adhesives, which are intrinsically single component products and will polymerize in the absence of air. Various aerobic adhesive compositions were made from alkyl 2-bromoacrylates and diethyleneglycol bis(2-bromoacrylate). Their tensile bond strengths were determined using the Hounsfield tensiometer between metal surfaces according to ASTM D-897-78 specifications. A two-part adhesive composition consists of a solution of acrylic monomer, polymerization catalyst and inhibitor in the first part, and an activator in the second part. A typical composition is given below: Table 5. GPC data for poly (alkyl 2-bromoaerylates) Poly(alkyl 2-bromoacrylates) ,~w Mn MWD Methyl 38,200 5120 7.45 Bromoethyl I0,300 2460 4.18 Cyclopcntyl 76,060 52,290 1.45 Methoxy 2-propyl 50,250 50,000 10.04 Benzyl 24,480 1780 13.77 2-Bromoallyl 50,030 3520 14.22 Polymers from the monomeric 2-bromoacrylates were prepared by following the above procedure, and their weight-average molecular weights (Mw), number-average molecular weights (20~), and molecular weight distributions (MWD) were determined by gel permeation chromatography (GPC) (see Table 5). From these data, it is clear that poly(cyclopentyl 2-bromoacrylate); polybromethyl and polymethyl(2-bromoacrylates) have narrow molecular weight distributions. Alkyl 2-bromoacrylates are not as reactive as alkyl 2cyanoacrylates. The difference is attributed to the difference between cyano and bromo groups in electron-attracting nature. The carbanion formed in anionic polymerization of 2-cyanoacrylates is stabilized by CN and COOR groups, Br H2C = base C \ Ingredients Part one Part two Br / P- B CH2-- C-- COOR I O-- C~N ~- B-- CH2-- C ~ x~- COOR C--OR II 2-Cyanoacrylate 0 // B -- CH 2 - C---~- N C \ C--OR II 0 Scheme 5 Br C--OR II base \ B--CH2--C 0 CN H2C = ~ / \ C--OR 2-Bromoacrylate / C ~ Parts by weight Monomer (2-bromoacrylate) Saccharin Cumene hydroperoxide Hydroquinone Activator (N,N-dimethyl p-toluidine) B--CH2--C / \ C~N C--OR I O-- 5.000 0.200 0.050 0.025 0.025 Alkyl 2-bromoacrylates 1327 I;5,3G 2?.SS ,r. Br 5 131.22 I o 120.32 16t.01 I 220 I I I 200 180 160 140 I I I I I I 100 120 80 60 40 20 0 6 ppm Fig. 2. ~3C-NMR spectrum of bromoethyl 2-bromoacrylate. Using the quoted quantities, aerobic adhesives were prepared using methyl, bromethyl, 2-bromoallyl, cyclopentyl, methoxy-2-propyl, acetoxyethyl and benzyl 2-bromoacrylates and ethyleneglycol bis(2-bromoacrylate). First, the activator was applied as a thin film to one of the surfaces to be joined and the adhesive was applied to the other mating surface. The two surfaces were brought together and bonded. The glued specimens were left for 24 hr Br at room temperature (25°), and tested for resistance to a uniform direct pull. Setting time and tensile shear strength of aerobic adhesives are given in Table 6. It has been found that, with increase in the amounts of saccharin and dimethyl p-toluidine (DMPT), the set-time decreased. Too much activator lowered the bond strength. The quantity of stabilizer could be increased to a certain extent to make the compositions more stable. In all cases, Br o o G8.70 SS.&S r, 130.81 i ,.,.,2 3 I JL 120,|$ 12 I I I I I I I I 160 140 120 100 80 60 40 20 8 ppm Fig. 3. ~3C-NMR spectrum of diethyleneglycol bis(2-bromoacrylate). 1328 V. VIJAYALAKSHMIet al. ~100 l,- Z kd 80 R" CzHs _~ ~o I,- -, iJl 20 0 I &O II I I 60 I Lf,j ll: '* I 80 I i ' c=c-T, T o ] " W Ore.-I IR Sl I 100 I D t -.~m ' ,/;~ F,C=;-' I--*,, A * O* ~--II --' 113 C L!3*S3 I 120 I I l&0 N-Or ~ I 161 i" "'1" 160 I 180 I I 200 I i 220 ! M _ it0 II 2&0 i I 260 I 280 mlz Fig. 4. Mass spectrum of benzyl 2-bromoacrylate. Table 6. Setting times and tensile strengths of alkyl 2-bromoacrylate bonds between metal surfaces Tensile strength (MPa) Alkyl 2Setting bromoacrylate time (min) M-M S--S A-A C-C B--B Methyl 15 7.03 5.31 2.28 2.20 3.82 Acetoxy ethyl 18 4.92 4.20 2.42 1.84 2.90 Methoxy 2-propyl 20 5.64 4.24 2.64 1.94 3.24 2-Bromoethyl 12 4.42 3.78 2.08 1.98 3.18 Cyclopentyl 20 3.79 3.74 1.82 1.80 2.28 Benzyl 15 2.28 1.52 1.04 1.02 1.50 2-Bromo allyl 10 9.31 6.41 4.24 3.22 4.28 Bis (2-bromoacrylate) Diethyleneglycol 10 8.08 6.04 3.04 2.52 3.79 M, Mild steel; S, stainless steel; A, aluminium; C, copper; B, brass. the adhesive bond strength was highest between mild steel surfaces followed by stainless steel, brass, aluminium and copper. The aerobic adhesive composition based on 2-bromoallyl 2-bromoacrylate and diethyleneglycol bis(2-bromoacrylate) gave highest adhesive bond strength between the metal surfaces, because of the crosslinking between the unsaturated groups. 3. CONCLUSIONS Alkyl 2-bromoacrylates were f o u n d to be useful in aerobic adhesive formulations, giving good adhesive b o n d strengths between various metal surfaces. O f these aikyl 2-bromoacrylates, 2-bromoallyl 2-bromoacrylate a n d diethyleneglycol bis(2-bromoacrylate) gave stronger adhesive b o n d s because o f the crosslinking taking place in the polymers. Acknowledgement--VV and JNR fellowship. thank CSIR for a REFERENCES I. C. S. Marvel, J. Dec, H. G. Cooke Jr and J. C. Cowan. J. Am. chem. Soc. 62, 3495 (1940). 2. M. A. Askarov, K. A. Aylylanov and A. B. Alovitdinov. Uzbeksk. Khim. Zh. 7(5), 50 (1973) Chem. Abstr. 60, 5326 (1974). 3. A. A. Vardapetyan, D. S. Khachatran, G. A. Panosyan and N. M. Morlyan. Zh. Org. Khim. 22, 2262 (1986). 4. P. K. Dhal, G. B. Babu and J. C. W. Chien. Polym. Degrad. Stab. 18, 1 (1987). 5. J. D. Buckman, S. J. Buckman, B. S. Johnson and J. D. Dera. U.S. Pat. 4 293 559 (1981). 6. H. Helmut, H. Adoff and K. Guenter. Ger. Pat. 2 105 174 (1971).