The polymerization of acrylates using a combination of a carbonyl compound and an amine as a photo-initiator system

Eur. Polyra. J. Vol. 18, pp. 597 to 606, 1982

0014-3057/82/070597-10503.00/0 Copyright © 1982 Pergamon Press Ltd

Printed in Great Britain. All rights reserved

THE POLYMERIZATION OF ACRYLATES USING A COMBINATION OF A CARBONYL C O M P O U N D AND AN AMINE AS A PHOTO-INITIATOR SYSTEM R. S. DAVIDSON and J. W. GOODIN Department of Chemistry, The City University, Northampton Square, London, EC1V 0HB (Received 10 November 1981)

Abstract--Photo-initiator systems for the polymerization of acrylates, based on a mixture of an aryl ketone and an ~,co-diaminoalkane, have been investigated. Rate constants for the photoreduction of fluorenone by ct,co-diaminoalkanes have been evaluated; it was found that CH3N groups are more relative than CHaCH2N groups. The relative rates of photo-oxidation of ct,co-diaminalkanes sensitised by benzopbenone have been determined. Surprisingly, little correlation exists between the susceptibility of an amine towards oxidation and its ability to reduce excited carbonyl groups. Several mixtures composed of an aryl ketone and an ~t,to-diaminoalkane were found to initiate the polymerization of methyl methacrylate. The efficiency of initiation appears to be related to the efficiency of reaction of the triplet carbonyl compound with the amine. The efficiency of a particular combination of an aryl ketone and an amine to cure films of acrylate oligomers is also governed to some extent by the efficiency of reaction of the triplet carbonyl compound with the amine. However, the structure of the amino alkyl radical produced in the initiation reaction appears to be of greater importance in determining the efficiency of polymerization. Radicals of the type R2NCHCH2OR were found to be highly efficient.

Earlier work has established that excited carbonyl compounds can react with tertiary amines via an electron transfer process [5]. A feature which distinguishes the reduction of carbonyl compounds by amines from reductions by alkanes and ethers, is that reaction occurs irrespective of whether the carbonyl

The u.v. initiated polymerization of acrylates is the basis of many u.v. curing of inks [1]. A number of initiator systems have been developed including the use of (i) the a-cleavage reactions of benzoin ethers [2], (ii) hydrogen abstraction from donor molecules [3] and (iii) photo-induced electron transfer reactions [4].

(i)

PhCO' C(OR)~Ph Ph~O + M

(ii) Ph2CO + ~

(iii)

Ar2C O + R3N

Ph~O + Phi(OR) 2

hv = hF

hv

= Ph2~OH

+ •

~

Ar2~O--

+

R2NCH 2 R l

1

Ar 2 ~OH

R~N~HRt + M M:Monomer

:

RICH2 -" R

+ R 2 NC°H R t

Pt"

P:': First chain

This paper describes an investigation of the factors which can affect the efficiency of the photo-induced electron transfer initiator systems.

carrying

radical.

excited state is an mz* or nn* state. Thus such compounds as fluorenone, xanthone, 1-naphthaldehyde, 2-acetophenone [6] and 4-aminobenzophenone [7] 597

598

R.S. DAVIDSON and J. W. GOODIN

are reduced by tertiary amines. In principle therefore a wide variety of carbonyl compounds, in conjunction with tertiary amines, can be used as photo-initiators. The identity of the radical intermediates produced in the reaction of ketones with tertiary amines has been examined by flash photolysis a n d electron spin resonance spectroscopy [8]. Kinetic studies have shown that m a n y of the reactions have high rate constants [9] a n d as a consequence, if the amine concentration is sufficiently high, the reaction of the triplet carbonyl c o m p o u n d with amine can successfully compete with deactivation by oxygen. In the presence of oxygen, oxygenation of the amines occurs [10]. Since amines are also photo-oxidized by singlet oxygen [11] to give the same a m i n o alkyl radical as is produced by attack of a triplet carbonyl c o m p o u n d upon an amine viz. Ar2COT + R2NCH2R'----, Ar2COH + R 2 N C H R ' 1A~O2 + R 2 N C H 2 R ' - - , HO2 + R2N(~HR' R 2 N C H R ' + M --, P'I photo-initiator systems containing a carbonyl comp o u n d a n d an amine are not deactivated by oxygen. Thus, with these systems, there is no need for blanketing the materials to be printed by a n inert gas such as nitrogen. One of the less attractive features of the carbonyl amine systems is their tendency to form cross coupled products [12] (probably as a result of cage recombination) a n d of d i s p r o p o r t i o n a t i o n of the initially produced radicals [13, 14]. Such reactions decrease the efficiency of initiation. We now describe our attempts to elucidate the factors which govern the efficiency of a carbonyl comp o u n d - a m i n e photo-initiator system. Both m o n o - and polyamines have been used. The use of polyamines was investigated because these c o m p o u n d s are particularly efficient at forming excited complexes with aromatic h y d r o c a r b o n s [15] a n d dyes [16] a n d therefore should react efficiently with excited carbonyl compounds.

EXPERIMENTAL Melting points, uncorrected, were determined on either a Kofler Block or a Gallenkamp MFB-600. i.r. Spectra for samples as thin films or KBr discs were obtained with Perkin-Elmer 237 and 457 grating spectrophotometers. ~H n.m.r, were recorded for solutions in CDCI3 (with tetramethylsilane as internal standard) on a Varian T60 and a Jeol PS100 spectrometer, 13C and 31p n.m.r, were recorded on a Jeol FX60 n.m.r, spectrometer. Mass spectra were recorded using a Micromass MS30/76 Kratos instrument. H.p.l.c. was carried out using a Waters model 6000A solvent delivery system and a Cecil CE2012 variable wavelength u.v. monitor. Elemental analyses were determined by C,H,N. Analyses Ltd at T.C.U. using a Carlo Erber Model 1106 C, H and N analyser.

Determination of the rate constants for photoreduction of fluorenone by amines A stock solution of fluorenone (0.897g) in benzene (500 cm 3) was prepared. 20 cm 3 of the stock solution was mixed with 5 ml of a benzene solution of the amine. Three concentrations of amine were employed (0.1, 0.05 and 0.01 mol equivalents of amine in 5 cm 3 of benzene). The benzene solution of the amine and fluorenone was transferred

to a "pyrex" tube of dimensions 240 x 13 mm. The solution was degassed by purging with dry N2 for 0.5 h. An aliquot (1 cm 3) was removed for analyses to give a t = 0rain value. The tube contents were irradiated in a Rayonet reactor (Southern New England Ultra-violet Corporation) fitted with back-light lamps (emission 330-390n.m.) and a merry-go-round apparatus. Several tubes containing various amounts of the amine and tubes containing fluorenone and triethylamine as an actinometer [24] were simultaneously irradiated [22]. Quantum yields were calculated from plots of rate of reduction of the ketone against time and were linear. Further aliquots of 1 cm 3 were taken at intervals and the concentration of fluorenone determined by HPLC (Column: Hypersil ODS 5, Solvent; methanol; water 75:25v/v; detection wavelength 294 n.m.). From the rates of reduction, the quantum yields of photoreduction were obtained. Using the relationship [25] : 1 q~

1 + - -kd a ak,(amine)

a = yield of triplets and for benzene solution is taken as 126; kd = rate constant for deactivation of triplet fluornenone. A value of 2.2 × 105 sec 1 was used [24].

Photo-oxidation studies The equipment used has been described previously [11]. Oxygenations were carried out using a benzene solution of benzophenone, (optical density 0.4 at 350 n.m.--16 ml) containing the amine (4 x 10 -4 mol). A medium pressure Hg lamp (250 W) was used as the irradiation source.

Bulk polymerization of methyl methacrylate The general procedure is illustrated by the method employed for the benzophenone-triethanolamine system. Benzophenone (2.5 x 10-3mol) was dissolved in MMA (20g). Triethanolamine (1.25 x 10 -3 mol) was added and the mixture thoroughly shaken. The solution was irradiated in an open-topped pyrex tube 240 × 13 mm which was circulated around a water-cooled medium pressure Hg lamp (Applied Photophysics Ltd, 450 w). On completion of irradiation, the solution was syringed into a flask containing petroleum ether (100 ml) which was vigorously agitated throughout the addition. The precipitated polymer was collected on a pre-weighed sinter, washed with petroleum ether and dried in a vacuum desiccator to constant weight. When different photo-initiators were used, the concentrations of the carbonyl compound and the amine were as those used in the example. To study the effect of 02 on the reaction, solutions were purged with argon or air as necessary for 0.5 h prior to irradiation.

Photo-initiated polymerization of thin films The varnishes and inks were cured using an MB Conveyor Drier fitted with a single Primarc medium pressure lamp having an output of 200W per inch. The conveyor speed for the polymerizations was 500' per min unless otherwise stated. Following each pass through the drier, the varnishes and inks were tested for tack-free and through-cure. To compare the effect of various amines on the rate of cure in non-pigmented systems, a varnish was made up of 70% Bisphenol "A" epoxy acrylate prepolymer, and 30% Bisphenol "A" epoxy acrylate monomer. Benzophenone (5% weight for weight) was added and the mixture heated until the benzophenone dissolved. The amines were added at a 3% level and mixed in. A layer of varnish, 38p thick was coated onto art paper and then submitted to curing. Similar experiments were performed using a 5% level of other aromatic carbonyl compounds.

The polymerization of acrylates

Synthesis

599

mic acid b.p. 100-102 ° at 0.9mm (lit. 117 ~ at 1 0 m m H g )

[35]. The diamines were prepared from the appropriate ~,todibromoalkane and diethylamine. The following example illustrates the general procedure. A mixture of 1,3-dibromopropane (20g) and diethylamine (excess) was heated under reflux for 6 hr. A white crystalline deposit formed during the heating. After cooling, water (5 ml) was added. The solution was saturated with K O H and the amine extracted with diethyl ether (3 x 15 ml). The combined extracts were dried over anhydrous MgSO,. The solvent was removed in vacuo and the products distilled under vacuum to effect purification. The spectra (*H n.m.r, and i.r.) were in accord with the proposed structures and the boiling points agreed with the literatures values 1,3-N,N,N,N-tetraethyldiaminopropane 80-82 ° at 7 mm Hg (lit. 95-100 ~ at 15 mm Hg) [27], 1,2-N,N,N,Ntetraethyldiaminoethane, 68 ° at 6.5 mm Hg (lit. 82-85 ~ at 15 mm Hg) [27], 1,6-N,N,N,N-tetraethyldiaminohexane 88 at 1 m m H g (lit. 135 ~ at 1 3 m m H g ) [28], l,lO-N,N,N,N-tetraetbyldiaminodecane, 120-122 ° at 1 m m H g (lit. 186 188 ~' at 1 6 m m H g ) 1-29] 1,12-N,N,N,Ntetraethyldiaminodecane, 140 c at 0.5 mm (lit. 125-128 at 0.01 mm Hg) [28]. 1,10-N,N,N,N-tetraethyldiaminooctane was prepared in a similar manner and found [30] to have b.p. 112-114 ° at l m m H g , 6 (CDC13), 2.4 (12Hm), 1.4 (10 H broad S), 1.0 (12 H t, J, 7 Hz), 2970, 2930, 2850, 2790, 1465, 138, 1280, 1200, 1065, 760 and 730cm -~. 1,5-N,N,N,N-tetraethyladiaminopentane could not be obtained from 1,5-dibromopentane and diethylamine. It was prepared from 1,5-N,N,N,N-tetraethylpentandioic acid diamide. Pentanedioic dichloride (20g) was added dropwise with stirring to excess diethylamine. After the addition, the mixture was heated under reflux for 18 hr. On cooling, water (20 ml) was added and the organic components extracted with diethyl ether (4 x 30ml). The extracts were dried and the solvent removed to give the crude diamide which was distilled, (b.p. 150 ° at 0.2 mm Hg, lit: 205' at 760mm Hg) [31]. The diamide (13.0 g) in ether (20 ml) was added, with stirring, to a slurry of lithium aluminium hydride (4.1 g) in ether (150ml). After the addition the mixture was heated under reflux for 4 hr. After cooling, water (caution!) was added to destroy the excess lithium aluminium hydride. The liquid portion was decanted from the alumina and the water separated off. The ethereal layer was dried and the solvent removed to give a yellow oil (7.2 g) which was distilled: 80 c at 1-5 mm (lit. 68 71': at 0.2ram) [32]. N,N,N,N,N,N-hexamethyltriethylenetetramide was prepared from triethylenetetramine. To the tetramine (10 g) in water (100 ml), formaldehyde (40~ aqueous solution 21 g) and formic acid (61.9 g) were added and the mixture heated under reflux for 6 hr. The solution was evaporated in vacuo to one third of the original volume and then saturated with KOH. A dark red oil separated and was extracted with ether (3 × 25 ml). The extracts were combined, dried and the solvent evaporated to leave a red oil which was distilled to give the desired product (7.1 g), b.p. 120-122 ~ at 6 m m (lit. 130 at 11 mm Hg) [33]. N,N,N,N,N,N,N-heptamethyltetraethylenepentamine was prepared from tetraethylenepentamine. To the pentamine (10g) in water (30ml), formaldehyde (40~o aqueous solution 28.2 g) and formic acid (73.2 g) were added. The mixture was heated under reflux for 4 hr. The solution was evaporated to half the original volume and saturated with KOH. The organic components were extracted with diethylether (3 × 30 ml). The combined extracts were dried, and the solvent removed to leave a yellow oil which distilled to give a colourless oil (7.5g): b.p. 129 ° at 3mm, [34] 6 (CDCI3) 2.5 (21H,S), 2.2 (16H,S), 3350, 2920, 2760, 1650, 1450, 1050cm- l [30]. 1,8-N,N,N,N, tetramethyldiaminooctane was prepared similarly from 1,8-diaminooctane, formaldehyde and for-

bis (2-N,N-diethylaminoethyl) ether was prepared from bis (2-hydroxyethyl) ether via the dibrorno compound. Phosphorus tribromide (20g) was added dropwise with stirring to a mixture of bis (2-hydroxyethyl) ether (7.8 g) and pyridine (5ml). The mixture was stirred for 16hr, poured into water (30 ml), and th.e yellow oil which separated was extracted with ether (2 x 25 ml). The extracts were dried and the solvent removed in vacuo to give an oil which was distilled to give bis (2-bromoethyl) ether (12.2 g) b.p. 75-77 '~ at 5 mm Hg. 6(CDC13), 3.83(4HtJ6Hz), 3.43(4HJ6Hz), 2980, 2890, 1450, 1360, 1280, 1120, 670 cm-~. The dibromo-compound was heated under reflux with excess diethylamine for 12 hr. A white crystalline mass formed and was removed by filtration. The filtrate was made basic by the addition of NaOH (5M, 30 ml). The amine was extracted with diethyl ether (2 × 30 ml). The extracts were combined, dried and the solvent removed to leave a yellow oil which was distilled to give a colourless oil (1 g) b.p. 65 ° at 0.045 mm Hg [30], 3(CDC13), 3.55(4Ht, J6Hz), 2.55 (12Hq, J6Hz) 1.0 (12Ht, J6Hz) 2960, 2860, 1450, 1380, 1290, 1205, 1120 and 1065 cm - l . 1,2-bis (2-N,N-diethylamino)ethoxy) ethane was prepared from 1.2-bis (2 hydroxyethoxy)ethane via the dibromo compound. 1,2-Bis (2-hydroxyethoxy) ethane (10g) was reacted with phosphorus tribromide (30g) as in the previous synthesis to give 1,2-bis(2-bromoethoxy) ethane (yield of crude material 12.7 g), 6(CDC13) 3.74 (4H,t J6Hz), 3.59 (4HS), 3.39 (4H,t J6Hz). The crude dibromo-compound (4.5 g) was reacted with excess diethylamine to give the diethylamino compound (3.0g), b.p. 92 ° at 0.1 mm Hg [30]. 6(CDC13), 3.5(8Hm), 2.5(12HM), 1.0 (12Ht J6Hz), 2960, 2860, 2790, 1465, 1360, 1290, 1206, 1130 and 1070 cm ~. bis (2-(N,N-diethylamino-2-ethoxy)ethyl) ether was prepared from bis ((2-hydroxyethoxy)ethyl) ethyl) ether via the dibromo compound. The hydroxy compound (29 g) was transformed into the dibromo compound using phosphorus tribromide (29.8 g). A colourless oil (26.25 g) was obtained b.p. 110-120 ° at 0.5 mm Hg. 3(CDC13) 3.7(16Hm). The title compound was prepared from the dibromo compound (6 g) by reaction with excess diethylamine. A colourless oil (4.1g) was obtained, b.p. 126 ° at 3 m m H g , 6(CDC13) 3.6(12Ht, J6Hz) 2.55(12Hq, J6Hz) 1.0 (12Ht, J6Hz). ct,(o-dimorpholinoalkanes were prepared from the appropriate ct,~o-dibromo-alkane and morpholine. Thus, 1,2-dibromoethane (5.0 g) was added to excess morpholine and the mixture heated at 70 ° for 2 hr. To the cooled reaction mixture, water (15 ml) and NaOH pellets were added. The mixture was extracted with diethyl ether (4 x 30 ml); the combined extracts were dried and the solvent removed in vacuo to give the dimorpholino compound: m.p. 69-70 ° (lit. b.p. 285 ° [36]) C, 58.37; H, 10.08; N, 14.09; Ct0H20N202 requires C60.00 H10.00, N,14.0°o), 3(CDC13) 3.7(8Hm), 2.5(12Hm), i.r. (CH2CI2), 2845, 2800, 1270, 1110, 860 c m - ~. The following compounds were prepared similarly: 1,3-dimorpholinopropane, b.p. 124-127 ° at 1.5 mm Hg (lit. 317 °) [36]; 1,6-dimorpholinohexane, m.p. 40-41' (lit. m.p. 35.5-38.5 °) [37]; 1,8-dimorpholinooctane, m.p. 44-45 c (lit. m.p. 46.5-47.5 °) [37]; 1,10-dimorpholinodecane, m.p. 47-48 ° (lit. m.p. 48-49 °) [37]; 1,12-dimorpholinododecane, m.p. 46-47 °, (C, 70.55; H, 11.68; N, 8.15; C2oH4oN202 requires C, 70.59; H, 11.76; N, 8.24°~,). 6(CDC13), 3.78(8Ht, 5Hz) 2.4(12Hm) 1.15(20Hm). 1,3-di(N-methylpiperazino)-propane. 1,3-Dibromopropane (5 g) was added, dropwise with stirring, to N-methylpiperazine (20g). Stirring was continued for 16hr. Water (10 ml) was added and the mixture made strongly basic by the addition of N a O H (10g). Extraction with ether (4 x 30 ml) followed by drying and removal of the ether

R. S. DAV1DSONand J. W. GOODIN

600

gave the crude product as a yellow oil which was distilled to give the pure material as a colourless oil (4.6 g), v.p. 130° at 0.5mmHg. (C, 64.05; H, 11.59; N, 23.01; CIaHaN4 requires C, 64.95; H, 11.74; N, 23.31%), 6(CDC13), 2.4(20Hs) 2.1(6Hs) 1.6(2Hm), 2960, 2795, 1510, 1375, 1280, 1170, 1015, 805 cm- 1. bis(2-dimorpholinoethfl)-ether. Bis(2-chloroethyl) ether (5 g) was added dropwise with stirring to morpholine (four molar excess). The mixture was heated at 80 ° for 4 hr. After cooling, water (10 ml) was added and the mixture saturated with NaOH. The amine was extracted with ether, in the usual way. The product, a colourless oil, was distilled to give the pure material (1.7g), b.p. 164 ° at 2mm Hg. (C, 58.81; H, 9.82; N, 11.48; C12H24 N20 3 requires C, 58.99; H 9.9; N, 11.47~o), tS(CDCIa) 3.6 (12Hm), 2.4 (12Hm), 2800, 1660, 1440, 1300, 1270, 1100, 915, 855cm -1. RESULTS In order to ascertain the reactivity of a wide variety of amines, the rate constants for reaction of the amines with triplet carbonyl compounds were determined. The photoreduction of benzophenone by the amines was found to be very fast and little distinction

between the amines could be made. The use of a less reactive carbonyl compound, fluorenone, was found to be suitable. Table 1 shows the rate constants for the photoreduction of fluorenone by the amines in benzene solution. The results lead to an order of reactivity of the amines towards a triplet carbonyl compound. The ability of carbonyl compounds to sensitize the photooxygenation of the amines was determined by measuring the rate of oxygen absorption when benzene solutions of the amine (at a fixed concentration: 2 x 10 -4 mol) containing benzophenone were irradiated. The results are shown in Table 2. The rate of photo-oxygenation with fluorenone as sensitizer was too low to give accurate results and therefore the comparison of reactivities is made with benzophenone as sensitizer. The efficiency of various benzophenone-amine systems to photo-initiate the polymerization of methyl methacrylate (MMA) was determined by irradiation of M M A containing benzophenone and the amine. Following cessation of illumination, the amount of

Table 1. Rate constants for the photoreduction of fluorenone in benzene by amines Amine

k.r. (sec- l)

(1) triethanolamine (2) N-methyldiethanolamine (3) N,N-dimethylethanolamine (4) 1,3-dimorpholinopropane (5) 1,6-dimorpholinohexane (6) 1,10-dimorpholinodecane (7) 1,3-N,N,N,N tetraethyldiaminopropane (8) 1,8-N.N.N.N tetraethyldiaminooctane (9) 1,10-N,N,N,N tetraethyldiaminodecane (10) 1,8.N.N.N.N.tetramethyldiaminooctane (11) 1,8-diaminooctane (12) bis (2-N,N-diethylaminoethyl) ether (13) 1,2-bis-(2-(N,N-diethylamino)ethoxy)-ethane (14) bis (2-(N,N-diethylamino 2-ethoxy)ethyl)-ether (15) N,N,N,N,N,N-hexamethyltriethylenetetramine (16) N,N,N,N,N,N,N-heptamethyltetraethylenepentamine (17) bis-(2-morpholinoethyl)ether (18) 1,3-di(N-methylpiperazino) propane R2N (CH2CH2OH) (1)-- (3

~ O

N ~

(CH2)n ~

8.37 x 1.53 x 5.61 x 1.47 x 6.15 × 1.03 x 2.41 x 1.37 x 9.19 X 4.13 x 4.92 X 7.25 x 2.75 x 1.8 x 5.62 × 1.79 x 3.69 × 2.63 x /"'-X N O

\

/

l0 s _+ 2.93 x 105 106+ 6.7 x 105 10T + 3.06 x 107 106 + 1.17 x 106 106 "k 1.94 x 106 106 _+ 0.46 x 106 107 _ 1.55 x 107 107 _ 0.38 x 107 106"+ . 0.11 x 106 107 _+ 2.69 x 107 106 q- 1.75 X 106 107 _+ 3.75 x 107 107 _+ 1.75 x l0 T 107 _+ 0.39 x 107 106 ___ 3.46 x 106 107 _ 1.02 x 107 106 "k 1.23 x 106 107 _+ 1.42 x 106 Et 2 N (CH2) n NEt~I

( 7 ) - - (9)

(4).-- (6) (19)

Me2N(CH2)I NMe2

H2N (CH2)$ NH2

(10)

(11)

Me2N (CH2CH 2 NMe)n CH2CH2NMe2

\

k

(18)

(20)(21)

/---k /---k O N CH2CH2OCH2CH 2 1~ O

(17)

N (CH2) 3 N

/

(12)--(14)

_

(15)-- (16)

MeN

Et 2 N (CH2CH20) n CH 2 CH 2 N Et 2

/

NMe

/

601

The polymerization of acrylates Table 2. Rate of photo-oxygenation of amines sensitized by benzophenone in benzene Amine

Oxygen uptake ( x 10- a cm a sec- 1)

(1) triethanolamine (2) N-methyldiethanolamine (3) N,N-dimethylethanolamine (4) 1,3-dimorpholinopropane (5) 1,6-dimorpholinohexane (6) 1,10-dimorpholinodecane (7) 1,3-N,N,N,N-tetraethyldiaminopropane (8) 1,8-N,N,N,N-tetraethyldiaminooctane (9) 1,10-N,N,N,N-tetraethyldiaminodecane (10) 1,8-N,N,N,N-tetramethyldiaminooctane (11) 1,8-diaminooctane (12) bis(2-N,N-diethylaminoethyl)ether (13) 1,2-bis(2-(N,N-diethylamino)ethoxy)ethane (14) bis(2-(N,N-diethylamino-2-ethoxy)ethyl)ether (16) N,N,N,N,N,N,N-heptamethyltetraethylenepentamine (17) bis(2-morpholinoethyl)ether (18) 1,3-di(N-methylpiperazino)propane (19) 1,2-dimorpholinoethane

030 1.55 1.45 6,60 3.45 3.00 2.90 3.50 4.15 4,55 0.20 1.40 2.55 3.80 3.35 2.00 3.85 2.35

poly MMA was measured by syringeing the solution into petroleum ether, filtering off the polymer and after drying determining its weight. The results are shown in Table 3. The relative efficiencies of a variety of carbonyl compounds were determined (see Table 4). However, it should be noted that in Table 4 no correction has been made for the different absorption characteristics of the various ketones. Since a Hg lamp was used for the reactions, the yield of polymer

does reflect how the various carbonyl compounds would behave in a u.v. curing apparatus. The effect of oxygen upon the efficiency of polymerization was also investigated (see Table 5). The efficiency of the various ketone-amine systems to photo-initiate acrylate oligomers under u.v. curing conditions was also investigated. A clear varnish was prepared from a bisphenol "A" epoxyacrylate. A preliminary study was made to investigate the optimum

Table 3. Amount of polymethyl methacrylate produced from methyl methacrylate using various amine/benzophenone mixtures Amine (1) (2) (3) (7) (20) (21)

triethanolamine N-methyldiethanolamine N,N-dimethylethanolamine

(22) (16) (4) (6)

tetraethylenepentamine

% Polymethyl methacrylate

1,3-N,N,N,N-tetramethyldiaminopropane 1,5-N,N,N,N-tetraethyldiaminopentane 1,6-N,N,N,N-tetraethyldiaminohexane (8) 1,8-N,N,N,N-tetraethyldiaminooctane (9) 1,10-N,N,N,N-tetraethyldiaminodecane N,N,N,N,N,N,N-heptamethyltetraethylenepentamine 1,3-dimorpholinopropane 1,6-dimorpholinohexane

13.6 14.6 34.0 28.7 20.3 20.5 21.0 21.0 2.3 31.9 22.8 19.5

Table 4. Amount of polymethyl methacrylate produced from methyl methacrylate using various amine/carbonyl compound mixtures Amine (4) (4) (4) (1) (1) (1) (23) (23) (23)

1,3-dimorpholinopropane 1,3-dimorpholinopropane 1,3-dimorpholinopropane triethanolamine triethanolamine triethanolamine triethylamine triethylamine triethylamine

Photoinitiator

~o Polymethyl methacrylate

benzophenone 2-isopropylthioxanthone fluorenone benzophenone 2-isopropylthioxanthone fluorenone benzophenone 2-isopropylthioxanthone fluorenone

21.0 17.5 2.2 11.9 11.0 2.5 30.2 28.0 1.1

602

R.S. DAVIDSON and J. W. GOODIN

• -Through cure Xl Track free e~ 0 O. O.

6

o • Through cure x • Track free

5

5 4 --

4

X

._J 3

•

•

3

X

X

2

Q

all

I

0

I

I

I

I

I

2

3

4

0

% THethan01amine

I

I

I

I

I

2

3

4

% 1,3 dimorpholinoprol~ne

Fig. 1. Passes to effect cure in clear varnish with varying triethanolamine content.

Fig. 2. Passes to effect cure in clear varnish with varying 1,3 dimorpholinopropane content.

amine content in the varnish when the diaryl ketone content of the varnish was kept at 5 ~ weight by weight. The results are shown in Figs 1 and 2. For utilization in a printing, the optimum amount of

amine is a balance between the greatest cure rate and the cost of additional quantities of amine. For triethanolamine, the optimum percentage is 3~o and therefore experiments with other amines were carried out at this level of loading. Various ketones were examined and the results are shown in Tables 6-9. The terms tack-free and through-cure, used in these Tables, can be related to the extent of polymerization in the system. Tack-free is the condition of the film when the surface of the printed varnish or ink has polymerized to such an extent that the surface is no longer tacky to touch. This condition is associated with the formation of long cross-linked chains at the surface of the printed material. Through-cure is the condition which accrues when the whole film consists

Table 5. Effect of oxygen upon the amount of polymethyl methacrylate produced from methyl methacrylate using triethanolamine/ carbonyl compound initiator systems benzophenone benzophenone 2-isopropylthioxanthone 2-isopropylthioxanthone

air argon air argon

22.0 26.3 15.0 19.2

Table 6. Assessment of the efficiency of various benzophenone/amine mixtures as photo-initiators for curing a clear acrylate varnish Amine none (24) 1,2-N,N,N,N-tetraethyldiaminoethane (7) 1,3-N,N,N,N-tetraethyldiaminopropane (20) 1,5-N,N,N,N-tetraethyldiaminopentane (21) 1,6-N,N,N,N-tetraethyldiaminohexane (8) 1,8-N,N,N,N-tetraethyldiaminooctane (9) 1,10-N,N,N,N-tetraethyldiaminodecane (25) 1,12-N,N,N,N-tetraethyldiaminododecane (1) triethanolamine (19) 1,2-dimorpholinoethane (4) 1,3-dimorpholinopropane (5) 1,6-dimorpholinohexane (26)' 1,8-dimorpholinooctane (6) 1,10-dimorpholinodecane (27) 1,12-dimorpholinododecane (12) his (2-N,N-diethylaminoethyl) ether (13) 1,2-bis(2-(N,N-diethylamino)ethoxy)ethane (14) bis(2-(N,N-diethylamino 2-ethoxy)ethyl)ether (18) 1,3-di(N-methylpiperazino) propane (16) N,N,N,N,N,N,N-heptamethyltetraethylenepentamine

Lamp passes to tack free 8 2 2 2 3 3 3 3 2 3 2 3 3 3 3 2 2 2 1-2 2

Lamp passes to through cure 2 2 2 3 3 3 4 2 4 2 3 3 3 3 2 2 2 2 2

603

The polymerization of acrylates Table 7. Assessment of the efficiency of various 2-chlorothioxanthone/amine mixtures as photoinitiators for curing a clear acrylate varnish Lamp passes to tack free

Amine none 1,2-N,N,N,N-tetraethyldiaminoethane 1,3-N,N,N,N-tetraethyldiaminopropane 1,5-N,N,N,N-tetraethyldiaminopentane 1,6-N,N,N,N-tetraethyldiaminohexane 1,8-N,N,N,N-tetraethyldiaminooctane l,lO-N,N,N,N-tetraethyldiaminodecane 1,12-N,N,N,N-tetraethyldiaminododecane triethanolamine 1,2-dimorpholinoethane 1,3-dimorpholinopropane 1,6-dimorpholinohexane 1,8-dimorpholinooctane 1,10-dimorpholinodecane l, 12-dimorpholinododecane bis (2-N,N-diethylaminoethyl) ether 1,2-bis(2-(N,N-diethylamino)ethoxy) ethane (14) bis(2-(N,N-diethylamino 2-ethoxy)ethyl) ether (16) N,N,N,N,N,N,N-heptamethyltetraethylenepentamine

(24) (7) (20) (21) (8) (9) (25) (1) (19) (4) (5) (26) (6) (27) (12) (13)

of long cross-linked polymer chains. To test for through-cure, one applies one's thumb to the print with a screwing downward motion. If this treatment makes no impression upon the print, it is considered that a through-cure has been effected.

Discussion of results Photoreduction studies. We have previously shown [17] that amino-alcohols participate less readily than amino-ethers in electron transfer reactions. The hydrogen bonding in systems containing amino-alcohols (1-3) raises the ionization potential of the nitrogen and as a consequence the energy of activation for electron transfer is increased. 2-Amino-ethers (4-6) are less reactive than straight trialkylamines and this

8 2 2 2 2-3 2 3 3 3 2 2 1 2 2 2 2 1 1-2 2 2

Lamp passes to through cure .... 4 5 4 5 5 6 5-6 5-6 6 6 4-5 4 2 2 2 2 3 3 2 3 4 4 6

is probably due to the inductive effect of oxygen increasing the ionization potential of the amine. Thus the increase in reactivity in going from triethanolamine (1) to N,N-dimethylethanolamine (3) can be explained in terms of there being less hydrogen bonding in the latter system. Another observation of relevance is the finding that N-methyl groups are more reactive than N-ethyl groups in electron transfer reactions [18]. That 1,8-N,N,N,N-tetramethyldiaminooctane (10) is more reactive than 1,8-N,N,N,N-tetraethyldiaminooctane (8) is in accord with this earlier work. Rather disappointingly there appears to be little correlation between the length of the intervening chain of the ~,og-diaminoalkanes (7-10) and the rate constants for reduction. However, one cannot assume

Table 8. Assessment of the efficiency of various Michlers ketone/amine mixtures as photo-initiators for curing a clear acrylate varnish Amine

Lamp passes to tack free

none 1,2-N,N,N,N-tetraethyldiaminoethane 1,3-N,N,N,N-tetraethyldiaminopropane 1,5-N.N.N.N-tetraethyldiaminopentane 1,6-N,N,N,N-tetraethyldiaminohexane 1,8-N,N,N,N-tetraethyldiaminooctane 1,10-N,N,N,N-tetraethyldiaminodecane 1,12-N,N,N,N-tetraethyldiaminododecane triethanolamine 1,2-dimorpholinoethane 1,3-dimorpholinopropane 1,6-dimorpholinohexane 1,8-dimorpholinooctane 1,10-dimorpholinodecane 1,12-dimorpholinododecane bis(2-N,N-diethylaminoethyl)ether 1.2-bis(2-(N,N-diethylamino)ethoxy)ethane (14) bis(2-(N,N-diethylamino 2-ethoxy)ethyl)ether (16) N,N,N,N,N,N,N-heptamethyltetraethylenepentamine

8 2 2 2 3 3 3 3 2 2 2 2 2 2 2 1 2 2 2

(24) (7) (20) (21) (8) (9) (25) (1) (19) (4) (5) (26) (6) (27) (12) (13)

3

3

3 3 3 2

Lamp passes to through cure 4-5 4~ 5 8 9 10 10 10 8 8 4 7 7 6 7 4 4-5 5 6 5

R. S. DAVID,SONand J. W. GOODIN

604

Table 9. Assessment of the efficiency of various fluorenone/amine mixtures as photo-initiators for curing a clear acrylate varnish Lamp passes to tack free

Amine none

> 10 > 10 > 10 > 10 > 10 > 10 > 10

1,2-N,N,N,N-tetraethyldiaminoethane 1,3-N,N,N,N-tetraethyldiaminopropane 1,5-N,N,N,N-tetraethyldiaminopentane 1,6-N,N,N,N-tetraethyldiaminohexane (8) 1,8-N,N,N,N-tetraethyldiaminooctane

(24) (7) (20) (21)

(9) 1,10-N,N,N,N-tetraethyldiaminodecane (25) 1,12-N,N,N,N-tetraethyldiaminododecane (1) triethanolamine (13) 1,2-bis(2-(N,N-diethylamino)ethoxy)ethane (14) bis(2-(N,N-diethylamino 2-ethoxy)ethyl)ether (16) N,N,N,N,N,N,N-heptamethyltetraethylenepentamine

that this lack of correlation is due to the rate constants for photoreduction not truly reflecting the efficiency of interaction of the triplet ketone with the amine due to disproportionation between the geminate radicals [13, 14]. Photo-oxidation studies. Inspection of Table 2 shows that there is little correlation between the rate of benzophenone sensitized oxidation of the amines and the rate constants for photoreduction of fluorenone by the amines. This may well be due to two reasons; (1) the presence of oxygen may reduce the efficiency of disproportionation of the radicals produced by reaction of the triplet ketone and the amine, and (2) the rate of oxidation will to some extent reflect the ability of an amine to undergo auto-oxidation. In a u.v. curing system, factor (1) will increase efficiency whereas factor (2) will decrease the efficiency.

The bulk polymerization of methyl methacrylate.

Lamp passes to through cure

m

> 10 7 6 6

7

7-8 6 6 7

The results shown in Table 3 refer to bulk polymerizations performed without any degassing of the systems, whereas in Table 5 a distinction is made between oxygen free and the normal aerated conditions. The results of Table 5 are in agreement with those obtained by Van Neerbos [19-] who showed that oxygen does retard the polymerization reactions. This may be due to it deactivating the triplet carbonyl compound, but this should not be a particularly marked effect since the deactivation produces ringlet oxygen which can attack the amine to give the desired radicals. Figure 3 shows a plot of the weight of poly MMA formed in the benzophenone sensitized reaction (from Table 3) and the rate constants for photoreduction of fluoroenone by amines (Table 1). Considering the crudity of the measurement of extent of polymerization, there is a reasonably good correlation between the efficiency of polymerization and the rate

35

S

3O

(1) Triethanolamine (2) N-Methyldiethanolamine (3) 1,3-Dimorpholinopropane (4) 1,6-Dimorpholinopropane o

5

(5) 1,10.N,N,N,N-Tetraethyldiaminodecane (6) 1,8-N,N,N,N-Tetraethyldiaminooctane (7) 1,3-N,N,N,N-Tetraethyldiaminopropane (8) N,N,N,N,N,N,N-Heptamethyltetraethylenepentamene

6

(9) N,N-Dimethylethanolamine 15

tO

I

I

I

IxlO 6

5xlO $

IxlO 7

I 5xlO 7

kr

Fig. 3.

The polymerization of acrylates constants for photoreduction. The results shown in Table 4 indicate the relative efficiencies of a number of carbonyl compounds to sensitize polymerization using the radiation of wavelength > 300 n.m. from a Hg arc. In all cases, benzophenone and 2-isopropylthioxanthone display similar efficiencies whereas fluorenone is far less efficient. It is known that the rate constants for the photoreduction of fiuorenone by amines are lower than those for the photoreduction of benzophenone by amines [9] and, although the results in Table 4 are not quantum yields, they are strongly indicative (as are the conclusions reached in the earlier part of this section) that the efficiency of polymerization is markedly affected by the efficiency of reaction between the carbonyl compound and the amine. U.V. curing studies. The most surprising finding of these studies was that although triethanolamine (1) reacts relatively inefficiently with triplet carbonyl compounds and is a poor amine for use in conjunction with a carbonyl compound for sensitizing the polymerization of MMA, it performs very well when used to u.v. cure a clear varnish [20]. This result is at variance with the suggestion, made by Cardenas and O'Driscoll [20], that there should be little difference between acrylate polymerizations in the bulk and cure of commercial formulations. Walling [21] has found that gel points occur at around 5~o conversion of the prepolymer in typical commercial formulations. The more recent study by Brown [22] showed that a conversion of about 63% of prepolymer to polymer was required to obtain a dry coating. Thus the results of bulk polymerizations may not be truly representative of what occurs in thin films. Nevertheless, the results obtained from bulk polymerizations etc. are of value in predicting the performance of a carbonyl compound-amine mixture as an initiator system. F r o m the finding that fluorenone is an inefficient sensitizer when compared with benzophenone, suggests that the efficiency of generation of amino-alkyl radicals is of importance in the u.v. curing of varnishes. However, the ability of the amino-alkyl radical to react with the acrylate oligomer is obviously of more importance in the u.v. curing of varnishes than in the bulk polymerization of MMA. This ability to act as an initiator appears to be associated with an appropriately situated oxygen atom i.e. > N - - C H z C H 2 O H . The presence of an hydroxyl group, but not an ether group, in an amine often lowers the reactivity of the amine towards carbonyl compounds having charge-transfer triplet states [23]; it was for this reason that systems leading to the formation of amino-alkyl ether radicals ~ N - - C H 2 C H z O R were investigated. The greater reactivity of the amino-alkyl ethers e.g. ( ~ 6 , 12--14, 26) when compared with triethanolamine using 2-chlorothioxanthone, Michlers ketone and fluorenone (Tables 7, 8 and 9) as sensitizers, is, we believe, due to this effect. It is possible that many of the amino ethers e.g. the morpholino compounds, may be used with advantage in many u.v. curing systems. In particular, the ability to vary the water solubility of these compounds, by altering chain length, should enable them to be used in lithographic processes in which triethanolamine cannot be used because of its water solubility.

605 CONCLUSION

The results show that the efficiency of reaction of triplet carbonyl compounds with amines is of importance in determining the efficiencY of these systems as mxtmtors for the u.v. curing of acrylate oligomers and the bulk polymerization of MMA. However, in the u.v. curing of varnishes the structure of the initiator radical (amino-alkyl radical) appears to be of great importance. It is hoped that further work will lead to a greater understanding of these structural effects. Although oxygen retards the curing process, there appears to be little correlation between the ease of photo-oxidation of the amine and the cure rate. Thus the scavenging of initiator radicals by oxygen does not appear to be of great importance.

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606

R.S. DAVIDSON and J. W. GOODIN

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31. 32. 33. 34. 35. 36. 37.

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