The influence of H-donors on the photodecomposition of selected water-soluble photoinitiators

Polymer Photochemistry 6 (1985) 59-70

The Influence of H - D o n o r s on the P h o t o d e c o m p o s i t i o n of Selected Water-Soluble Photoinitiators Rawdon

A. Bottom

Norsk-Hydro Polymers Ltd, Ayelitte Industrial Estate, Newton Ayclitie, Co. Durham DL5 6EA, Great Britain James T. GuthrieT Department of Colour Chemistry, The University of Leeds, ~ d s Great Britain

LS2 9JT,

and Peter N. Green Ward-Blenkinsop and Co. Ltd, Widnes WA8 8NS, Great Britain (Received: 5 October 1983)

ABSTRACT Evidence is presented which shows that aqueous solutions of 4(trimethylammoniummethyl)benzophenone chloride, BP2, are photochemically stable within experimental conditions applied. However, in aqueous solutions of B P 2 which contain compounds capable of H-atom donation, then BP2 undergoes rapid decomposition. In terms of the enhancement of photodecomposition, the observed order is poly(vinyl alcohol) > propan-2-ol > ethanol > propan-l-ol > glucose > 1,3-propanediol > 1,2-ethanediol > methanol. By contrast, 4-(sulphomethyl)benzil sodium salt, BZ1, is rapidly photodecomposed when irradiated in aqueous solution even in the absence of additional H-atom donors. Information is given which shows that various monomers have the ability to reduce the level of photodecomposition of aqueous solutions of BP2 which contain propan-2-ol. Thus, 2-hydroxyethyl acrylate> 2-hydroxypropyl acrylate > acrylic acid in terms of reducing the extent of photodecomposition. In addition, details are presented ret To whom all correspondence should be addressed. 59

Polymer Photochemistry 0144-2880/85/$3-30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Northern Ireland

60

Rawdon A. Bottom, James T. Guthrie, Peter N. Green garding the concentration dependence of this photodecomposition process with regard to the presence of 2-hydroxyethyl acrylate. Finally, aspects of the photodecomposition of BP2 and BZ1, present in regenerated cellulose film, are considered.

INTRODUCTION We have recently been concerned with the photoinitiated graft copolymerisation of vinyl and acrylic monomers to a range of polymeric substrates. Examples include cellulose 1 and cellulosics,2 polyolefins3"4 and woollen substrates. 5 Much of the effort has been devoted to reactions carried out in aqueous media. For this reason, a series of water-soluble, highly effective photoinitiators was developed. One severe difficulty with photopolymerisations carried out in aqueous media lies in the much reduced activity of conventional photoinitiators when they are placed in aqueous solution. In addition, problems of solubility commonly arise. Thus, many of the conventional photoinitiator assemblies have little to offer to grafting reactions carried out in aqueous solution. Enhancement of the solubility of photoinitiators using substituted solubilising groups often leads to further reduction in the photosensitising ability. It has been shown that suitably modified benzophenone and benzil derivatives are highly effective in their photoinitiation of graft copolymerisation from aqueous media. 6"7 However, it was thought that this enhanced photoreactivity did not simply arise from the inherent water solubility. Because of their affinity for aqueous media, we feel that natural polymeric systems have most to benefit from the availability of photoinitiators of this type. This report considers, from the practical point of view, the nature of the photodecomposition reactions which take place during photografting reactions. We are concerned especially with the nature and lifetimes of reactive species, the role of the aqueous medium and influence of H-donors. The approach taken was designed to provide an insight into the nature of the mechanism of active site formation during photografting reactions involving cellulose and other polysaccharides. The photodecomposition of 4-(trimethylammoniummethyl)benzophenone chloride (BP2), in aqueous solution in the presence of the

Photodecomposition of water-soluble photoinitiators

61

following H-donating compounds, was investigated: methanol, ethanol, 1,2-ethanediol, propan- 1-ol, propan-2-ol, 1,3-propanediol, poly(vinyl alcohol) and glucose. In addition, the photodecomposition of BlW2 and 4-(sulphomethyl)benzil sodium salt (BZl) on regenerated cellulose film was investigated. These two water-soluble photoinitiators are taken as being representative members of individual families of photoinitiators, each of which is effective in photografting. However, each photoinitiator possesses certain individual characteristics. T h e effect of the presence of selected monomers, acrylic acid (AA), 2-hydroxyethyl acrylate (2HEA), and 2-hydroxypropyl acrylate (2HPA), on the photodecomposition of BP2 was investigated. This was carried out since, in an investigation of the polymerisation of 2 H E A in the BIr2/propan-2-ol system, no h o m o p o l y m e r formation was detected. 7 EXPERIMENTAL

Reagents Details relating to the preparation of BZ1 have been given in an earlier publication, s 4-(Trimethylammoniummethyl)benzophenone chloride (BP2) was prepared as follows. 4-Methylbenzophenone was converted to 4-(chloromethyl)benzophenone by reaction with sulphuryl chloride under reflux for 2 h. T h e photoinitiator (BP2) was obtained by overnight reflux of the 4-(chloromethyl)benzophenone with trimethylamine in ethanol. It was recrystaUised from ethanol. The solubility of BP2 and BZ1 in each of the aqueous systems was determined in advance of photodecomposition studies. BZ1 has a )Lmax(30*c)(H20) of 268 nm, 8rnax(30*C)= 26600 litre mo1-1 cm -1 and a melting point of 273°C. BP2 has a hm=(H20) of 256 nm, e ~ = 16200 litre mo1-1 cm -1 and a melting point of 207 °C. T h e monomers, acrylic acid (supplied by Aldrich Chemical Co.), 2 - H E A and 2 - H P A (supplied by BASF, West Germany), were purified by standard procedures and vacuum distilled prior to use. The H-donating compounds referred to above (Aldrich Chemical Co.) were of analytical grade and were used as supplied. T h e regenerated cellulose film (British Cellophane Ltd, Bridgewater, Great Britain) was deplasticised by hot aqueous extraction.

62

Rawdon A . Bottom, James T. Guthrie, Peter N. Green

This involved boiling the film samples in four changes of distilled water, each for 10 min. T h e films were part-dried on a photographic plate drier and then dried to constant weight u n d e r v a c u u m at 313 K. Films were stored in a v a c u u m desiccator until required.

Photodecomposition procedure F r o m 1 0 0 c r n 3 Of a 0 - 0 1 7 3 m o l l i t r e -1 solution of photoinitiator a i cm 3 aliquot was t a k e n and m a d e up to 50 cm 3 with distilled water. T h e stock solution was irradiated via a 1 2 5 W m e d i u m - p r e s s u r e H g lamp. A t regular intervals i cm 3 aliquots w e r e r e m o v e d and m a d e up to 50 cm 3 with distilled water. Such extracts were taken for irradiation times from 0 to 180 min. T h e 50 cm 3 samples w e r e diluted as necessary to provide optical density values, at the particular Am=, in the range 0.1 to 1-0 and the optical density values w e r e measured. The percentage decomposition of the photoinitiator is expressed as D e c o m p o s i t i o n = ( O D ~ J O D m = ) × 100

(1)

w h e r e ODs~m is the a b s o r b a n c e of the irradiated sample at ?tm~ and ODm~x is the a b s o r b a n c e of the unirradiated sample at hm~. In studies of the p h o t o d e c o m p o s i t i o n of B P 2 in the presence of the H - d o n a t i n g compounds, solutions were assembled containing 0.5g of B P 2 and the required mass of H - d o n a t i n g c o m p o u n d , in 100 cm 3 of a q u e o u s solution. All the H - d o n a t i n g c o m p o u n d s w e r e used in equimolar proportions (0.31 mol litre -1) with respect to the n u m b e r of hydroxyl groups present, so that their relative effectiveness could b e assessed. T h e irradiation procedures and spectroscopic m e a s u r e m e n t s were c a r d e d o u t as before. In studies of the p h o t o d e c o m p o s i t i o n of B Z 1 and B P 2 on regenerated cellulose film, the tared samples of deplasticised film were impregnated with a solution of photoinitiator (1.73 × 1 0 - 2 mol litre -1) for 60 s at 303K. Previous experimentation had shown that this was sufficient time to provide equilibrium u p t a k e at this temperature. T h e film samples were r e m o v e d from the solution, surface dried to r e m o v e surplus liquid, weighed and sealed in polyethylene bags. T h e sealed films were e x p o s e d to U V light using a Wallace-Knight photocuring assembly, for total exposure times ranging f r o m 0 to 10 s at a belt s p e e d of 1-0 m s -x. T h e W a U a c e - K n i g h t unit used had a 200 W in-~ m e d i u m - p r e s s u r e H g lamp.

Photodecomposition of water-soluble photoinitiators

63

After exposure, the films were removed from the sample bags and placed in 10 cm a sample vials which were then filled with distilled water. The samples were left overnight in the dark. The liquid from the sample bottles was transferred to a 25 c m 3 graduated flask and the irradiated cellulose washed further with two separate 5 c m 3 aliquots of distilled water. These were added to the flask and the volume made up to 25 cm 3 with distilled water. The optical densities of these solutions were measured, at the hm~ of the photoinitiator, using a P y e - U n i c a m SP800 spectrophotometer, after controlled dilution as necessary, to provide optical density values in the range 0-1 to 1-0. The percentage decomposition was calculated as shown in eqn. (1).

R E S U L T S A N D DISCUSSION Figure 1 gives data relating to the decomposition of BZ1 and BP2 when irradiated in aqueous solutions under the conditions described above. BP2 is unaffected by irradiation both in terms of decomposition and wavelength shifts. With BZ1, a steady decline in the amount of residual photoinitiator is seen. This is accompanied by a hypsochromic shift in the )tm~ of the photoinitiator of 11 nm over 180 min of irradiation. The photodecomposition of Bit2 in aqueous solutions of the aforementioned H-donating compounds produced the results shown in Fig. 2. The study was carried out since BP2 was thought to operate via hydrogen abstraction from C-atoms carrying - - O H groups. In Fig. 2 the results are expressed as the percentage of residual photoinitiator at each irradiation time. It can be seen that BP2 is rapidly decomposed when irradiated in aqueous solutions of poly(vinyl alcohol) and propan-2-ol. Methanol and 1,2-ethanediol produce the smallest extent of decomposition over the irradiation period. 1,3-Propanediol is the next least reactive compound after 1,2-ethanediol. Propan-l-ol, ethanol and glucose show similar behaviour in the decomposition, lying in the middle of the range of decomposition patterns. Clearly the structural features of the H-donating compound have a marked influence on the decomposition behaviour of the photoactivator. A striking feature is that the two secondary alcohols used,

Rawdon A. Bottom, James T. Outhrie, Peter N. Green

64

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propan-2-ol and poly(vinyl alcohol) are by far the most effective with regard to the photodecomposition of BP2. (We should recall that Bit2 is the b e n z o p h e n o n e derivative, 4-(trimethylammonium methyl) benzophenone chloride.) T h e photoreduetion of b e n z o p h e n o n e in propan-2-ol is a wellknown reaction, s If BP2 reacts photochemically in the same way as b e n z o p h e n o n e then Scheme 1 is appropriate, where, in BP2, R = ---CH2--1~I(CH3) 3C1-. We can follow this reaction by monitoring the U V absorption

Photodecomposition of water-soluble photoinitiators

65

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Fig. 2. Effect of various alcohols on the photodccomposition of aqueous solutions of BP2. Q, Methanol; (D, ethanol; A, 1,2-ethanexiiol; ~7, propan-l-ol; Q, propan-2-ol; O, poly(vinyl alcohol); A, 1,3-propanexiiol; E), glucose.

associated with the carbonyl group which falls in intensity as the photoreduction proceeds. Figure 2 indicates that this type of photoreduction does occur with BP2. However, Fig. 1 dearly indicates that BP2 is not photoreduced in water alone. Thus, the normal benzophenone-type of photoreduction with propan-2-ol is predominant with BP2 and is not prevented, or greatly interfered with, by the presence of the water-solubilising group.

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66

Rawdon A . Bottom, James T. Guthrie, Peter N. Green

In Scheme 1 the ease with which the alcohol radical, CH3 .C(OH)CH3, can be formed will affect the rate of photoreduction. This ease is determined by the stability of the radical which is, in turn, determined by the structure of the parent alcohol. Radicals derived from secondary alcohols are, in general, more stable than radicals from primary alcohols because of the larger inductive electron release to the carbon atom carrying the unpaired electron. The results shown in Fig. 2 can thus be explained in terms of the stabilities of the alcohol radicals formed. With methanol, the radical formed is relatively unstable since there are no electron-releasing groups to stabilise the radical. Also the electron-withdrawing power of the oxygen atom in the hydroxyl group will further destabilise the radical. The radical from ethanediol would be expected to have similar stability to the methanol radical. The results in Fig. 2 support this expectation. The radical from 1,3-propanediol should be more stable than those formed from methanol or ethanediol because of the electron-releasing methylene group. Ethanol would be expected to form a less stable radical than propan-l-ol because of the shorter length of its alkyl chain. Propan-2-ol and poly(vinyl alcohol), being secondary alcohols, will form more stable radicals than the primary radical species discussed above. /3-Glucose contains both a primary and secondary hydroxyl groups. Its structure is such that the stabilising effect on the secondary alcohol groups will be less than for propan-2-ol, so glucose would be expected to form a less stable radical than propan-2-ol. Figure 2 would suggest that the glucose radical, whatever its form, is slightly less stable than the radicals formed from ethanol or propan-l-ol, when generated in the system outlined above. Figure 3 give the results of the photodecomposition of BZ1 and BP2 on regenerated cellulose films. As can be seen, both photoinitiators are effectively completely decomposed after 10 s of exposure. Both photoinitiators appear to decompose at a constant, rapid rate up to approximately 80% decomposition. It has already been shown that BP2 does not decompose when irradiated in aqueous solution alone. When irradiated in aqueous solutions containing propan-2-ol, BP2 decomposes rapidly. The results shown in Fig. 3 indicate, therefore, that BP2 is being photoreduced when irradiated in swollen cellulose film. One can conclude that BP2 is in very close affinity with the cellulosic substrate and that a consequence of photo-

Photodecomposition of water-soluble photoinitiators o~

67

100 BO

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Fig. 3. The photodecomposition of BP2 and BZ1 on regenerated cellulose film. Initiator concentration in bulk solution, 0.0173 tool litre-1; Q, BZ1; O, BP2.

reduction will be the formation of radical sites on the cellulose. These findings are important since they point to the possibility of forming true graft copolymers in photografting experiments undertaken with these water-soluble photoinitiators. With BZ1, the picture is not as clear, since it has been found that BZ1 decomposes at the same rate in water as in aqueous propan-2-ol. However, the rate of decomposition on regenerated cellulose film is much more rapid than was seen in either water or propan-2-ol. The photochemistry of benzil-based photoinitiators in aqueous media seems to be significantly different from that of such photoinitiators in organic solvents. It is generally stated that benzil and its derivatives undergo photoreduction, in a similar way to benzophenone, to yield a pinacol as one of the products. However, in some cases, several compounds have been isolated from photodecomposition studies of benzil, including benzoins and benzoic acids. 9"1° The formation of benzoins, which could themselves undergo photocleavage reactions, may further complicate the situation. LeMwith et al. have confirmed the absence of photocleavage reactions with benzil. 1~ In the initiation of polymerisation reactions, they have postulated a mechanism not involving H-abstraction. They suggest the formation of a diradical, directly as a result of light absorption, which initiates free radical polymerisation through reaction with a suitable monomer. The effect of good H-donors has been found to

68

Rawdon A. Bottom, James T. Outhrie, Peter N. Green 100 ~0 60

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~0 20 0

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I

I

100

~0

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80

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Fig. 4. The effect of various monomers (monomer concentration 8 . 6 x 1 0 -3 mol litre -1) on the photodecomposition of BP2 in aqueous propan-2-ol. O, 2Hydroxyethyl acrylate (2HEA); O, 2-hydroxypropyl acrylate (2I-IPA); O , acrylic acid (AA); 0 , no monomer present.

increase the rate of photoreduction of benzil. However, we have found that the decomposition of benzil-based photoinitiators used in our total study is not affected by such H-donating compounds. Thus, the position regarding the method of radical formation on cellulosic substrates, in the presence of henzil-hased photoactivators, is still far from clear. Figures 4 and 5 give data relating to the effect of incorporating 100 A

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~0 20 0

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Fig. $. The effect of the 2-hydroxypropyl acrylate (2HPA) concentration on the photodecomposition of Bit2 in aqueous propan-2-ol: &, 0.0172mol litre-~; A, 0-0086 tool litre-1; 0 , 0.0043 mol litre-1; O, no monomer present.

Photodecomposition of water-soluble photoinitiators

69

water-soluble acrylic monomers into solutions of BP2 in aqueous solutions of propan-2-ol and subsequent irradiation. Figure 4 shows the effect of equal molar concentrations of 2-hydroxyethyl acrylate (2HEA), 2-hydroxypropyl acrylate (2HPA) and acrylic acid (AA) on the photodecomposition of BP2 in aqueous propan-2-ol. Figure 5 shows the effect of various concentrations of 2 H P A on the photodecomposition of BP2. A general feature in Fig. 4 is the presence of an induction period, the length of which varies in the order 2 H E A > 2 H P A > AA. On cessation of induction, the photoreduction proceeds at the same rate as occurs in the absence of monomer. Figure 5 shows that, for 2-HPA, the concentration of m o n o m e r affects the length of the induction period in that an increase in this concentration leads to an increase in the induction period. We can recall that the photoinitiator concentration was maintained at 0-0173mollitre -a, the propan-2-ol concentration at 0 . 3 1 m o l litre -1 and the range of 2 - H P A concentrations used was 4.3 × 10 -31-72 x 10 -2 mol litre -1. It would appear that the acrylic monomers are acting as quenching agents in the photoreduction of BP2 in aqueous propan-2-ol. Since the effect is seen only as an induction period it is clear that the quenching power of the m o n o m e r must be destroyed during the induction period. Thus, when all the m o n o m e r is consumed, the quenching stops and photoreduction proceeds normally. It is possible that the monomers react in some way with either the excited triplet state of the photoinitiator or with the radical formed after H-abstraction from the propan-2-ol. We favour the latter possibility because the ease of the H-abstraction process will result in the excited state of the initiator having a very short lifetime. In the quenching process a m o n o m e r radical will be formed, which will, in turn, be consumed through further transfer or combination reactions involving the alcohol, water or initiator radical. It is unlikely that the m o n o m e r radical will initiate homopolymerisation owing to the large number of competitive transfer possibilities arising from the low m o n o m e r concentration of the system.

ACKNOWLEDGEMENT We are indebted to S.E.R.C. and British Cellophane Ltd, Bridgewater, for their support through financial assistance to R.A.B.

70

Rawdon A. Bottom, James T. Guthrie, Peter N. Green

REFERENCES 1. Abdel-Hay, F. I., Barker, P. and Guthrie, J. T., Makromol. Chem., 181 (1980) 2063. 2. Guthrie, J. T., Ryder, M. and Abdel-Hay, F. I., Polym. Bull., 1 (1979) 501. 3. Tazuke, S. and Kimura, H., J. Polym. Sci., Polym. Letters Edn., 16 (1978) 497. 4. Tazuke, S. and Kimura, H., Makromol. Chem., 179 (1978) 2063. 5. Barker, P., Guthrie, J. T., Davis, M. J., Godfrey, A. A. and Green, P. N., J. Appl. Polym. Sci., 26 (1981) 521. 6. Barker, P., Ph.D. Thesis, University of Leeds, Great Britain, 1982. 7. Bottom, R. A., Ph.D. Thesis, University of Leeds, Great Britain 1982. 8. Pappas, S. P. and McGinniss, R. M., U.V. curing science and technology, Technical Marketing Corporation, Stardord, USA, 1978, Chapter 1. 9. Bunbury, D. L. and Wang, C. T., Canadian J. Chem., 46 (1968) 1473. 10. Ogata, Y., Takayiki, K. and Fuji, Y., J. Org. Chem., 37 (1972) 4026. 11. Ledwith, A., Russell, P. J. and Sutclitte, L. H., J. Chem. Soc. Perkin 11, (1972) 1925.