Synthesis of crosslinked polymers containing benzoyl peroxide groups and their use as initiators

Eur. Polym. J. Vol. 25, No. 4, pp. 327-330, 1989 Printed in Great Britain. All rights reserved

0014-3057/89 $3.00+ 0.00 Copyright ~" 1989 Pergamon Press plc

SYNTHESIS OF CROSSLINKED POLYMERS C O N T A I N I N G BENZOYL PEROXIDE GROUPS AND THEIR USE AS INITIATORS P. HODGE* and A. A. NAIM Chemistry Department, University of Lancaster, Lancaster LA1 4YA, England

(Received 5 October 1988)

Abstract--Crosslinked polystyrene beads (prepared using 2 and 20% of divinylbenzene) were chemically modified to introduce benzoyl peroxide residues in which both of the aroyl groups were attached to the polymer backbone. Using these as initiators, methyl methacrylate and N,N-dimethylacrylamide were grafted onto the beads; attempts to graft on styrene derivatives were unsuccessful.

I N T R O D U C T I O N

--

In recent years there has been considerable interest in the use of functional polymers as catalysts and reagents in organic chemistry [1-6]. Crosslinked polymers are generally preferred for such applications because they can be separated particularly easily from the other reaction products. The appropriate functional polymers are usually prepared either by chemical modification of preformed crosslinked polymers of good physical form or by copolymerisations which include appropriate functional monomers. The latter approach is less popular because substantial study may be necessary to obtain a product of good physical form. A third approach, which has only been used occasionally and which has features of both of the above, is to graft functional polymers onto crosslinked polymers. Important examples are the 7-ray initiated grafting of 4-diphenylphosphinylstyrene [7,8] and of acryloylsarcosine methyl ester [9] on polypropylene. We now report the preparation of crosslinked polymers containing pendant benzoyl peroxide groups (1) and some preliminary studies of their use as initiators. It was anticipated that having both benzoyl moieties bound to the polymer would mean that there would be very little soluble polymer produced during the grafting. Polymers with pendant diacyl peroxide residues (2) have been described previously [10], but in these cases only one acyl group was bound directly to the polymer.

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EXPERIMENTAL

Unless indicated otherwise, polymer samples were dried to constant weight using a vacuum oven (2 mm of Hg) at 60°. Infrared spectra were measured for KBr discs using a Nicolet MX-1 FT-IR instrument. Elemental analyses were carried out by Butterworth's Microanalytical, Teddington. Preparation of polymers with residues (3) Using the published procedures [11], carboxyl groups were introduced into microporous 2% crosslinked polystyrene beads (200-400 mesh; from Lancaster Synthesis, Lancaster) by 2-chlorobenzoylation followed by quantitative cleavage of the 2-chlorobenzophenone residues; then the carboxyl residues (5) were converted into carbonyl chloride residues (3) using oxalyl chloride. The polymer with carboxyl residues (5) had DF 15% and Vr~x 1732 and 1685 cm -I. The polymers with residues (3) had DF 15% and Vma~ 1770 and 1734 cm- i. Similarly macroporous polymers with carboxyl groups (5; DF 9%) and carbonyl chloride groups (3; DF 9%) were prepared from a 20% crosslinked macroporous polystyrene (Biobeads SM-2; Biorad, Richmond, Calif.)

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Treatment of the 2% crosslinked polymer with residues (3) with hydrogen peroxide (a) The above 2% crosslinked polymer with carbonyl chloride residues (3; 8.3 g) was stirred in dry benzene

*To whom all correspondence should be addressed. 327 25/4--A

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328

P. HODGE and A. A. NA1M

(150ml) and cooled to approximately 0 °. Hydrogen peroxide (5.67 g of 30%) was added and was followed by the dropwise addition of pyridine (3.95g) in dry benzene (60 ml). The mixture was then stirred for 18 hr at 20 °. The polymer was treated with dilute HCI, filtered off, and washed on the filter with 5% KHCO.~, water, tetrahydrofuran, and ether. The polymer was dried in vacuum at 20° . The product (8.50g) had Vmax 1790, 1762, 1724 and 1689cm -~. To determine the activity of the product, a portion (0.20 g) was treated with dichloromethane (5 ml), KI (0.6 g) and H~SO~ (10ml, 3M), The mixture was set aside with occasional shaking for 3 days. The liberated iodine was then titrated against sodium thiosulphate (0.05 M), starch indicator being added near the end-point. The product had an activity of 0.47 mmol of peroxide/g. The polymer was collected, washed, and dried. It had Vm~x 1721 and 1685 cm -~. Treatment of a portion of the original product with diazomethane using the usual procedure [11] gave a polymer with Vm~x1790, 1762 and 1721 cm -~. (b) The preparation was repeated. The peroxyacid polymer had a similar i.r. spectrum to the above product and contained 0.49 mmol of peroxide/g.

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Oxidation of cyclo-octene by peroxide polymer A mixture of cyclo-octene (74 mg), dry tetrahydrofuran (4 ml) and the peroxide polymer prepared in (a) above (0.521 g) was stirred at 40 ° for 4 hr, then cooled and filtered. The polymer was washed several times with small portions of tetrahydrofuran. The combined filtrate and washings were analysed by GLC using a column packed with 10% SE-30 liquid phase on Chromosorb W. The analysis showed that 91% of the cyclo-octene was unchanged and that 9% was converted into cycloooctene oxide. This corresponds to the original polymer having 0.12 mmol of peroxyacid residues per g. The recovered polymer had Vm,~ 1790, 1762, 1718 and 1688 cm-L

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Treatment of the 20% crosslinked polymer with residues (3) with hydrogen peroxide Using the procedure described above for the 2% crosslinked polymer, the 20% crosslinked polystyrene containing residues (3) was treated with hydrogen peroxide and pyridine. The product had vn~,x1785, 1770, 1725 and 1700 cm -~ and, by titration, 0.15 mmol of peroxide/g.

RESULTS

AND

DISCUSSION

Benzoyl peroxide can be p r e p a r e d in high yield by treating benzoyl chloride with h y d r o g e n peroxide in the presence o f bases such as pyridine [12]. Accordingly 2 % crosslinked polystyrene beads c o n t a i n i n g benzoyl chloride groups (3) were prepared [11]. T h e

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Grafting experiments The results are summarised in Table I. The following procedure (entry 1) is typical. The 2% crosslinked peroxide resin (0.60 g) in dry benzene (10 ml) was mixed with methyl methacrylate (6.0 g, freshly distilled) in dry benzene (10ml) at 20°. The mixture was placed in a strong glass ampoule and de-gassed. The tube was then sealed and heated at 70-80 ° with stirring for 3 days. The cooled mixture was added to cold methanol (200 ml) and the polymer collected and washed exhaustively with tetrahydrofuran and with dichloromethane. The product was dried to constant weight (5.12 g). A portion (1.21 g) of this product was stirred in tetrahydrofuran (25ml) for 3 days. The product was then collected, dried and weighed. This washing procedure was repeated twice by which stage the weight did not change further. The final product (0.98 g) had Vm,~ 1731 cm -~ and C = 62.8%; H = 7.7%.

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Synthesis of benzoyl peroxide crosslinked polymers

329

Table 2. Polymerscontainingbenzoylperoxideresidues (1) Starting polymer~ % Of acid chlorideresidues(3) converted into various residues Entry Benzoylperoxide Peroxyacid Carboxylic acid number Crosslinking DF of residues(3) residues (1)b residues (4) residues (5)c 1 2% 15% 51% 9% 40% 2 2% 15% 53%d 10%d 37% 3 20%~ 9% 29%a 4%d 67% ~Styrene~:livinylbenzenecopolymers. bNote that each benzoylperoxidemoietycorresponds to two acid chlorideresidues(3). ~By difference. aAssumesproportionsof residues(I) and (4) were the same as in entry 1. ~BiobeadsSM-2. degree of functionalisation (DF) was deliberately kept to a modest level (15%) since it was anticipated that once initiation occurred at a site the polymer chain formed would sterically hinder initiation at neighbouring sites. The beads in benzene at 20 ° were treated with 30% hydrogen peroxide and pyridine for 20 hr. On the basis of the evidence presented below, the product contained the desired residues (1), together with peroxyacid residues (4), and carboxylic acid residues (5) in the proportions shown in Table 2, entry 1. The presence of benzoyl peroxide residues (1) was indicated by the i.r. spectrum of the product which contained carbonyl bands at 1790 and 1760cm -~. After the product had been heated in benzene at 80 ° for 2 days or had been treated in dichloromethane with an excess of acidified potassium iodide, these two carbonyl bands were no longer present. By titration with acidified potassium iodide, the oxidative capacity of the polymer [due to both residues (1) and (4)] was 0.47 mmol/g. In a repeat preparation (Table 2, entry 2) the oxidative capacity of the polymer was 0.49 mmol/g. Reactions between pendant groups on crosslinked polymers usually do not proceed to completion [13]. It was therefore anticipated that the present product would contain some peroxyacid residues (4). Polymers containing such groups have been described before and it is known that, with tetrahydrofuran as the solvent, they oxidise cis-cyclo-octene to the epoxide in near quantitative yield [14-16]. Carrying out a similar reaction with the present product and assuming the yield was quantitative indicated that the polymer contained 0.12 mmol/g of residues (4). The i.r. spectrum of the present polymeric product had a substantial broad carbonyl band at 1724 cm -~ due to carboxyl groups (5) which could well have contained the carbonyl band due to residues (4) in the same envelope. The carboxyl groups (5) also displayed a small carbonyl band at 1689 cm -~. Esterification of the polymer using ethereal diazomethane caused this band to disappear and the larger carbonyl band to sharpen and shift to higher frequency. The product did not contain any of the original residues (3), those not reacting to give residues (1) and (4) being hydrolysed to residues (5). A polymer containing benzoyl peroxide residues (1) was similarly prepared from a commercial 20% crosslinked macroporous polystyrene. The content of residues (1) was substantially less than with the 2% crosslinked polymer. The results are summarised in Table 2, entry 3. Attempts to convert the 2% crosslinked polymer (3) into a polymer with residues (1)

using potassium superoxide in benzene, a procedure which is successful with several low molelcular weight acid chlorides [17], failed. Attempts to catalyse the polymer reaction with dicyclohexano-18-crown-6 also failed [18]. Attempts were made to graft several monomers onto the polystyrenes with benzoyl peroxide groups (1). For this initial study, those monomers were selected which would produce a product with characteristic i.r. (e.g. methyl methacrylate) and/or which would introduce an element not previously present that could be monitored easily by elemental analysis (e.g. phosphorus). Experiments were carried out by heating a degassed solution of the monomer in benzene with the polymer at 80 ° for 20 hr. The polymer was then recovered and exhaustively washed to remove any soluble polymer. The results are summarised in Table 1. It is clear that significant grafting occurred only with methyl methacrylate and N,N-dimethylacrylamide. Trace amounts of grafting occurred with styrene and maleic anhydride together but not with the styrene monomer alone. In order to determine whether any thermal polymerisation occurred during the grafting process, solutions of methyl methacrylate and N,N-dimethylacrylamide in benzene were separately heated at 80 ° for 20 hr. The yields of polymer were insignificant ( < 0.33%). Polymerisations initiated by the benzoyl peroxide residues (1) in the crosslinked polymer beads would be expected to differ considerably from those using benzoyl peroxide itself. For example, the fraction of residues (1) producing initiation will be substantially less than with benzoyl peroxide in solution. Thus, the relatively low mobility of the polymer backbone will mean that once the peroxide bond is broken the two aroyloxy radicals produced will not diffuse away from each other rapidly. If one or both of these radicals loses carbon dioxide and the radicals remaining then combine, a benzoyl peroxide residue (1) will have been destroyed without producing initiation. When initiation does occur, the growing polymer chain is likely sterically to block other potential initiation sites in the vicinity. A further complication is that the concentration of monomer in the beads will depend on the distribution of the monomer between the beads and the solution. As the monomer in the beads is consumed, the rate of replacement from solution will depend on how rapidly it can diffuse into the beads. Finally, as the concentration of all the active species in the beads is relatively high compared with the analogous solution polymerisation, reactions between the bound species are

P. HODGEand A. A. NAIM

330

likely to be more important, in particular, termination is likely to be easier. In view of the above differences from polymerisation in solution, it is clearly not possible to explain in detail why the pattern of results found in the present work was obtained. We suggest, however, that one important factor is that since the polymer beads were moderately polar [due to the presence of groups (1), (3) and (4)], methyl methacrylate and N,N-dimethylacrylamide were present at higher concentration in the beads than the other monomers. REFERENCES

1. Polymer-Supported Reactions in Organic Synthesis (Edited by P. Hodge and D. C. Sherrington). Wiley, Chichester (1980). 2. N. K. Mathur, C. K. Narang and R. E. Williams. Polymers as Aids in Organic Chemistry. Academic Press, New York (1980). 3. A. Akelah and D. C. Sherrington. Polymer 24, 1369 (1983). 4. Polymeric Reagents and Catalysts (Edited by W. T. Ford). American Chemical Society, Washington D.C. (1986). 5. P. Hodge. R. Soc. Chem.; Ann. Rep. B 283 (1986).

6. Syntheses and Separatlions Using Functional Polymers (Edited by D. C. Sherington and P. Hodge). Wiley, Chichester (1988). 7. F. R. Hartley, S. G. Murray and P. N. Nicholson. J. Polym. Sci. Polym. Edn. 20, 2395 (1982). 8. J. L. Garnett, R. Levot and M. A. Long. J. Polym. Sci.; Polym. Lett. 19, 23 (1981). 9. A. Akelah, J. G. Hefferman, S. B. Kingston and D. C. Sherrington. J. appl. Polym. Sci. 28, 3137 (1983). 10. G. S. Bylina, L. K. Burykina and Y. A. Oldekop. V.gsokomolek. Soedin., Ser. B 12, 670 (1970); Chem. Abstr. 74, 3896K (1971). 11. C. R. Harrison, P. Hodge, J. Kemp and G. M. Perry. Makromolek. Chem. 176, 267 (1975). 12. S. R. Sandier and W. Karo. Polymer Synthesis, Vol. I, p. 445. Academic Press, New York (1974). 13. See, for example, P. Hodge in Ref. [5]. 290, and Ref. [6] p. 60. 14. C. R. Harrison and P. Hodge, J. chem. Soc. Chem. Commun. 1009 (1974). 15. C. R. Harrison and P. Hodge. J. chem. Soc. Perkin Trans. 1 605 (1976). 16. J. M. J. Fr6chet and K. E. Haque. Macromolecules 8, 130 (1978). 17. R. A. Johnson. Tetrahedron Left. 17, 331 (1976). 18. See, for example, B. N. Kolarz and A. Rapak. Macromolek. Chem. 185, 2511 (1984).