Synthesis and photooxidation of styrene copolymer bearing camphorquinone pendant groups

(±)-10-Methacryloyloxycamphorquinone (MCQ) was synthesized from (±)-10-camphorsulfonic acid either by a known seven-step synthetic route or by a novel, shorter five-step synthetic route. MCQ was copolymerized with styrene (S) and the photochemical behavior of the copolymer MCQ/S was compared with that of a formerly studied copolymer of styrene with monomers containing the benzil (BZ) moiety (another 1,2-dicarbonyl). Irradiation (λ > 380 nm) of aerated films of styrene copolymers with monomers containing the BZ moiety leads to the insertion of two oxygen atoms between the carbonyl groups of BZ and to the formation of benzoyl peroxide (BP) as pendant groups on the polymer backbone. An equivalent irradiation of MCQ/S led mainly to the insertion of only one oxygen atom between the carbonyl groups of camphorquinone (CQ) and to the formation of camphoric anhydride (11) covalently bound to the polymer backbone. While the decomposition of pendant BP groups formed in irradiated films of styrene copolymers with pendant BZ groups leads to crosslinking, only small molecular-weight changes in irradiated MCQ/S were observed.


S2
Experimental Materials (±)-10-Camphorsulfonic acid, phosphorus pentabromide, methacryloyl chloride, anhydrous dioxane (Aldrich), triethylamine (Fluka), iodine, triphenylphosphine (Lachema), and all solvents (p.a.) were used as received. Selenium dioxide (Reachim) was resublimed from a nitric acid suspension. Potassium acetate was freshly molten before use. Acetic acid was dried over P 2 O 5 and distilled. Diethyl ether was dried over anhydrous Na 2 SO 4 for several days, filtered and dried with a sodium wire and distilled. Xylene was dried with a sodium wire and distilled. Stabilized styrene (Aldrich) was washed with diluted sodium hydroxide solution and water, dried with anhydrous MgSO 4 and distilled. Azobisisobutyronitrile (AIBN) (Fluka) was recrystallized from methanol.
Characterization 1 H NMR (600 MHz) and 13 C NMR (150 MHz) spectra were recorded with a Varian Inova 600 spectrometer in CDCl 3 , with the solvent signal as a reference. Chemical shifts are given in the -scale (ppm), with coupling constants J given in Hz. COSY, HSQC, and HBMC sequences were used for an assignment of signals. FTIR spectra were recorded on a Nicolet Impact 8700 FT spectrophotometer. UV-vis absorption spectra were measured on a Shimadzu 1650PC spectrophotometer (Japan). Mass spectra were recorded with a Hewlett Packard 5989B instrument. Melting points were determined by a hot-stage apparatus (Nageman, Germany) and are uncorrected. PS equivalent molar masses were estimated by GPC with THF as a mobile phase, series-connected PSS SDV 500 Å and PSS SDV 10 5 Å columns (d = 8 mm, l = S3 300 mm), Knauer 64 pump (1 mL min −1 , 3.3-3.4 MPa), and RI WATERS 410 detector. Loop 100 L and polymer concentration 2 mg mL −1 were used.

Syntheses
Syntheses of the compounds 6, 7, 9, and MCQ were carried out in darkness or under illumination at a wavelength at which CQ does not absorb.

Potassium (±)-10-camphorsulfonate (2)
A solution of (±)-10-camphorsulfonic acid (1) (250 g, 1.08 mol) in water (300 mL) was neutralized with K 2 CO 3 (79.3 g, 0.57 mol). Water was evaporated under reduced pressure and acetone was added and the salt was filtered. The residue was evaporated and combined with filtered salt. Pulverized 2 was dried for several days in a vacuum desiccator over P 2 O 5 until a constant weight was measured and was used without further purification.

(±)-10-Bromosulfonylcamphor (3)
A stirred suspension of 2 (62.0 g, 0.229 mol) in anhydrous diethyl ether (300 mL) under argon was cooled to −60 °C. PBr 5 (100 g, 0.232 mol) was added at once. The cooling bath was removed and the reaction mixture was stirred for 1 h at ambient temperature and then heated under reflux for 3 h. The reaction mixture was poured into a mixture of ice (600 g) and water (600 mL). The ethereal layer and the white solid were separated and the aqueous layer was extracted with diethyl ether (2 × 100 mL). The combined ethereal layers were washed with water (50 mL), dried over Na 2 SO 4 and evaporated to give a yellow solid 3 (58.0 g, 86%), which was used without further purification.
The cooled reaction mixture was dissolved in water (200 mL), neutralized with an aqueous solution of Na 2 CO 3 and extracted with diethyl ether (3 × 100 mL). Combined S5 ethereal layers were washed with water (50 mL), dried over Na 2 SO 4 and concentrated under reduced pressure. Distillation (75-80 °C and 0.13 Torr) afforded 5 (26.1 g, 96%) as a colorless oil.

(±)-10-Hydroxycamphorquinone (7)
A solution of 6 (3.1 g, 14 mmol) in 20% HCl was heated under reflux for 2 h. The cooled reaction mixture was neutralized with an aqueous solution of Na 2 CO 3 and extracted with ethyl acetate (3 × 50 mL). The organic layer was dried over CaCl 2 and concentrated. The residue was purified by column chromatography (silica gel, ethyl acetate/petroleum ether 1:2) to afford 7 as a yellow solid in 91% yield (2.3 g), which is a sublimating compound with an extremely wide melting range ≈100-255 °C determined in a sealed capillary.

(±)-10-Iodocamphorquinone (9)
A suspension of 8 (13.9 g, 50 mmol) and freshly resublimed SeO 2 (6.9 g, 62 mmol) in bromobenzene (70 mL) was heated under reflux for 22 h. The cooled reaction mixture was concentrated at reduced pressure and filtered through a short silica column to remove arose selenium, the column was washed with ethyl acetate, and the solution was concentrated at reduced pressure. The residue was purified by column chromatography (silica gel, ethyl acetate/petroleum ether 1:5) to afford 9 as a yellow solid in 84% yield (12.25 g), mp 109-113 °C.

Irradiation
Films of MCQ/S copolymer (40 mg) with good optical quality about 40 µm thick were prepared by casting from 1 mL benzene solutions onto a glass plate (10 cm 2 area).
The plate was covered by a Petri dish to slow down the evaporation of the solvent.
The self-supporting polymer films were separated from the glass by dipping into distilled water. Films were dried to a constant weight at room temperature under vacuum and were irradiated at  > 380 nm at ambient temperature in air. A homemade carousel apparatus was employed. It consisted of a 125 W mediumpressure mercury arc placed in a circulating-water-jacketed quartz tube, which was surrounded by a 1 cm thick layer of a liquid filter (500 g NaBr and 3 g Pb(NO 3 ) 2 per 1000 mL aqueous solution) transmitting at  > 330 nm. The distance of each sample (placed in rotating eight-rectangular holders) from the arc was about 8 cm. Finally, a plastic filter UV CL SR HPR (LLumar, USA) transmitting at  > 380 nm was placed in front of the sample.