Synthesis and photooxidation of styrene copolymer bearing camphorquinone pendant groups

Article abstract of DOI:10.3762/bjoc.8.37

Abstract (±)-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.

Full text of DOI:10.3762/bjoc.8.37

Synthesis and photooxidation of styrene copolymer
bearing camphorquinone pendant groups
Branislav Husár*1, Norbert Moszner2 and Ivan Lukáč1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 337–343.
1Polymer Institute, Slovak Academy of Sciences, Dúbravská cesta 9,
SK-845 41 Bratislava 45, Slovakia and 2Ivoclar Vivadent AG,
Bendererstrasse 2, FL-9494 Schaan, Liechtenstein
doi:10.3762/bjoc.8.37
Received: 09 November 2011
Accepted: 13 February 2012
Published: 06 March 2012
Email:
Branislav Husár* - branislav.husar@savba.sk; Norbert Moszner -
norbert.moszner@ivoclarvivadent.com; Ivan Lukáč -
ivan.lukac@savba.sk
Associate Editor: H. Ritter
© 2012 Husár et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
camphorquinone; 1,2-diketone;
(±)-10-methacryloyloxycamphorquinone; photooxidation; polystyrene
Abstract
(±)-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 inser-
tion 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.
Introduction
Camphorquinone (CQ) in the presence of H-atom donors such triplet state of the carbonyl group of CQ abstracts an H-atom
as ethers (H abstraction), or more efficiently tertiary amines from a hydrogen donor. The two primarily formed radicals
(electron/proton transfer), is known to be an effective photoini- undergo subsequent reactions leading to photoproducts [21].
tiator for curing methacrylate-based dental restorative resins The rate-determining step in photoinitiation by CQ/amine is
[1-9]. CQ photochemistry in solution in the absence of oxygen hydrogen transfer by the excited n→π* triplet state of the car-
[10-16] and in the presence of oxygen [10,13,17-20] has been bonyl group of CQ from the alkylamino group [8,9]. The photo-
studied extensively. In an inert atmosphere, the excited n→π* chemistry of the low molecular CQ in the polystyrene (PS) film
337

Beilstein J. Org. Chem. 2012, 8, 337–343.
was the subject of previous studies [21,22]. It is reasonable to literature [31]. The goal of this work was to prepare a more
compare the photochemical properties of polymer-bound CQ easily accessible racemic MCQ and to compare the photochem-
with a polymer matrix containing another well-studied 1,2- istry of MCQ/S with that of low molecular CQ in PS films in
dicarbonyl compound, namely benzil (BZ). BZ can be the presence of oxygen.
converted almost quantitatively to benzoyl peroxide (BP) in an
Results and Discussion
aerated polymer matrix by irradiation at λ > 400 nm (Scheme 1)
[23,24]
Synthesis of MCQ
Racemic MCQ was synthesized in seven steps from (±)-10-
camphorsulfonic acid (1) as a starting material in 18% overall
yield (Scheme 2, see Supporting Information File 1 and
Supporting Information File 2 for full experimental data). Total
synthesis of (1R)-10-methacryloyloxycamphorquinone from
(1S)-10-camphorsulfonic acid [31] as well as of enantiopure
stable intermediates 2–7 [8,32-34] are known. Though the phys-
iological activity of optically active compounds depends on
their configuration, the photochemical activity of CQ does not
depend on the configuration. Without the need for an enan-
Scheme 1: Photoperoxidation of BZ in an aerated glassy polymer
matrix.
The pendant BZ groups of the styrene copolymers may be tiopure product, the reaction economics could be improved both
transformed into pendant BP groups [25,26]. Subsequent by the use of a cheaper racemic starting material and by a
decomposition of BP pendant groups results in the formation of simplified multistep synthesis.
highly crosslinked films [27-30].
A shorter alternative synthetic route to 6 from 1 was proposed
Owing to the interesting properties of styrene copolymers (Scheme 2, see Supporting Information File 1 and Supporting
formed from BZ-containing monomers, a CQ-bearing mono- Information File 2 for full experimental data). (±)-10-Iodocam-
mer (±)-10-methacryloyloxycamphorquinone (MCQ) (another phor (8) was prepared in one step from 1 with iodine and PPh3
monomer with a 1,2-dicarbonyl moiety) was prepared and in toluene under reflux in 39% yield (lit. [35] 85% yield). Com-
copolymerized with styrene to give MCQ/S copolymer bearing pound 8 was selectively oxidized with SeO2 in bromobenzene
CQ pendant groups. Enantiopure MCQ is known from the under reflux to (±)-10-iodocamphorquinone (9) in 84% yield. A
Scheme 2: Synthesis of MCQ from (±)-10-camphorsulfonic acid (1). Only one enantiomer of each compound is depicted.
338

Beilstein J. Org. Chem. 2012, 8, 337–343.
suspension of 9 in anhydrous acetic acid with freshly molten but the corresponding compounds in the literature are pure
potassium acetate was heated to 170 °C for 8 h to provide 6 in enantiomers. The racemates can crystallize as a racemic mix-
37% yield.
ture (lower mp), as a racemic compound (lower or higher mp),
or rarely as a racemic solid solution (slightly lower or higher
The advantage of this synthetic pathway lies in the reduction of mp).
synthetic steps as well as avoiding the use of rather expensive
PBr5. Although we obtained MCQ in overall yield of only 5%, Polymerization
optimization of conditions will likely provide a higher value. As introduced in Supporting Information File 1, the copolymer
For example, iodocamphor 8 was previously obtained in 85% MCQ/S was synthesized by copolymerization of styrene
yield (much better than the 39% yield expressed here). Low (99.54 mol %) and MCQ (0.46 mol %), initiated by AIBN at
yield in this step is blamed on the poor quality of reactants. The 60 °C and resulting in 10% conversion. FTIR spectroscopy was
yield of 6 from 9 was lower due to the formation of side prod- used to estimate the content of CQ units in the MCQ/S
ucts. Thus, iodocamphor 8 should be transformed first to 5 and copolymer by interpolation of the peak area of the carbonyl
afterwards to 6.
band (1740–1790 cm−1) using a calibration curve consisting of
five different concentrations of MCQ in CCl4 solution. UV–vis
All attempts to transform 9 directly to MCQ were unsuccessful. spectroscopy was used in a similar way by interpolation of the
For example, such a transformation was tested by stirring com- n→π* peak area (390–510 nm) using a calibration curve. The
pound 9 at ambient temperature with the following reagents: content of MCQ units in MCQ/S copolymer was determined to
Methacrylic acid/Cs2CO3/DMF, methacrylic acid/DBN/ be 0.72 mol % from FTIR and 0.62 mol % from UV–vis. MCQ
benzene, methacrylic acid/NaH/n-hexane, methacrylic acid/ is therefore more reactive than styrene, which is in agreement
NaHCO3/DMF, potassium methacrylate/acetone, and silver with the copolymerization parameters of structurally similar
methacrylate/toluene.
monomers.
From the last step of the MCQ synthesis, a side-product 10 was Photooxidation of MCQ/S
isolated and identified. It was previously reported that a Since it is difficult to follow the structural changes of the CQ
commercial sample of methacryloyl chloride contains its oxa- structures of the MCQ/S copolymer during photochemical
Diels–Alder dimer, responsible for the formation of 10 [36]. transformation, an analogous study with low molecular CQ
This side-product could be avoided by using freshly distilled doped in a PS matrix was first performed. Elucidation of the
methacryloyl chloride. A significant difference in the deter- structures of low molecular photoproducts was conveniently
mined melting points of the racemates and enantiomers of 6, 7, followed by spectral methods [22] and by isolation from the
and MCQ is caused by the configuration of these compounds polymer matrix followed by spectral identification [21]. The
(Table 1). The decrease of melting point of the racemate is results of the CQ photooxidation in PS are summarized in
10–12 °C in the case of 6 and 43–45 °C in the case of MCQ Scheme 3 [21].
compared to the pure enantiomer. Racemic 7 melts over a broad
range of temperatures in contrast to enantiomeric 7, which has a The addition of molecular oxygen to the excited n→π* triplet
sharp melting point. The compounds in this work are racemates, state of ketones and 1,2-diketones to form 1,4-biradicals is a
Table 1: Values of melting points of synthesized racemic camphor derivatives and corresponding pure enantiomers (R or S) from the literature.
Compound
Melting point of racemate
(°C)
Melting point of enantiomer
(°C)
Configuration of enantiomer
(R or S)
Reference
4
77–78
78
S
R
R
R
R
R
R
S
R
[35]
[32]
[8]
4
77–78
78
4
77–78
76–78
88
6
76–78
[33]
[8]
6
76–78
87–90
201–203
205
7
≈100–255
≈100–255
67–72
[8]
7
[34]
[35]
[31]
8
71
MCQ
46–49
91–92
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Beilstein J. Org. Chem. 2012, 8, 337–343.
Scheme 3: Photooxidation of CQ in aerated glassy PS matrix.
generally accepted mechanism, which has been theoretically
treated and reviewed [37]. The oxygen atom released during the
formation of 11 can oxidize another molecule of CQ to 11. It is
likely that common biradical intermediates are responsible for
the formation of lactones 12 and 13 in solution [10] and acids
14a and 14b formed in the PS matrix [21]. The intramolecular
recombination of biradical intermediates is favored in benzene
solution. However, in glassy PS matrix the intramolecular
abstraction of a hydrogen atom and formation of a double C=C
bond occurs. The glassy polymer matrix should retard the
intramolecular recombination of biradicals.
During the irradiation of MCQ/S film at λ > 380 nm in air, the
changes were followed by FTIR (Figure 1) and UV–vis
(Figure 2) spectroscopy. The evolution of both spectra of low
molecular CQ doped in the PS matrix and that of copolymer
MCQ/S during irradiation (beside ester carbonyl absorption in
FTIR spectra) are similar. Absorption bands of the CQ 1,2-
dicarbonyl group vibrations (1776, 1759 cm−1) decreased
Figure 1: FTIR spectra of MCQ/S film after irradiation in a carousel for
the indicated periods. Spectrum of PS film was subtracted.
quantitatively. This decrease is accompanied by the formation reduction of the n→π* absorption band of the 1,2-dicarbonyl
of bands at 1815 and 1770 cm−1 assigned to anhydride 11 group of the CQ structure is seen.
(Figure 1). Increased absorption near 1700 cm−1 was assigned
to acids 14. After thermal treatment of the irradiated MCQ/S The GC–MS results [21] allow the rough estimation of the
film at 90 °C for 2 h, no change was observed by FTIR spec- distribution of CQ photooxidation products in PS from the GC
troscopy. This signifies that no thermally unstable eight- record. About 54% of the whole area corresponds to 11 and its
member ring peroxide (in analogy with BP formed from BZ as secondary products formed during isolation (secondary prod-
shown in Scheme 1) was present in the PS matrix after irradi- ucts are not formed in the MCQ/S film during irradiation). 16%
ation. FTIR vibration bands for such cyclic diacylperoxide corresponds to 14a–c and the remaining 30% are unidentified
would be expected to be found near 1800 cm−1. Also in UV–vis compounds. A similar distribution of products is most probably
spectra of the MCQ/S film after irradiation (Figure 2), complete also present in the photooxidized MCQ/S copolymer.
340

Beilstein J. Org. Chem. 2012, 8, 337–343.
nation of macroradicals occurred during irradiation. However,
the crosslinking of styrene copolymers with the monomers
containing BZ is significantly more efficient than those of
MCQ/S.
While in the case of styrene copolymers with the monomers
containing BZ decomposition of pendant BP groups produces
only one polymeric benzoyloxy radical that is efficient in
crosslinking, in the MCQ/S copolymer two acyloxy radicals
could be produced from the postulated covalently bound cyclic
diacylperoxide and could cause extensive crosslinking. As the
changes in the chemical and molecular weights show, the mech-
anism of MCQ/S crosslinking (increase of molecular weight) is
clearly different from that in the presence of BZ structures.
Hence, the covalently bound cyclic diacylperoxide as an inter-
mediate is most probably not formed. Similar to BZ, the n→π*
triplet state of the CQ structure may also add molecular oxygen
to form a 1,4-biradical. In the case of the BZ structures, forma-
tion of the 1,4-biradical is followed by BP formation. In com-
Figure 2: UV–vis spectra of MCQ/S film after irradiation in a carousel
apparatus for the indicated periods.
Whereas the previously published irradiation of styrene copoly- parison, CQ structures react with oxygen forming 1,4-biradi-
mers with the monomers containing BZ (Mn = 84000, PD = 1.8) cals, which decompose to camphoric anhydride 11 structures
with 0.54 mol % of the BZ-containing component led to exten- covalently bound to the polymer backbone (Scheme 4a). Liber-
sive crosslinking with 69% gel fraction [21], MCQ/S with ated atomic oxygen during the formation of covalently bound
0.72 mol % of CQ moieties did not reach the gel point under 11 may oxidize another pendant CQ unit to the covalently
the same conditions of irradiation; MCQ/S film retained its bound 11. Cyclopentenecarboxylic acid 14 structures shown in
solubility in organic solvents. PS equivalent molar masses were Scheme 3 according to the low molecular study [21] were
then determined by GPC before and after irradiation. The mole- formed to a much lesser extent. Since the phototransformation
cular weights and polydispersities of the irradiated copolymer of CQ to 11 is not quantitative, n + m = 99.7; the remaining 0.3
(Mn = 190000, PD = 2.7) were higher than those of the initial consists of cyclopentenoic acids 14 and other unknown photo-
copolymer (Mn = 170000, PD = 2.0). Therefore, some recombi- products.
Scheme 4: Proposed mechanism of MCQ/S photochemistry.
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Beilstein J. Org. Chem. 2012, 8, 337–343.
The increase of the molecular weight of MCQ/S during irradi- Acknowledgements
ation under low oxygen conditions is induced primarily by an Financial support from grants VEGA 2/0072/11 and APVV-
abstraction of hydrogen from the polymer backbone by the CQ 0562-07 is gratefully acknowledged. Authors sincerely thank
structure in the triplet state under the formation of ketyl and Ms. Katarína Borská for GPC measurements, Mr. Jean-Louis
alkyl radicals. The formed macroradicals can recombine and/or Canet for MS measurements, and Mr. Samuel C. Ligon for
disproportionate (Scheme 4b). Recombination of macroradicals reading the manuscript.
leads to an increase of the molecular weight and polydispersity.
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