J. Arct

Some photo-reactions of substituted poly(acrylophenones)

Abstract Poly(p-methoxyacrylophenone) (PPMA) and poly(3′,4′-dimethoxyacrylenephenone) (PDMA) were exposed in the form of thin films to long-wave u.v. radiation (λ ⩾ 300 nm) under high vacuum. The low molecular mass products from both were ethane and methane, the quantum yields (Φi) for their formation being in the range 1 × 10−4 π → π ∗ (S 0 → S 1 ) transition of the phenyl group is sufficiently red-shifted, by the presence of the methoxy substituents, to cause absorption in the long-wave region (λ > 300 nm). In addition, there is a considerable interaction between n → π ∗ and π → π ∗ orbitals, and this produces a composite chromophore. For the purposes of comparison, the plausible model compound 3, 4 dimethoxybenzylalcohol (DMBA) was irradiated under identical conditions. Not only were the gaseous products identical and the quantum yields similar, but DMBA underwent similar spectral changes, including coloration. In this case the S0 → S1 absorption is the chromophore. The results have some bearing on the coloration of lignin, which contains similar structural elements.

Studies of the photochemistry of poly(p-propionylstyrene)

Abstract Films of poly(p-propionylstyrene) were exposed to long-wave u.v. radiation (λ ⩾ 300 nm) under high vacuum at 25°. The volatile products were quantitatively analyzed by mass spectrometry. The principal initial photoreactions are Norrish Type I α-cleavages but some β-cleavage also occurs with the formation of methyl radicals. The resulting free radicals participate in abstraction reactions with the polymer. Cross-linking occurs by the interaction of a variety of macroradicals. The importance of the carbonyl triplet is demonstrated and its lifetime, as determined by time-resolved spectroscopy and laser flash photolysis, is 147 ± 10% nsec. The polymer is less susceptible to photodegradation than poly(p-acetylstyrene).

Photodegradation of lignin model compounds: part 2—substituted stilbenes

Three stilbenes—St 1 (3,5-dimethoxy-4-hydroxystilbene), St 2 (3,3′,5,5′-tetramethoxy-4-hydroxystilbene) and St 3 (3,3′,5-trimethoxy-4-hydroxystilbene)—were exposed under high vacuum conditions and in the form of thin films to long-wave UV (λ ≥ 300 nm) radiation. In all cases very rapid changes occurred in the UV and emission spectra. In particular, intensities of both π → π∗ absorptions and fluorescences decreased while new long-wave chromophores and fluorophores were produced. Rapid colouration (yellow) accompanied these changes. Methane and ethane were the only detectable low molecular weight products, and quantum yields for their formation were estimated (±20%). It was concluded that OCH3 fission was occurring with concomitant formation of phenoxyl radicals. These, in turn, are relatively long-lived and can undergo further photolysis to yield precursors of o-quinones, for which other spectroscopic evidence (NMR, infrared and UV/visible) was obtained and to which the colouration was ascribed. Gel permeation chromatography indicated the presence of products of cyclization (i.e. substituted dihydrophenanthrenes) and of higher molecular weight material, and it was suggested that the latter is the product of phenoxy radical interactions. Plausible reaction schemes which account for the experimental data are given.

The photolysis of poly(p-hydroxystyrene)

Abstract The photochemistry of poly( p -hydroxystyrene) (PPHS) has been investigated, thin films being exposed to 254 nm radiation under high vacuum at 298K. H 2 is the principal volatile product, but small amounts of H 2 O are also formed. The polymer becomes rapidly coloured, but unlike other styrene polymers, which remain yellow, PPHS undergoes a progressive colour change from yellow through orange to brown. Infrared spectral data indicate the depletion of OH group concentration, and a concomitant formation of carbonyl, possibly quinonoid species. Crosslinking is rapid and no chain scission was detected. It is likely that the polymeric phenoxy radicals formed by photolysis or by subsequent H-abstraction contribute to the formation of both quinonoid species and crosslinks. The mechanism of the photolysis is discussed.

Photodecomposition of lignin model compounds: Part I

The plausible lignin model compounds, 3,4-dimethoxy-α-(2-methoxyphenoxy)-β-hydroxy-propiophenone (I) and 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-propandiol-1,3 (II) were irradiated in the form of thin films with long-wave ultraviolet (λ ≥ 300 nm) radiation under high vacuum. Methane and ethane were the only detectable low molecular mass products, the quantum yields for their formation being low (around 10−4 mol einstein−1). The compounds also underwent coloration and a number of other changes, as indicated by spectroscopy, and these were attributed to the formation of quinonoid species, their precursors being phenoxy radicals formed by direct photolysis of the OCH3 bonds in the benzene rings. In the case of I, the long-wave chromophore is composite, involving both the carbonyl (n→π∗ transition) and the phenyl group (π→π∗ transition). In II the absorption due to the S0→S1 transition of the benzene ring extends into the long-wave region (λ > 300 nm). It is suggested that OCH3 fission and subsequent reactions of the phenoxy radicals contribute to the coloration of lignin, which contains identical structural elements.

Long-wave photochemistry of di-methoxylated compounds related to lignin

Abstract Four methoxylated compounds which resemble moieties either found in lignin or produced in lignin, i.e. poly(3,4-dimethoxyacrylophenone) (Poly 34), guaiacylacetoveratrone (34Keto), 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propan-1,3-diol (34Hydroxy) and 3,3′,5,5′-tetramethoxy-4-hydroxytilbene (ST2), were irradiated in the form of thin films under vacuum, wavelengths being restricted to 300 nm and greater. In all cases new absorptions were formed in the long-wave UV and visible regions and the samples became coloured (yellow), ST2 being particularly reactive. In all cases methane and ethane were the only two low molecular weight products, and it was proposed that OCH 3 bond fission was occurring, all of the compounds having appreciable long-wave absorptions before irradiation. 13 C NMR spectra showed that the depletion of the methoxy C-atoms was accompanied by the formation of new carbonyls. Further evidence for OCH 3 fission was obtained from the photolysis of 3-ethoxy-4-hydroxystilbene in which ethane, ethylene and n -butane were formed. Spectral observations indicated that the new carbonyls were quinones and o -quinones, these being formed from the phenoxy radicals in turn formed by OC fissions, and the coloration was attributed to quinonoid species. The work demonstrates that lignin-type compounds can undergo coloration reactions, which are independent of oxidation, and are totally photo-induced. Plausible reaction mechanisms are proposed.

Long-wave photodegradation of poly(3,5-dimethoxyacrylophenone) in the solid state

Abstract The photolysis of poly(3′,5′-dimethoxyacrylophenone) was studied in the solid state, thin films being exposed to long-wave (i.e. λ ⩾ 300 nm) u.v. radiation under high vacuum at 25 °C. The polymer underwent colouration (initially yellow) and new carbonyl species were formed. The degraded polymer reacted with o -phenylenediamine to produce a phenazine derivative which exhibited strong fluorescence at 350 nm. The intensities of NMR signals due to the methoxy carbons decreased, and, at the same time, methane and ethane were formed (quantum yields in the 10 −4 region). It was concluded that OCH 3 bond scission was occurring, the methyl radicals producing ethane and methane and the phenoxy radicals being transformed into quinonoid compounds to which the colouration was attributed. The polydispersity of the polymer increases during photodegradation, and this was ascribed to simultaneous chain scission and crosslinking, the former being associated with a type II decomposition and the latter being brought about by combination reactions of macro-phenoxy radicals. A plausible reaction scheme is presented.

Photodegradation of poly(3,5-dimethoxyacrylophenone) in the long-wave UV region

The photophysics and long-wave (λ ≥ 300 nm) photochemistry of poly(3,5-dimethoxyacrylophenone) (P3,5DMAP) were investigated. The quantum yield for fluorescence is low (< 10−4) and the principal excited species is the triplet carbonyl. Emission (phosphorescence) occurs at 450 nm, the triplet energy (i.e. ET(0-0)) is 270 kJ mol−1 and the phosphorescence lifetime is long (23 μs). These observations are consistent with the formation of a low-lying π,π∗ triplet which, in turn, is the result of substitution of the aromatic ketone by electron donating methoxy groups. The principal photoprocess is a Norrish type II decomposition which results in random chain scission. The low value of the quantum yield, i.e. Φcs = 10 4, is further evidence for a π,π∗ triplet which is typically unreactive in abstraction reactions. In the case of P3,5DMAP, H-abstraction is a prerequisite for biradical formation (and for chain scission). Inhibition of chain scission was investigated, using naphthalene and cis-1,3-pentadiene as quenchers, and the data could be represented by Stern-Volmer kinetics. However, the quenching rate constants (kO) were lower than the corresponding diffusion controlled values, and it was proposed that the actual encounter of triplet and quencher is subject to adverse steric interactions associated with the proximity of the polymer chains.

CHROMOPHORE PRODUCTION IN LIGNIN MODEL COMPOUNDS - ANAEROBIC REACTIONS

The anaerobic photochemistry of a number of plausible lignin model compounds (i.e.I: 3,4-Dimethoxy-α-(2-methoxyphenoxy)-β-hydroxypropiophenone;II: 1-(3,4-Dimethoxyphenyl)-2-(2-methoxyphenoxy)-propan-1,3-diol;Pol A: Poly(4-methoxyacrylophenone);Pol B: Poly(3,4-dimethoxyacrylophenone);St 1: 3,5-Dimethoxy-4-hydroxystilbene; andSt 2: 3,5,3′,5′-Tetramethoxy-4-hydroxystilbene) was studied, thin films of these materials being exposed to long-wave (λ≥300 nm) radiation under high vacuum conditions (10−6 torr). In all cases, the only low molecular weight products formed were methane and ethane, and quantum yields were estimated for these reactions. All materials underwent colouration (yellow) and a number of changes were also observed in both the absorbing and emitting characteristics. The colouration was attributed to the presence ofo-quinones which were formed (by further photolysis) from the phenoxy radicals, which were, in turn, produced by O−CH3 fission, the resulting methyl radicals being the precursors of methane and ethane. The stilbenes were in all cases much more reactive (by a factor of about 100); however, they also absorbed higher intensities of radiation in the 300<λ<350 nm region on account of the greater extent of red-shifting of the longest-wave π-π′ aromatic transitions. Gel permeation data indicate the formation of products of cyclization and isomerization of stilbenes and the dimerization of phenoxy radicals while new absorbances in the infrared and13C NMR confirm the presence ofo-quinones in all the models.

Thermal degradation of poly(O-propionylstyrene)

Abstract The thermal degradation of poly( o -propionylstyrene) (POPS) was studied at 385°C under high vacuum. The principal reactions are removal and decomposition of the propionyl substituents, depolymerization, oligomer formation and chain scission. While the mechanism of degradation is qualitatively similar to that of poly(styrene), the probability of transfer reactions occurring with the polymer is considerably greater, on account of the presence of ethyl and methyl radicals (derived from the propionyl groups). The resulting macroradicals undergo β-scission, and this reaction accounts for most of the chain scission (yielding a terminally unsaturated molecule and another macroradical). These two species further decompose to give (respectively) oligomeric products and monomer, the relative abundance of oligomers to monomer being about twice that observed for PS, and this has been attributed to shorter zip lengths for depolymerization and to the more likely occurrence of transfer reactions in POPS.