N.A. Weir

Photochemistry of ring substituted polystyrenes—Part III: Photolysis of poly(para-methoxystyrene)

Abstract The photodegradation of films (4 × 10 −4 cm thick) of poly( p -methoxystyrene) with 254 nm radiation under high vacuum at 25°C has been studied. The principal gaseous product is hydrogen, but smaller quantities (in decreasing yield) of methane, methyl alcohol and ethane are also formed, indicating that fission of bonds in the para methoxy group is also involved. Ultraviolet and visible spectra of degraded films indicate the presence of unsaturated groups and of coloured species. Solubility data indicate that crosslinking and chain scission occur simultaneously. Rates of chain scission of a number of p -substituted styrenes are compared and a reasonable correlation between these and the electron donating character of the para group is observed, explainable in terms of the stabilising effect of such groups on the radicals formed during chain scission. Rates of crosslinking are greater than those for polystyrene and this is attributable to the participation of the substituted phenoxy radicals (formed by CH 3 O fission) in addition reactions. Quantum yields for the gaseous products and for chain scission and crosslinking have been determined and a mechanism has been advanced to account for the experimental data.

Photoreactions of poly(vinylacetophenone) in solution

Abstract Excitation of poly(vinylacetophenone) (PVAP) at 308 nm produces a transient (λmax = 362 nm) which was assigned to the n → π ∗ triplet. Time resolved spectroscopy indicates a triplet lifetime of 184 ± 10 nsec. Dilute solutions of PVAP (in Ch2Cl2) were exposed to long-wave u.v. radiation (λ ⩾ nm) under high vacuum at 25 ± 1°C. The principal gaseous product was methane but the quantum yield for its formation (≈ 10−4 mol einstein−1) was considerably lower than that found from similar studies of films. Ethane was formed in trace amounts only. A Norrish Type I decomposition was involved. Molecular weight measurements indicated that PVAP undergoes chain scission, attributed to β-scission of the tertiary radicals, produced by H-abstraction at the α-C-atom by the carbonyl triplet and, to a lesser extent, by the methyl radicals. The presence of hydrogen donors (isopropanol) reduces the extent of chain scission, and alters the balance between this process and cyclization (intramolecular crosslinking). Solvent quality plays an important role, particularly if the added poorer solvent (e.g. methanol) is not an efficient transfer agent. Tighter coiling of the polymer leads initially to more intramolecular photoreduction, and with it, more chain scission. However at higher concentrations of non-solvent, diffusive separation of the macro-fragments becomes more difficult, and the balance shifts to cyclization and to intermolecular cross-linking, resulting in increases in molecular weight.

Initial steps in the photolysis and photooxidation of polystyrene

Abstract The effects of exposure of a number of differently prepared polystyrenes to long-wave u.v. ( λ ⩾ 300 nm) in O 2 were investigated. The polymers undergo two distinct phases of degradation, one associated with the initial decomposition of photo-labile structures, and the other with the photolysis of oxidation products, such as hydroperoxides. Polymers prepared under conditions of high free radical concentrations (higher temperatures and initiator concentrations) were the most reactive and contained more photo-labile structures. Hydroperoxides were prepared from these polymers and their degradations investigated. While hydro-peroxides act as initiators of the oxidative phase of the degradation, by producing radicals on photolysis, no convincing evidence was obtained for their participation in the initial photodecomposition. On the other hand, experimental data favoured the involvement of in-chain peroxides in this process. Differences of reactivity of polymer solutions and films were also observed, the apparent relative stability of films being attributed to the difficulty associated with the separation of large fragments in a more viscous medium.

Some aspects of the long-wave photo-oxidation of polystyrenes

Abstract The degradation which is brought about by exposing a number of polystyrenes prepared in various ways to long-wave ultra violet radiation in the presence of oxygen, has been investigated. Radically prepared polymers undergo two distinct degradation processes: one is associated with the initial photo-decomposition of labile species in the chains, the other with the photolysis of oxidation products, such as hydroperoxides. Anionically prepared polystyrene, by comparison, is unreactive. Earlier work suggested that the photo-labile entities were in-chain peroxides, and the present work lends support to the proposal. Possible photo-labile species were investigated. Hydroperoxides were prepared from anionically and radically prepared polymers. While rates of degradation were increased, the initial chain scission characteristics were very similar to those of the polymers themselves. Similar results were obtained when polymers were exposed to high oxygen pressures, under which conditions charge transfer complexes may be formed. It can be seen that if hydroperoxides and/or charge transfer complexes are present, the rates of degradation are increased. However, their contributions to the initial phases of the degradation, and to the actual initiation of photo-degradation of polystyrene on exposure to terrestrial sunlight in air, are not significant.

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).

Reactions of free radicals with polymers—II: Abstraction reactions of methyl and acetyl radicals with poly(vinyl acetophenone)

Abstract The kinetics of H abstraction by methyl and acetyl radicals from poly(vinyl acetophenone) (PVAP) films (4 × 103 mm thick) have been investigated, both radicals being derived from the polymer by photolysis (λ ≥ 300 nm) under high vacuum conditions (pressure

Reactions of methyl radicals with polymers—part 1: Abstraction reactions with poly(p-methoxystyrene)

Abstract The kinetics of hydrogen abstraction by methyl radicals from poly( p -methoxystyrene) have been investigated, methyl radicals being produced by the photolysis ( λ = 254 nm) of the polymer under high vacuum. A new differential equation has been derived to describe simultaneous diffusion and reaction of methyl radicals; its solution, along with experimentally determined yields of methane, have been used to obtain a value for the rate constant for H abstraction from the polymer. The value of this constant is compared with previous data for poly(styrene) and the apparent agreement is found to be fortuitous. Relative to similar gas phase abstractions, the rate constants are lower by about four orders of magnitude; some of the factors contributing to this difference are discussed.

Co-photolysis of poly(styrene) and poly(vinylacetophenone)—I. Vacuum photolysis

Abstract Films made from blends of poly(styrene) (PS) and poly(vinylacetophenone) (PVAP) (up to 12% mol PVAP) were exposed under high vacuum to long wave u.v. radiation (λ > 300nm). Spectral changes (phosphorescence, NMR and i.r. indicated that PVAP was photolysed: the nature of gaseous products (in order of abundance, methane, carbon monoxide, ethane and acetaldehyde) confirmed that PVAP was undergoing a Norrish Type I decomposition. There was also evidence of photoreduction, OH absorptions appearing in the i.r. The blended films undergo cross-linking but the insoluble material formed after about 10hr contains little carbonyl. The effects of PVPA concentration and molecular weight on the rate of product formation were also investigated. Experiments with poly(α-D-styrene) confirmed that methyl radicals were abstracting H atoms at the α-position in PS. The implications of such reactions in the initiation of photo-oxidation by polymeric ketonic species are discussed.

Reactions of radicals with polymers—III. Abstraction reactions of methyl and tert. butyl radicals with poly(p-tert. butyl styrene)

Abstract The kinetics of H abstraction by methyl and tert.butyl radicals from poly( p -tert.butyl styrene) (PTBS) films (5 × 10 −3 mm thick) have been investigated at 298 K, both radicals being produced from the polymer by its exposure to short-wave u.v. radiation ( λ = 254 nm) at pressures −4 Pa. Differential equations describing simultaneous diffusion and reaction of each of the radicals have been developed, and their solutions (for both steady and non-steady state conditions) used along with the experimentally determined abstraction products (CH 4 and C 4 H 10 ) to evaluate rate constants for abstraction, which occurs predominantly at the α-carbon atoms in the polymer. the values of the rate constants for the two radicals were unexpectedly different (by at least a factor of 10 3 ) and explanations are advanced. Steady and non-steady state solutions rapidly converge. Experimental and theoretical product evolution (based on the solutions of the equations) were compared; deviations between the two sets of data, appearing after about 2 hr photolysis, are discussed.