Kenneth Whiting

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.

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).

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.

Photoreactions of poly(p-propionylstyrene) in solution

Abstract Dilute solutions (2 × 10−3 M) of poly(para-propionylstyrene) were exposed to long-wave u.v. radiation (λ ⩾ 300 nm) under high vacuum at 25°C. Methane and ethane were formed, their quantum yields [

Thermal degradation of poly(o-acetylstyrene)

Abstract The thermal degradation of poly( o -acetylstyrene) (POAS) was studied at 390°C under high vacuum. The principal reactions are removal and decomposition of acetyl groups, depolymerization, random chain scission and oligomer formation. The degradation mechanism resembles that of polystyrene (PS) but the presence of methyl radicals increases the number of transfer reactions with the polymer. The resulting chain radicals undergo β-scission (chain scission), one of the two species formed yielding oligomeric products and the other yielding monomer. The ratio of oligomer to monomer concentration is greater than that observed for PS: this effect has been attributed to shorter zip lengths, and to the occurrence of additional transfer reactions in POAS.

Polymers with enhanced photostability: photoreactions of poly(o-isobutyrylstyrene) in the long-wave region

Abstract Poly( o -isobutyrylstyrene) (POIS) was exposed in the solid state to long-wave UV radiation ( λ ≥300 nm) under high vacuum conditions at 25±1 °C. Transient spectral measurements indicate the presence of tow distinct enols, ie. the syn and anti , which have lifetimes of 163 ns and 4.5 μs respectively. The most important photoprocess is photoenolization, and although the syn carbonyl triplet is principally involved, experimental data indicate that the corresponding singlet may also contribute to the process. The anti carbonyl triplet appears to play a relatively passive part in photoenolization; however, it is involved in α-scission which leads to the formation of isopropyl radicals which, in turn, give rise to propane (principal product), propene and 2,3-dimethylbutane. Quantum yields for the formation of these products are low (10 −5 ), and this reflects the extent of competition from photoenolization. Similarly, the extent of cross-linking is very small. The carbonyl concentration is depleted to a minimal extent; thus, in terms of photodegradation, POIS appears to be a relatively photostable polymer.