Styrene photooxidation

This reaction is considered the most important initiation mechanism for styrene photooxidation in the presence of aromatic ketones. 

The product of sensitised photooxidation of c7.v-2-mcthoxy styrene is explosive. 

Photooxidation of poly(styrene) has been a subject of considerable interest over the past 25 years (14,15). However, the surface photooxidation of poly(styrene), the aspects of which are most pertinent to the proposed photooxidative scheme, has only been examined recently (16,17). Free radical intermediates have been proposed to account for the formation of oxidized groups upon 254 nm irradiation of poly(styrene). 

A photooxidative scheme has been developed to pattern sub half-micron images in single layer resist schemes by photochemical generation of hydrophilic sites in hydrophobic polymers such as poly(styrene) and chlorinated poly(styrene) and by selective functionalization of these hydrophilic sites with TiCU followed by O2 RIE development. Sub half-micron features were resolved in 1-2 pm thick chlorinated poly(styrene) films with exposures at 248 nm on a KrF excimer laser stepper. The polymers are much more sensitive to 193 nm (sensitivity 3-32 mJ/cm2) than to 248 nm radiation (sensitivity -200 mJ/cm2) because of then-intense absorption at 193 nm. 

The observed ambient organic aerosol formation rates are also consistent with those estimated by extrapolation of smog-chamber kinetic data. Other heavy unsaturates, such as styrene and indene, are present in the atmosphere and may contribute, in part, to the formation of benzoic acid and homophthalic acid, respectively. Diesel exhaust and industrial emission are possible sources of such heavy unsaturates. Diolefins of C6+ are not present in gasolines and exhaust gases and have not been found in the atmosphere, and their possible role as precursors of the Cs-7 difiinctional acidic compounds is seriously challenged. Lower diolefins are emitted in automobile exhaust. Examination of vapor-pressure data indicates that the bulk of their expected photooxidation products remains in the gas phase, including most of the less volatile C3-4 dicarboxylic acids. 

Atkinson (1985) reported a photooxidation reaction rate of 5.25 x lO " cmVmolecule-sec for styrene and OH radicals in the atmosphere. A reaction rate of 1.8 x 10 L/molecule-sec at 303 K was reported for the reaction of styrene and ozone in the vapor phase (Bufalini and Altshuller, 1965). 

Grosjean, D. Atmospheric reactions of ortho cresol gas phase and aerosol products, Atmos. Environ., 19(8) 1641-1652,1984. Grosjean, D. Photooxidation of methyl sulfide, ethyl sulfide, and methanethiol, Environ. Sci. Technol, 18(6) 460-468,1984a. Grosjean, D. Atmospheric reactions of styrenes and peroxybenzoyl nitrate, ScL Total Environ., 50 41-59, 1985. 

During weathering, phenolic antioxidants are photooxidized into hydroperoxycy-clohexadienones, such as 59 (Pospisil, 1993 Pospisil, 1980). The presence of peroxidic moieties in 57 and 59 renders them thermolabile at temperatures exceeding 100 °C and photolysable under solar UV radiation. Both processes account for homolysis of the peroxidic moieties. As a result, the oxidative degradation of the polymeric matrix is accelerated by formed free-radical fragments (tests were performed with atactic polypropylene and acrylonitrile-butadiene-styrene terpolymer (ABS) (PospiSil, 1981 PospiSil, 1980). Low-molecular-weight products of homolysis, such as 60 to 63 are formed in low amounts. 

After the examination of the PS photooxidation mechanism, a comparison of the photochemical behavior of PS with that of some of its copolymers and blends is reported in this chapter. The copolymers studied include styrene-stat-acrylo-nitrile (SAN) and acrylonitrile-butadiene-styrene (ABS). The blends studied are AES (acrylonitrile-EPDM-styrene) (EPDM = ethylene-propylene-diene-monomer) and a blend of poly(vinyl methyl ether) (PVME) and PS (PVME-PS). The components of the copolymers are chemically bonded. In the case of the blends, PS and one or more polymers are mixed. The copolymers or the blends can be homogeneous (miscible components) or phase separated. The potential interactions occurring during the photodegradation of the various components may be different if they are chemically bonded or not, homogeneously dispersed or spatially separated. Another important aspect is the nature, the proportions and the behavior towards the photooxidation of the components added to PS. How will a component which is less or more photodegradable than PS influence the degradation of the copolymer or the blend We show in this chapter how the... 

SAN is constituted of styrene and acrylonitrile units copolymerized statistically in the ratio 80 20 mol%. Previous studies on the photooxidation of PS [7,8] and PAN [3] have shown that the photooxidation rates of these polymers were very different PS degrades about 20 times faster than PAN. Consequently, the first steps of photooxidation of the copolymer SAN is presumed to involve mainly the styrene units. SAN samples have been irradiated and analyzed under the same conditions as PS samples. 

Fourier transform (FT) IR analysis of the photooxidized SAN samples shows that the oxidation products formed in the copolymer may result not only from the oxidation of the styrene units, even in the first few hours of irradiation [11]. Figure 30.4 shows that the absorbance of the carbonylated photoproducts in the photooxidized SAN samples is different compared with PS (Figure 30.1). Substantial evidence for the contribution of the acrylonitrile units in the photooxidation was obtained by chemical and physical treatments carried out on pre-photooxidized samples as described above. For example, the SF4 treatment of a SAN photooxidized sample led to a partial decrease in absorbance in the hydroxyl region, corresponding to the disappearance of alcohols, hydroperoxides and acids. The absorbance remaining after treatment may be assigned to... 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

We conclude that the photooxidative degradation of SAN is initiated by the styrene units. The mechanism of SAN presents three ways of degradation a classical photooxidation pathway of PS, an oxidation of the acrylonitrile units initiated by the oxidation of the adjacent styrene units and a degradation of the acrylonitrile units due to the formation of acids among the photoproducts. 

Under these conditions of irradiation, the oxidation of the acrylonitrile component does not proceed, as shown by the invariance of the band (2237 cm-1) of acrylonitrile units. The calculated photooxidation rate of a model ABS is also reported in Figure 30.5. In this reference system, no interaction occurs between the photooxidation of the elastomeric and styrenic phases. The photochemical evolution of the model ABS has been determined for various irradiation times the increase in absorbance of the BR homopolymer has been added to that of the PS homopolymer and corrected by a multiplicative factor that takes into account the percentage of each component in the experimental ABS blends. 

The comparison of the photooxidation rates reported in Figure 30.5, along with analysis of FTIR and UV spectra, suggests that the BR component was implicated as the prime site for the photooxidation of ABS and that the photooxidation of the styrenic phase was enhanced by the presence of the polybutadiene in ABS, compared with PS homopolymer. 

In secondary steps, radical species formed in BR photooxidation are able to initiate the oxidation of the neighbouring styrenic moieties the photooxidation of the styrenic phase is enhanced by the presence of polybutadiene in ABS, compared with PS homopolymer. Butadiene grafting sites, containing tertiary allylic carbon atoms (A), are preferentially oxidized in tertiary hydroperoxides in the first stages of ABS photooxidation rather than in secondary allylic carbon atoms (B) ... 

AES (acrylonitrile-EPDM-styrene) is a blend of SAN and EPDM. SAN is a statistic copolymer of styrene and acrylonitrile. EPDM is an elastomeric terpo-lymer of ethylene, propylene and a nonconjugated diene. The diene studied here was 5-ethylidene-2-norbomene. The total content of EPDM was 34mole% [6]. The diene represented 8 mole% of the EPDM and the SAN phase was composed of 80 mole% of styrene and 20 mol% of acrylonitrile. The AES films were irradiated at / > 300 nm at 60 °C in the presence of oxygen. The photoproducts resulting from the photooxidation of each components of AES were identified by FTIR spectroscopy coupled with the same chemical and physical treatments as mentioned above for the previous studies. As pointed out in the literature [17], the EPDM component is more reactive than the copolymer SAN towards photooxidation. 

Oxidation of the styrene moiety and changes in the nitrogen environment in benzotriazole took place in 94. Available data confirm that the surface photooxidation of polymers containing aromatic moieties can differ from that of the bulk material [86]. This is due to the higher partial pressure of oxygen and incident photon flux in the surface area. Transmission IR data indicate that the quenching capabilities of benzotriazole moieties in the copolymer 94 are not active in inhibiting surface oxidation. 

DUV exposure of poly( p-substituted styrenes), such as poly( p-chloro-styrene), poly( p-chloromethylstyrene), and poly( p-hydroxystyrene) [poly(p-vinylphenol)] (structure 3.11), in air leads to photocross-linking and photooxidation. Consequently, DUV hardening is applicable to resists based on these polymers as well as novolac-based resists (168, 169). 

Weak Links and Energy Sinks in the Photooxidation of Styrene Polymers and Copolymers in Solution... 

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