Poly , radical species

We have reported that ESR and ENDOR (52) examination of degassed, irradiated samples of a number of polymers (PDHS, PDBS, poly(di-a-tetra-decylsilane) (PDTDS), poly(di-n-octylsilane (PDOS), poly(di-n-decylsilane) (PDDS) and poly(di-4-methylpentylsilane) (PDMPS) shows the formation of radical species which are persistent for hours and even days. The radical spectra (see Figure 4) are, however, clearly not consistent with those expected from the simple cleavage of the polymer backbone as described by Equation [2]. The structure of these radicals... 

Although N-halogenated pyrazoles are known, they are quite unstable compounds, increasing in stability from chloro to iodo. They may be implicated in the formation of poly halogenated derivatives (76MI2). There are many examples of competition between nuclear and side-chain halogenation, particularly when radical species can form (90CHE301). 

There is evidence that both ionic and free radical species are involved in the degradation and depolymerization of poly (olefin sulfone) s by high energy radiation (70). Thus, the yields of olefins from poly (1-butene sulfone) at 30 °C (the sample was heated to 70 °C during removal of the gaseous products) are shown in Table II. The butene is not solely 1-butene, but comprises significant proportions of all three isomers, 1-butene, 2-butene and isobutene. 

The poly (amino acid)s with aromatic side chains behave somewhat differently. In poly(phenylalanine) the a-carbon radical is the major radical species observed, but radicals formed by hydrogen atom addition to the ring are also found. Benzyl radicals formed by side-chain cleavage are present, but only in very low yield. In poly (tyrosine) the only radical species observed is the tyrosyl phenoxyl radical formed by loss of the hydroxyl hydrogen. There is no evidence for formation of significant concentrations of a-carbon radicals. Thus, the nature of the substituents can strongly influence the radiation sensitivity of the backbone chain. 

In carbon tetrachloride poly(a-methylstyrene) were degraded, even without oxygen, by the irradiation of ionizing radiation [58], The decay of the charge transfer radical complex, observed in this solvent, may be due to the reaction of a chlorine atom with p-site H of poly(a-methylstyrene) the reaction leads to the formation of P-position radicals of poly(a-methylstyrene). The produced polymer radicals were unstable and dissociated into neutral and radical species. 

In the pulse radiolysis of aqueous solutions of poly(styrenesulfonate), various radical species have been identified [83]. OH radicals reacted with this polymer to produce a mixtures of OH adducts. When several OH radical adducts were produced on the same polymer molecule, the intramolecular radical-radical reactions occurred. A radical cation was also produced from the OH adduct by the following acid catalyzed reaction. 

As reported in Table 15, the kinetic data clearly indicate that the photoinitiation activity of poly(BMOA-co-MtA) is not substantially affected by the content of BMOA co-units along the polymer chain and is of the same order of magnitude as that found for the model compound BMOAc. The absence of a polymer effect in the above photoinitiators has been interpreted [84] in terms of a photodegradation mechanism of the macromolecules involving the ftee radical species anchored to the main chain, even in the presence of acrylic monomers, analogous to what is reported in Scheme 18. Moreover, the induction period of the HDDA/BA photoinduced polymerization increases, on decreasing the content of... 

Structure in the ESR spectrum for peroxy radicals arises fromf-tensor ankotropy, and can give rise to different spectra under different situations. %>ectra for poly-propjdene peroj radicals at four different temperatures are shown in F. and are due to Chien and Boss Which spectrum obtains, depends on the nwbility of the molecules containii the peroi radical species. Increasii mobility leads to an averaging of the values and a reduction of the number of lines in the spectrum, until only a broad sin t is observed. 

In a study, copper-catalyzed radical copolymerization of nBA and methacryloxy-capped poly(MMA) was compared with conventional radical copolymerization.267 431 The graft copolymers G-l obtained with copper catalysts are more homogeneous in terms of MWD (Mw/Mn 1.6 vs 3) and the number of side chains. This is attributed to diffusion control being less important in the metal-catalyzed radical polymerization, where the growing radical species is rapidly converted into the dormant covalent species. 

As described In the Introduction and In references 3-5, poly-DCH has been found to react with bromine at 268 K. The purpose of these experiments was to see If there were any free radical Intermediates found during the reaction. Howeyer, none were found. Instead, the radicals observed In pristine poly-DCH disappear and two new radical species at lower concentrations reappear. 

The ESR spectrum observed for polycrystalline samples of poly-DCHBr, is shown in Figure 5. The most important question to be answered about this spectrum is whether it arises from a single species with a pseudo axial g-tensor or from two different radical species. Based on variable temperature and variable microwave power studies of this material we have concluded that the spectrum arises from two different radical species. However, the evidence in that regard leaves some uncertainty. Additional experiments are planned. 

In electrochemical polymerization a solution of monomer is oxidized or reduced at the electrode surface to generate reactive radical species which couple together and produce an adherent polymer film at the electrode. The films can be electronically conducting, as in the case of pyrroles, anilines, thiophenes, etc redox conductors in which conduction occurs by self-exchange between discrete redox sites attached to the polymer, as in metal poly(pyridine) complexes or insulating, as in the case of phenols, 1, 2-diaminobenzene, etc. 

The growing concern over the toxicity of residual ethylene oxide after sterilization of polymers for use in the field of medicine has led to the rapid growth of the field of radiation sterilization. The medical industry consumes a massive volume of polymeric material in both equipment and implants. As a consequence there is much interest in the effects of radiation on the physical properties and stability of irradiated polymers. The standard dose for radiation sterilization is 25 kGy, which is sufficient to alter the properties of many polymers, being for example close to or above the gel dose of many elastomers. There is also interest in the reaction of oxygen with long-lived radical species formed during irradiation. A common polymer used in medical equipment, poly(propylcne) is susceptible to oxidative degradation, and must be blended with appropriate stabilizers before radiation sterilization. 

---