Efficiency of radiation-induced main-chain scission of poly (methyl methacrylate) depends on the irradiation temperature because of coexisting monomer

Effect of irradiation temperature on the main-chain scission of poly (methyl methacrylate) (PMMA) caused by γ-irradiation was studied by means of gel permeation chromatography and ESR spectroscopy. Although no temperature dependency was observed on the scission efficiency for purified PMMA, the efficiency for crude or monomer-doped purified PMMA was decreased by decreasing the temperature below ca. 200 K. Above 200 K the efficiency was constant and did not depend on the purity of PMMA. ESR study of the irradiated PMMA revealed that the suppression of the scission below 200 K is induced by the addition of methyl methacrylate monomer to primary radical species, which otherwise cause the main-chain scission by warming the polymer above 200 K. The primary radical generated above 200 K immediately converts to the scission-type ? CH₂ ? ?(CH₃) COOCH₃ radical through the β-scission of the polymer main chain, so that the efficiency of the scission does not depend on both the impurity and the irradiation temperature. © 1994 John Wiley & Sons, Inc.     

Main chain scission reaction of poly(methyl methacrylate) caused by two-photon ionization of dopant

The main chain scission reaction of poly(methyl methacrylate) (PMMA) doped with N,N,N,′,N′-tetramethyl-p-phenylenediamine (TMPD) was examined by ESR spectroscopy and GPC measurement, and the scission mechanism was analyzed. The two-photon ionization of TMPD with excimer laser excitation at 77 K produced an ester radical anion of PMMA (PMMA·m?), which becomes the main chain tertiary radical ? CH₂? C˙(CH₃)? CH₂? after the detachment of the ester side group by annealing of the sample at room temperature. The main chain scission radical ˙C(CH₃)(COOCH₃)? (PMMA˙) which was produced by the β-scission from? CH₂? ˙C(CH₃)? CH₂? showed the 13-line ESR spectrum instead of the ordinary 9-line, due to the fast quenching of the sample to 77 K. The change of the molecular weight distribution was measured by GPC after several irradiation-and-annealing operations. The simulation of the GPC curve confirmed that the scission re-action occurs at random in the PMMA chain in the solid and the main chain scission yield from the ester radical anion, [PMMA˙]/[PMMA·m?], is 0.30. © 1995 John Wiley & Sons, Inc.     

The thermo-oxidative degradation of polyolefines—Part 10. Correlation between the formation of carboxyl groups and scission in the oxidation of polyethylene in the melt phase

Films of low density polyethylene have been degraded under an oxygen atmosphere at temperatures above the semicrystalline melting point. Time, conversion and temperature dependence of carboxyl group formation and chain scission have been studied. After induction periods we found linear dependences both in function of time and conversion. One third of absorbed oxygen forms carboxyl groups and the absorption of 3·57 mmol oxygen per monomer unit is needed for one chain scission. Maximum rates of carboxyl formation and chain scission have Arrhenius temperature dependence with 33·5 kcal/mole activation energy. The number of carboxyl groups and chain scissions are always practically the same; we assume that the isomerisation of secondary alkyl peroxy radicals simultaneously causes chain scission and carboxyl formation.     

Direct strand scission in double stranded RNA via a C5-pyrimidine radical

Nucleobase radicals are the major family of reactive intermediates produced when nucleic acids are exposed to γ-radiolysis. The 5,6-dihydrouridin-5-yl radical (1), the formal product of hydrogen atom addition and a model for hydroxyl radical addition, was independently generated from a ketone precursor via Norrish Type I photocleavage in single and double stranded RNA. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and only minor amounts of strand scission are observed at the initial site of radical generation. Strand scission occurs preferentially in double stranded RNA and in the absence of O(2). The dependence of strand scission efficiency from the 5,6-dihydrouridin-5-yl radical (1) on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. Internucleotidyl strand scission occurs via β-elimination of the 3'-phosphate following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical yields RNA fragments containing 3'-phosphate or 3'-deoxy-2'-ketonucleotide termini from competing deprotonation pathways. The ketonucleotide end group is favored in the presence of low concentrations of thiol, presumably by reducing the cation radical to the enol. Competition studies with thiol show that strand scission from the 5,6-dihydrouridin-5-yl radical (1) is significantly faster than from the 5,6-dihydrouridin-6-yl radical (2) and is consistent with computational studies using the G3B3 approach that predict the latter to be more stable than 1 by 2.8 kcal/mol.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 21: Improved product yields on true bimolecular hydroperoxide decomposition

The product yields from the reaction between two hydroperoxide groups have been re-calculated. This is a consequence of the fact that β-scission of secondary alkoxy radicals cannot be neglected in the high temperature range of the polyethylene processing experiments (170-200 °C). It must be taken into account in addition to disproportionation/combination and hydrogen abstraction by alkoxy radicals. The increased complexity caused by the additional reaction results mainly from the larger number of caged radical pairs involved in the reactions and also in the calculations. Among other products it becomes possible to calculate the yields of aldehyde and vinyl groups that would not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols, ketones and trans-vinylene groups are reduced to some extent in comparison with the values calculated if β-scission is neglected. The vinyl group yield corresponds to slightly more than 10% of the yield of trans-vinylene groups in the temperature range of the experiments. The aldehyde yield is significantly larger than the vinyl group yield and is important in the whole temperature range examined. Main-chain scissions are important at the temperatures of the experiments. They become more important than the sum of the different combination reactions from a temperature of 200 °C on.     

Mechanistic analysis and thermochemical kinetic simulation of the pathways for volatile product formation from pyrolysis of polystyrene, especially for the dimer

Simulations of the initial distribution of volatiles from pyrolysis of polystyrene were based on propagation rate constants estimated by thermochemical kinetic procedures. The voluminous database exhibits a disturbing lack of consistency with respect to effects of conversion level, temperature, and reactor type. It therefore remains difficult to assign the true primary distribution of the major products, styrene, 2,4-diphenyl-1-butene (“dimer”), 2,4,6-triphenyl-1-hexene (“trimer”), 1,3-diphenylpropane, and toluene, and its dependence on conditions. Probable perturbations by secondary reactions and selective evaporation are considered. The rate constant for 1,3-hydrogen shift appears much too small to accommodate the commonly proposed “back-biting” mechanism for dimer formation. Dimer more likely arises by addition of benzyl radical to olefinic chain-ends, followed by β-scission, although ambiguities remain in assigning rate constants for the addition and β-scission steps. With this modification, the major products can be successfully associated with decay of the sec-benzylic chain-end radical. In contrast, the minimal formation of allylbenzene, 2,4-diphenyl-1-pentene, and 2,4,6-triphenyl-1-heptene suggests a minimal chain-propagating role for the prim chain-end radical. Compared with polyethylene, the much enhanced “unzipping” to form monomer from polystyrene and the more limited depth of “back-biting” into the chain arise from an enthalpy-driven acceleration of β-scission coupled with a kinetically driven deceleration of intramolecular hydrogen transfer. In contrast, the greater “unzipping” of poly(isobutylene) compared with polyethylene is proposed to result from relief of steric strain.     

Thermal degradation of keto polymers: Part 1—Poly(vinylacetophenone)

The thermal degradation of poly(vinylacetophenone) (PVAP) was studied at 380°C under high vacuum. The degradation products were separated into gaseous and liquid fractions and identified (gc, nmr, ms). The principal reactions are removal and decomposition of acetyl groups, depolymerization and random chain scission, as evidenced by the pattern of number average molecular weight changes. While the basic mechanism of degradation resembles that of poly(styrene) (PS), i.e. initial random chain scission, the number of transfer reactions is considerably greater due to the presence of methyl radicals which abstract from C-atoms in the polymer chain. The resulting chain radicals undergo β-scission (the principal mode of chain scission) to yield a terminally unsaturated molecule and a macro radical. These two species further decompose to give oligomeric products and monomer. The relative abundance of oligomers to monomer is considerably greater than that observed for PS. This has been attributed to shorter zip lengths for depolymerization and to the occurrence of more transfer reactions in PVAP. Plausible mechanisms for the formation of the various reaction products are given.     

Mechanically induced scission and subsequent thermal remending of perfluorocyclobutane polymers

Perfluorocyclobutane (PFCB) polymer solutions were subjected to pulsed ultrasound, leading to mechanically induced chain scission and molecular weight degradation. (19)F NMR revealed that the new, mechanically generated end groups are trifluorovinyl ethers formed by cycloreversion of the PFCB groups, a process that differs from thermal degradation pathways. One consequence of the mechanochemical process is that the trifluorovinyl ether end groups can be remended simply by subjecting the polymer solution to the original polymerization conditions, that is, heating to >150 °C. Stereochemical changes in the PFCBs, in combination with radical trapping experiments, indicate that PFCB scission proceeds via a stepwise mechanism with a 1,4-diradical intermediate, offering a potential mechanism for localized functionalization and cross-linking in regions of high stress.     

Degradation of poly(vinyl chloride) by u.v. radiation—II: Mechanism

The mechanism of the light-induced degradation of solid poly(vinyl chloride) (PVC) has been investigated, and an overall reaction scheme has been developed, based on values of the quantum yields for the primary photoproducts. Only a very small fraction (0.2%) of the excited polyenes induces the degradation of PVC, primarily by photocleavage of the allylic CCl bond. The high instability of β-chloroalkyl radicals is responsible for the chain dehydrochlorination that leads to formation of polyenes. In the absence of O₂, chain scissions and crosslinking are postulated to originate mainly from α-chloroalkyl radicals through β-cleavage of CC bonds and radical coupling, respectively. In the presence of O₂, the chain dehydrochlorination still proceeds, together with an oxidative chain process which yields, via peroxy and alkoxy radicals, hydroperoxides, ketones and peroxide crosslinks. Cleavage of the polymer backbone results most probably from the decomposition of tertiary alkoxy radicals by a carbon-carbon β-scission process.     

Physico-chemical aspects of polyethylene processing in an open mixer. Part 20: Additional product yields on bimolecular hydroperoxide decomposition with an alcohol group

Most products formed on polyethylene oxidation result from hydroperoxide decomposition. The product yields can be calculated for various mechanisms of hydroperoxide decomposition. This work concerns the reaction of a hydroperoxide with an alcohol group thought to be dominant in the advanced stages of polyethylene processing in the high temperature range (170-200 °C). Besides hydrogen abstraction by caged alkoxy radicals already envisaged previously, the possibility of β-scission is taken into account. This additional reaction introduces significant complexity into the reaction schemes. This is especially so because additional caged radical pairs must be included into the schemes and the calculations. It becomes possible to calculate the yields of aldehyde and vinyl groups that do not result from hydroperoxide decomposition in the absence of β-scission. The yields of the main oxidation products such as alcohols and ketones are not much affected by taking into account β-scission. The yield of aldehydes is important in the whole temperature range and increases considerably if the temperature is raised from 170 to 200 °C. It becomes more important than the ketone yield. The vinyl groups are formed in amounts corresponding roughly to 10-15% of the trans-vinylene groups in the temperature range of 170-200 °C.     

The role of chain scission in fracture of amorphous polymers

We have investigated the role of chain scission in glassy polymers by monitoring the molecular weight changes induced by microtoming thin slices of monodisperse polystyrenes. The changes in number-average molecular weight allow determination of N_f, the number of bond scissions per unit area. It is found that N_f is independent of initial molecular weight and has the value 6.50 × 10¹³ scissions/cm² at room temperature; N_f decreases with increasing temperature, suggesting that chain pullout increases with temperature. The work required to create unit surface area in polystyrene is several orders of magnitude greater than the energy required to break N_f bonds, indicating that plastic deformation plays a major role in deformation and fracture of glassy polymers.     

Intramolecular 1,8-hydrogen atom transfer reactions in disaccharide systems containing furanose units

A previously developed 1,8-hydrogen atom transfer (HAT) reaction promoted by 6-O-yl alkoxyl radicals between the two pyranose units in Hexp-(1→4)-Hexp disaccharides has been extended to other systems containing at least a furanose ring in their structures. In Penf-(1→3)-Penf (A) and Hexp-(1→3)-Penf (B) disaccharides, the 1,8-HAT reaction and concomitant cyclization to a 1,3,5-trioxocane ring are in competition with radical β-scission of the C4-C5 bond and formation of dehomologated products. The influence of the stereoelectronic β-oxygen effect on the β-scission and consequently on the 1,8-HAT reaction has been studied using the four possible isomeric d-furanoses. d-xylo- and d-lyxo-derivatives afforded preferentially 1,8-HAT products, whereas d-arabino- and d-ribo-derivatives gave exclusively direct β-scission of the alkoxyl radical. When the 6-O-yl radical is on a pyranose ring, as occurs in Penf-(1→4)-Hexp (C), it has been shown to provide the cyclized products exclusively.     

Control of degradation of polypropylene during its radical functionalisation with furan and thiophene derivatives

The traditional melt radical functionalisation of isotactic polypropylene (iPP) with maleic anhydride (MAH) and peroxide affords functionalized samples with a severe decrease of the average molecular weight (MW) due to the β-scission reaction. In this work new push-pull unsaturated molecules were investigated, consisting of a heterocyclic ring conjugated with a double bond bearing an electron attracting group. These molecules were specifically designed as MAH substitute able to limit the iPP degradation, while providing functionalisation through grafting. Butyl 3-(2-furanyl) propenoate (BFA) and butyl 3-(2-thienyl) propenoate (BTA) were comparatively tested. The analysis of the reaction products indicated that both molecules are able to graft onto the iPP backbone by prompt reaction with the macro-radicals formed through H-abstraction from iPP chains, thereby significantly limiting the MW decrease, as the functionalized macro-radicals are stabilized by resonance. Nonetheless, some of iPP macro-radicals can give a parallel chain scission before reacting with the new molecules. In the case of BFA, coupling reactions of the formed macro-radicals can lead to the formation of branched high MW architectures, whereas in the case of the thiophene derivative (BTA) only a partial retaining of polymer chain fragmentation was observed due to the reduction of β-scission.     

Cyclic Acetal-Photosensitized Polymerization. VI. Photopolymerization of 2-Vinyl-1,3-dioxolane

The photopolymerization of 2-vinyl-l,3-dioxolane (VDO) was carried out in benzene at 40° C without use of the usual radical initiator. VDO was decomposed by means of photoirradiation to a cyclic acetal radical which transformed instantly into the ester radical by β-scission of dioxolane ring: the vinyl polymerization could be initiated by the ester radical. Because of the degradative chain transfer by allylidene group, the rate of polymerization and the molecular weight of polymer were very small.     

Macromolecular scission and crosslinking rate changes during polyolefin photo-oxidation

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of various thermoplastics exposed to ultraviolet irradiation (UV). Results are given for a high density polyethylene (HDPE); a low density polyethylene (LDPE); a linear low density polyethylene (LLDPE); a polypropylene homopolymer (PPHO); and a polypropylene copolymer (PPCO). As the oxidation progressed, it was observed that the scission rate for HDPE, LLDPE, PPHO and PPCO increased near to the exposed surface whereas for LDPE the rate remained almost unchanged. The crosslink rate fell near to the surface with HDPE and LDPE but increased with PPHO and PPCO. The reaction rates near to the bar centre (∼1.5 mm from the exposed surface) were low for HDPE, PPHO and PPCO; this is attributed to oxygen starvation, caused by consumption of oxygen by rapid reaction near the surface. Reaction was observed in the interior with LDPE and LLDPE, presumably because of a combination of a higher oxygen diffusion rate than for HDPE and a lower rate of consumption of oxygen near the surface than with the polypropylenes.     

Product and mechanistic analysis of the reactivity of a C6-pyrimidine radical in RNA

Nucleobase radicals are the major reactive intermediates produced when hydroxyl radical reacts with nucleic acids. 5,6-Dihydrouridin-6-yl radical (1) was independently generated from a ketone precursor via Norrish Type I photocleavage in a dinucleotide, single-stranded, and double-stranded RNA. This radical is a model of the major hydroxyl radical adduct of uridine. Tandem lesions resulting from addition of the peroxyl radical derived from 1 to the 5'-adjacent nucleotide are observed by ESI-MS. Radical 1 produces direct strand breaks at the 5'-adjacent nucleotide and at the initial site of generation. The preference for cleavage at these two positions depends upon the secondary structure of the RNA and whether O(2) is present or not. Varying the identity of the 5'-adjacent nucleotide has little effect on strand scission. In general, strand scission is significantly more efficient under anaerobic conditions than when O(2) is present. Strand scission is more than twice as efficient in double-stranded RNA than in a single-stranded oligonucleotide under anaerobic conditions. Internucleotidyl strand scission occurs via β-fragmentation following C2'-hydrogen atom abstraction by 1. The subsequently formed olefin cation radical ultimately yields products containing 3'-phosphate or 3'-deoxy-2'-ketouridine termini. These end groups are proposed to result from competing deprotonation pathways. The dependence of strand scission efficiency from 1 on secondary structure under anaerobic conditions suggests that this reactivity may be useful for extracting additional RNA structural information from hydroxyl radical reactions.     

Chain scission in polystyrene by impact fracture

The number of chain scissions n_s per unit fracture area by impact in high-molecular weight polystyrene is determined to be approximately 3.3 × 10¹⁴/cm² at room temperature. This is almost 20 times larger than would be expected if chain scissions took place only at, or very close to, fracture surfaces. This result was obtained by measuring the molecular weight decrease and the total fracture area of the impact fragments by using size exclusion chromatography and statistical particle size measurements, respectively. The large n_s strongly indicates that significant chain breakage occurs during crazing before the propagation of cracks. An average craze thickness before breakdown under impact is estimated from n_s to be around 2 μm. In a diluted polymer, n_s is found to be significantly lower than the extrapolated value, assuming a linear dilution of entangled chain crossings at the fracture surface. This low chain scission density, however, can be explained by taking into account the reduction of craze breakdown strain in the diluted polymers. Finally, the broken chain ends of polystyrene appear to be stable under ambient conditions. © 1992 John Wiley & Sons, Inc.     

Simulations on radiation-cured polyethylene

Simulations of the simultaneous crosslinking and chain scission reactions that are induced in bulk amorphous polyethylene by irradiation have been implemented with a computer. A detailed study of the various defect structures in the network is reported, to show that dangling ends have the highest population among these defects, especially at higher crosslink densities. The calculated sol fractions compare very favorably with previously published experimental studies. We find that there is about 1 chain scission for every 9 crosslinked units, i.e., one scission per 4.5 crosslinks. This is fewer scissions than had previously been deduced by application of the Charlesby-Pinner method.     

A Density Functional Theory Study of Poly (vinyl chloride) (PVC) Free Radical Polymerization

Summary: Quantum chemistry was applied to the free radical polymerization of Vinyl Chloride with the aim of elucidating the reaction kinetics and especially the formation of structural defects and low molecular weight polymer. The radical reactions were studied using the Density Functional Theory. All calculations were performed with B3LYP functionals and in particular the 6-31G(d,p) basis set was selected to evaluate the exchange and correlation energies. The computational method was first validated by predicting the rate constant of the propagation step and comparing the calculated values to experimental ones. Then intramolecular chain transfer, β-scission and branching reactions were also investigated, due to their direct connection with the production of defects in the growing chains. A comparison of the evaluated kinetic constants of such secondary reactions with other computational evaluations and experimental data was finally made.     

Mechanism of photo-oxidation of an elastomer

Ultraviolet irradiation in air of various elastomeric substances results in crosslinks, chain scissions and oxidation functions. The quantum yields of the different processes and the oxygen balance have been determined in the case of a model system. These results make it possible to propose a mechanism of photo-oxidation which agrees well with the experimental data. The rôle played by hydroperoxide functions has been recognised and demonstrated; their sensitised generation and decomposition have been explained in terms of energy transfer phenomena. Lastly, changes in the macromolecular chains and network formation have been followed. The results demonstrate quantitatively how light energy is absorbed by impurity in a polymer and is transferred to potential radical sites (hydroperoxides). Chain radical reactions develop in the material, leading predominantly to photocrosslinking simultaneously with a chain scission process which allows spatial reorganisation of the polymer medium.