Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene‐propylene copolymer (EPM), and ethylene‐propylene‐diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000     

REACTION MECHANISM OF BENZOPHENONE-PHOTOINITIATED CROSSLINKING OF POLYETHYLENE

The radical intermediates, the crosslink microstructures, and the reaction mechanism of benzophenone (BP)-photoinitiated crosslinking of low-density polyethylene (LDPE) and model compounds (MD) have been reviewed in detail.The spin-trapping electron spin resonance (ESR) spectra obtained from the LDPE/BP systems with spin-trap agents showthat two kinds of polymer radical intermediates are mainly formed: tertiary carbon and secondary carbon radicals. The spin-trapping ESR studies of MD/BP systems give further evidence that photocrosslinking reactions of PE predominantly takeplace at sites of tertiary carbon, secondary carbon, and especially allylic carbon when available. The high resolution ~(13)C-NMR spectra obtained from LDPE and MD systems show that the crosslink microstructures have H- and Y-type links andthat their concentrations are of the same order. The fluorescence, ESR ~(13)C and ~1H-NMR spectra from the PE and MDsystems demonstrate that the main photoreduction product of BP(PPB) is benzpinacol formed by the recombination of twodiphenylhydroxymethyl (K·) radical intermediates. Two new PPB products: an isomer of benzpinacol with quinoid structure,1-phenylhydroxymethylene-4-diphenylhydroxymethyl-2, 5-cyclohexadiene and three kinds of α-alkyl-benzhydrols have beendetected and identified. These results provide new experimental evidence for elucidating the reaction mechanism in the BP-photoinitiated crosslinking of polyethylene.     

Photocrosslinking of poly(methyl acrylate)s containing the 1,3-dioxolane structure as a pendant group

(2-Ethyl-2-methyl-1,3-dioxolan-4-yl)methyl- and (2-methyl-2-phenyl-1,3-dioxolan-4-yl) methyl acrylates were synthesized and polymerized. The photochemical behavior of the resulting polymers was investigated to determine whether the polymers pending on the 1,3-dioxolane structure were readily crosslinked with ultraviolet (UV) irradiation. The degree of crosslinking was estimated by the weight-loss method by immersion in acetone, with the result that the polymer with an aromatic substituent was more photocrosslinkable than the polymer that bore the aliphatic substituent. The catalytic effect on photocrosslinking of polymers was also studied by using benzoin and cobalt naphthenate. The infrared (IR) spectra of polymers irradiated in air that showed the new band at 3450 cm^?1 were attributed to a hydroxyl group; however, the spectra of polymers irradiated in vacuum displayed little absorption at 3450 cm^?1. To explain the mechanism of crosslinking model compounds were prepared and irradiated with UV light. It was concluded that crosslinking proceeds mainly from the fission of the 1,3-dioxolane ring and the coupling of the yielding radicals, together with autooxidation by atmospheric oxygen.     

Aminoxyl radicals as crosslinks for macromolecules of polyvinylpyrrolidone

Using polyvinylpyrrolidone as an example, it has been shown that photolysis of ceric ammonium nitrate at room temperature can result in crosslinking of macromolecules. This process correlates with the formation of stable aminoxyl radicals, which are registered by EPR. The mechanism involves photodissociation of nitrate radicals produced in the primary reaction into nitric oxide or nitrogen dioxide depending on the wavelength of the light, and simultaneous formation of macroradicals. The accumulation of aminoxyl radicals occurs owing to the acceptance of macroradicals by nitroso or nitro groups according to which mechanism of the nitrate radical photodissociation prevails. Similar radical reactions are observed in N-methyl-2-pyrrolidone.     

S′H type reactions of substituted allylic compounds

S′_H reactions of allyl sulfides and halides with phenyl radicals are reported. Thermal decomposition of phenylazotriphenylmethane with allyl sulfides and bromide has been shown to give allylbenzene. This apparent substitution reaction involves attack of a phenyl radical on the terminal unsaturated carbon atom of the allyl sulfide; the reaction in α,α-dimethylallyl ethyl sulfide produced 2-methyl-4-phenylbutene-2. To estimate the relative reactivities of allylic substrates towards phenyl radicals, competitive reactions of phenyl radicals with allylic compounds and carbon tetrachloride were investigated. The data indicate that the radical formed by addition of a phenyl radical to the allylic sulfide looses thiyl radicals almost quantitatively.     

Interaction of polypyromellitimide with nitrogen dioxide

The mechanism of interaction of nitrogen dioxide with aromatic polyimides is considered by the example of polypyromellitimide. The formation of stable radicals of acylarylaminoxyl, iminoxyl and phenoxyl types has been detected by electron paramagnetic resonance spectroscopy. Acylarylaminoxyl radicals were detected in polypyromellitimide after its exposure to nitrogen dioxide at room temperature followed by pumping nitrogen dioxide from the samples. Iminoxyl and phenoxyl radicals were formed during thermolysis of the nitration products of the polymer at 373 K. The proposed mechanism is based on the reaction of dimers of nitrogen dioxide in the form of nitrosyl nitrate. It was observed that intermediate radical cations and nitric oxide were formed in the primary reaction of electron transfer from the polyimide to nitrosyl nitrate. The subsequent cage reactions with participation of radical cations and nitric oxide give nitroso compounds and nitrates which are precursors of stable nitrogen-containing and phenoxyl radicals.     

Photocrosslinking reaction of vinyl‐functional polyphenylsilsesquioxane sensitized with aromatic bisazide compounds

Photochemical reactions of aromatic azide groups were applied for a novel photosensitive silicone ladder polymer, that is, partially vinyl‐substituted polyphenylsilsesquioxane sensitized with aromatic bisazide compounds as a photocrosslinker. The photocrosslinking reaction in this system was investigated from the viewpoint of the efficiency of the photocrosslinker, that is, the ratio of the photocrosslinker consumed for crosslinking. The numbers of photodecomposed azide groups and crosslinks in the polymer were determined by Fourier transform infrared measurements. At a higher bisazide concentration, the predominant reaction of nitrenes formed as the intermediary radical by the photolysis of azide was a coupling reaction that could not contribute to the gelation of the polymer. The ratio of the bisazide compound consumed for crosslinking showed the highest value at its concentration of 3 wt % and decreased with the addition of a larger amount. The semiempirical molecular orbital calculations were applied to the theoretical analysis of the photoreaction of nitrenes using phenylnitrene as a model structure. The calculation results indicated that the coupling reaction of nitrenes should proceed more easily than the photocrosslinking reaction in N₂ atmosphere, and the fact that the oxidation of nitrenes should proceed exclusively in the atmosphere including O₂ agreed with the experimental results. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 4196–4205, 2001     

A biocompatible nanocomposite based on allyl chitosan and vinyltriethoxysilane for tissue engineering

The synthesis, structure, and electrophysical properties of a polymer-inorganic biocompatible composite based on unsaturated chitosan ether, namely, allyl chitosan, and vinyltriethoxysilane are studied. During composite synthesis, allyl chitosan forms an individual nanophase with vinyltriethoxysilane and its condensation products in the polymer matrix of allyl chitosan. The size of nanoparticles embedded in a polymer matrix increases from 50 to 1000 nm as the fraction of the added vinyltriethoxysilane grows. Under exposure to UV radiation, both homopolycondensation and heteropolycondensation occur in the composite films via crosslinking according to the radical mechanism and the composite becomes insoluble in water. It has been shown that the resulting composites feature ionic conductivity under application of both direct current and high-frequency electric fields to the sample. Conductivity is provided by a proton–electron ensemble that concentrates at the nanoparticle/polymer matrix interface.     

Note Synthesis and Characterization of Poly-(2-Methallyl-p-cresol)

In spite of the low reactivity of allyl monomers, attempts have been made to prepare polymers by polyrecombination of allyl aromatic compounds. We were encouraged by the stability of allyl radicals [1-8]. The literature on the Claisen rearrangement of polyfunctional allyl aryl ethers contains some observations of the formation of tarry masses and resinification of polyfunctional aryl ethers. Details of the polymerization and the possible structures were not studied. We felt it interesting to synthesize this new monomer, 2-methyallyl-p-cresol, by the rearrangement, and to study in detail the polymerization and characterization of the polymer.     

A new mechanism of benzophenone photoreduction in photoinitiated crosslinking of polyethylene and its model compounds

The photolytic products and a new photoreduction mechanism of benzophenone (BP) as a photoinitiator in the photocrosslinking of polyethylene (PE) and its model compounds (MD) have been studied by means of fluorescence, ESR, ¹³C and ¹H NMR spectroscopy. The fluorescence spectra from the PE and MD systems demonstrate that the main photoreduction product of BP (PPB) is benzpinacol formed by the recombination of two diphenylhydroxymethyl (K^•) radical intermediates. The ESR spectrum obtained from the UV irradiation of the MD/BP system gives positive evidence of K^• radicals. Two new PPB products: an isomer of benzpinacol with quinoid structure, 1‐phenyl‐hydroxymethylene‐4‐diphenyl‐hydroxymethyl‐2,5‐cyclohexa‐diene and three kinds of α‐alkylbenzhydrols have been detected and identified for the first time by ¹³C and ¹H NMR spectroscopy from the MD systems. The latter could be formed by the reactions of K^• radicals with alkyl radicals produced by hydrogen abstraction of the excited triplet state ³(BP)* from polyethylene or its model compounds. These results provide new experimental evidence for elucidating the photoreduction mechanism of BP in the photoinitiated crosslinking of polyethylene. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 999–1005, 2000     

Cage effects in recombination of allyl radicals in irradiated polyethylene

A cage model has been presented to describe the kinetics of recombination of radicals in solid polymer. The theory includes Torrey's treatment for jump diffusion and radiative boundary condition in the diffusion equation to account for the hindrance to the diffusion of macroradicals and the finite cage process of recombination reaction, respectively. The result has been applied to the interpretation of data on the decay of allyl radical in irradiated polyethylene.     

Photocrosslinking of elastomer systems—III Ultraviolet light induced reactions in EPDM and EPR blended with or modified by grafting of derivatives of benzophenone

Photocrosslinking experiments were performed in elastomeric polymer films (EPR or EPDM) that either contained benzophenone or some of its derivatives, or had been previously modified (EPDM) by grafting of benzophenone. This latter modification was performed by a series of Friedel-Crafts reactions. The resulting materials were characterized by i.r. and u.v. spectroscopy in conjunction with physical property measurements. The results allowed relationships between the properties of the photosensitizers in their excited states and their efficiency in the crosslinking processes to be established. The results suggest that each coupled polymer-photoinitiator exhibits a particular behaviour. Thus it appeared that, in the presence of oxygen, the hexadiene branches of the terpolymer can undergo various types of reactions. Moreover, the grafting of the photosensitizer onto the elastomer backbone was shown to enhance considerably the crosslinking reaction. It is possible to conclude that oxygen plays a substantial role in the photocrosslinking. Indeed it always enhances crosslink formation. It can be assumed that the photooxidation of the polymer backbone yields alkoxy and hydroperoxy radicals which can act as crosslink sites.     

Photocrosslinking of silicones. VI. Photocrosslinking kinetics of silicone acrylates and methacrylates

The photoinduced radical crosslinking of silicones containing pendant acrylate and methacrylate groups has been investigated with calorimetric and ESR measurements. Oxygen very strongly influences this process, which leads to a prolonged induction period and a pseudo first-order termination reaction between polymer radicals and oxygen. Kinetically, such reaction steps are responsible for light intensity and monomer exponents, both of unity. In the absence of oxygen, second-order processes take place between polymer and primary radicals. The results of conversion-time and reaction-rate time measurements using stationary irradiations and postpolymerization experiments agree with the corresponding kinetic expressions. Long-lived polymer radicals and their decay have also been determined with ESR techniques. Long-chain spacer groups, which link the unsaturated ester moiety at the silicone backbone, increase the crosslinking rate. The final conversions of the double bonds exceed high values in each case. © 1992 John Wiley & Sons, Inc.     

Radiation induced crosslinking of poly(vinyl methylether) in aqueous solutions

Radiation induced crosslinking of poly(vinyl methylether) (PVME) has been investigated in aqueous solutions. The spectral and kinetic features of the transients involved in the crosslinking reaction have been studied by pulse radiolysis of dilute PVME solutions. H atoms reacts with PVME, like OH radicals, by abstracting an H atom predominantly from β-position with respect to ---OCH₃ group, but the rate of reaction of H atom is an order of magnitude slower than that of OH reaction. The PVME radicals formed by H attack have been found to decay by usual 2nd-order kinetics unlike PVME radicals produced by OH attack that are reported to decay by a complex time-dependent kinetics that deviates strongly from 2nd-order kinetics. The rate constant of e⁻_aq with PVME at pH 5.5 has been found to be 1.2×10⁸ dm³ mol⁻¹ s⁻¹. From the decay behaviour of the transient species formed by reaction of e⁻_aq with PVME, it has been shown that the transient initially reacts with solvent protons by a fast reaction to yield radical species which subsequently recombine by a slow mode. The dependence of gelation dose and radiation yields of crosslinking (Gx) of PVME on various factors such as polymer concentration, dose rate, pH, presence of oxygen and crosslinking agent has also been studied by steady-state radiolysis using an electron-beam accelerator.     

Photodynamic Crosslinking of Proteins. III. Kinetics of the FMN- and Rose Bengal-sensitized Photooxidation and Intermolecular Crosslinking of Model Tyrosine-containing N-(2-Hydroxypropyl)methacrylamide Copolymers

As part of a study on the role of Tyr residues in the photosensitized intermolecular crosslinking of proteins, we have surveyed the kinetics of the rose bengal- and flavin mononucleotide (FMN)-sensitized photooxidation and crosslinking of a water-soluble N-(2-hydroxypropyl)methacrylamide copolymer with attached 6-carbon side chains terminating in tyrosinamide groups (thus the -OH group of the Tyr is free, but both the amino and carboxyl groups are blocked, simulating the situation of a nonterminal Tyr in a protein). The intermolecular photodynamic crosslinking of the Tyr copolymer can result only from the formation of Tyr-Tyr (dityrosine) bonds, because the copolymer itself is not photooxidizable. Rose bengal, primarily a Type II (singlet oxygen) sensitizer, sensitized the rapid photooxidation of the Tyr residue in the Tyr copolymer only at high pH, where the Tyr phenolic group is ionized; crosslinking did not occur with rose bengal under any of the reaction conditions used. In contrast, FMN, which can sensitize by both Type I (free radical) and Type II processes, sensitized the photooxidation of the Tyr copolymer over the pH range 4-9.5. Also, significant photocrosslinking occurred, but only from pH 4 to 8, with a maximum rate at pH 6. Crosslinking required the presence of oxygen. Studies with inhibitors, D2O as solvent, catalase and superoxide dismutase indicated that the photooxidation and photocrosslinking of the Tyr copolymer with FMN at pH 6 were not mediated by singlet oxygen, superoxide or hydrogen peroxide. It appears that crosslinking involves the abstraction of an H atom from the Tyr phenolic group to give Tyr and FMN radicals. The Tyr radical in one Tyr copolymer can then react with a Tyr radical in another Tyr copolymer to give an intermolecular dityrosine crosslink.     

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.     

Kinetics and mechanisms of the allyl + allyl and allyl + propargyl recombination reactions

The kinetics and mechanisms of the self-reaction of allyl radicals and the cross-reaction between allyl and propargyl radicals were studied both experimentally and theoretically. The experiments were carried out over the temperature range 295-800 K and the pressure range 20-200 Torr (maintained by He or N(2)). The allyl and propargyl radicals were generated by the pulsed laser photolysis of respective precursors, 1,5-hexadiene and propargyl chloride, and were probed by using a cavity ring-down spectroscopy technique. The temperature-dependent absorption cross sections of the radicals were measured relative to that of the HCO radical. The rate constants have been determined to be k(C(3)H(5) + C(3)H(5)) = 1.40 × 10(-8)T(-0.933) exp(-225/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.088) and k(C(3)H(5) + C(3)H(3)) = 1.71 × 10(-7)T(-1.182) exp(-255/T) cm(3) molecule(-1) s(-1) (Δ log(10)k = ± 0.069) with 2σ uncertainty limits. The potential energy surfaces for both reactions were calculated with the CBS-QB3 and CASPT2 quantum chemical methods, and the product channels have been investigated by the steady-state master equation analyses based on the Rice-Ramsperger-Kassel-Marcus theory. The results indicated that the reaction between allyl and propargyl radicals produces five-membered ring compounds in combustion conditions, while the formations of the cyclic species are unlikely in the self-reaction of allyl radicals. The temperature- and pressure-dependent rate constant expressions for the important reaction pathways are presented for kinetic modeling.     

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.     

Development of a Kinetic Model for Thermal Crosslinking of Rubbery Polymers

Summary: A kinetic model for the thermal crosslinking of rubbery polymers is presented. The reaction mechanism used to develop the model includes thermal radical generation producing a polymeric radical and a primary radical, crosslinking from attack of a polymer radical to any inactive polymer molecule, bimolecular radical termination among chains of any degree of branching, and radical termination between a polymer radical and a primary radical from the thermal radical generation. The overall polymer population is divided into “generations”, which are defined in terms of the number of primary (linear) chains of original rubbery polymer attached to the polymer molecule. Mass balances for each generation are written down, and expressions to calculate fraction of branched molecules, gel fraction, and number and weight average chain lengths are derived. Model simulations, parameter sensitivity analyses and preliminary parameter estimation studies are presented, taking the thermal crosslinking of linear polybutadiene as a case study.

Development of a kinetic model for thermal crosslinking of rubbery polymers: reaction mechanism, model equations and parameter sensitivity analyses.     

Features of stable radical generation in lignin on exposure to nitrogen dioxide

The mechanism of stable radical generation in lignin under the action of nitrogen dioxide and NO₂ - air mixture is considered. The formation of phenoxyl, iminoxyl and acylaminoxyl radicals has been detected by EPR. The proposed mechanism involves a primary oxidative reaction of phenol groups with dimers of NO₂ (nitrosyl nitrate) resulting in the formation of phenoxyl radicals and nitric oxide. In the subsequent recombination of phenoxyl radicals and nitric oxide, nitroso compounds and oximes are formed. By reaction of oximes with radicals NO₂, stable iminoxyl radicals are formed. This mechanism is confirmed by kinetic dependencies obtained over a wide range of NO₂ concentrations. From IR spectroscopy measurements it follows that hydroxyl groups of non-phenolic structures of lignin are oxidised to aldehydes producing acylaminoxyl radicals by reaction with NO₂. The kinetic data show that the adsorption of NO₂ on the lignin surface is the rate-determining factor in stable radical formation.