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.     

Catalytic epoxidation of styrene–butadiene triblock copolymer with hydrogen peroxide

Styrene-butadiene-styrene linear block copolymer (SBS) can be epoxidized with hydrogen peroxide in the presence of methyltrioctylammonium tetrakis (diperoxotungsto) phosphate(3-) as the catalyst in a biphasic system. The effects of reaction time, temperature, solvent, the ratio of the organic phase to the aqueous phase, the concentration of the catalyst, hydrogen peroxide and polymer, respectively, are studied on the conversion of double bonds to oxirane groups. ¹H-NMR analysis confirms the absence of ring opening side reaction in this epoxidation reaction system up to at least 70% conversion of the double bonds. When the conversion of double bonds is over about 70% the resultant polymer is insoluble in chloroform even at reflux but soluble in polar solvents such as DMSO when heated. Toluene is a better solvent for the reaction than dichloroethane. The reaction rate constants are measured at four temperatures and the activation energy for the reaction with toluene as solvent is determined as 49.9 kJ/mol.     

Autoxidation of poly-4-methylpentene-1. I. The role of diffusion in autoxidation kinetics

An apparatus is described for the measurement of oxygen uptake into a polymer sample at constant oxygen pressures in the range 20–1000 mm Hg. Measurements of the rate of oxygen uptake into poly-4-methylpentene-1 show that the rate is accurately first-order in oxygen pressure over the range 50–800 mm pressure for temperatures ranging from 122 to 154°C and film thickness in the range 0.001–0.025 cm. A theoretical treatment of the kinetics of a reaction in which oxygen diffuses into both faces of a thin film, in which it is consumed by a first-order reaction shows that the oxidation rate ρ per unit area of film surface is given by ρ = ρ_∞ tanh ßL/2 where ρ_∞ is the limiting oxidation rate for a thick film, L is the film thickness, and ß = (k/D)^1/2, k being the oxidation rate constant and D the diffusion constant. Values of D and the activation energy for diffusion calculated from autoxidation data are in good agreement with values determined directly.     

Regioselectivity and Stereoselectivity in the Intramolecular Nitrile Oxide-Olefin Cycloaddition (INOC) Reaction and the Intramolecular Silyl Nitronate-Olefin Cycloaddition (ISOC) Reaction

Both INOC and ISOC reactions of 1 which has two different double bonds in a molecule (allylic and homoallylic double bond) showed high regioselectivity for allylic double bond to make [5,5] ring system. INOC reactions of 1 and 6 showed high diastereoselectivity for 4-substituents, whereas ISOC reaction did for 6-substituents of tetrahydrofuroisoxazolines.     

Die Copolymerisation von Vinylacetat und Divinyladipat. Vernetzung und Cyclisierung

The reactivity ratios for the vinyl acetate-divinyl adipate system are determined. Both valuesr _{V Ac} andr _{DV A} are near to unity. The reactivity of the pendent double bond is very low and soluble polymers are formed far over the critical conversion calculated with anr-value of the pendent double bonds equal to unity. Contrary to this a high portion of intramolecular cyclization occurs, presumably leading to chain units of the type amounting to 10% of the total number of divinyl adipate units in the chain in systems with an initial monomer concentration of 95% and to 23% in systems with a monomer concentration of 50%. The reactivity ratio of the pendent double bond is estimated. The main reason for the high conversion for gel formation is the low rate of intermolecular reaction of pendent double bonds. In this respect cyclization plays a minor role.     

Die Copolymerisation von Vinylacetat und Divinyladipat. Vernetzung und Cyclisierung

The reactivity ratios for the vinyl acetate-divinyl adipate system are determined. Both valuesr _{V Ac} andr _{DV A} are near to unity. The reactivity of the pendent double bond is very low and soluble polymers are formed far over the critical conversion calculated with anr-value of the pendent double bonds equal to unity. Contrary to this a high portion of intramolecular cyclization occurs, presumably leading to chain units of the type amounting to 10% of the total number of divinyl adipate units in the chain in systems with an initial monomer concentration of 95% and to 23% in systems with a monomer concentration of 50%. The reactivity ratio of the pendent double bond is estimated. The main reason for the high conversion for gel formation is the low rate of intermolecular reaction of pendent double bonds. In this respect cyclization plays a minor role.

---

---

Mit 2 Abbildungen     

Radiation-induced crosslinking and cyclization in 1,2-polybutadiene

In order to get information on the radiolytic changes in 1,2-polybutadiene (1,2-PB) the sol and gel fractions, the conversion of double bonds, the structure and concentration of radicals, the formation of dienes and the formation of gaseous products were measured. In addition, the dose rate dependence and temperature dependence for the conversion of double bonds were determined. G values for double bond conversion depend on molecular weight and range from 20 to 200. G values for crosslinking are about 10. A mechanism for the double bond conversion is proposed which involves initiation by a transformation of the primary radical ion in the vinyl group into a carbonium ion and a radical. This is supported by ESR measurement. Reaction of the carbonium ion with a vinyl group in the same chain gives rise to cyclization, whereas reaction with a vinyl group in a neighboring chain results in crosslinking. A comparison of the G values for conversion of double bonds with the G values for crosslinking shows that the formation of cyclic rings exceeds the formation of crosslinks by a factor of about 10. The corresponding values in 1,4-cis- and 1,4-trans-polybutadiene are much smaller [G(cl) ? 2; G(db) ? 7]. The pendent vinyl groups in 1,2-polybutadiene therefore are more reactive than the vinylidene groups in 1,4-polybutadienes.     

Control of reactions between surfactant reagents in miniemulsions. Surface nanoreactors

Emulsions may be used to speed up reactions of surface-active reagents. In this paper, a theoretical analysis of a simple catalytic reaction (A + B → P + B) is performed, where the substrate A in the presence of the catalyst B in an emulsion is converted into the product P, and both the substrate A and the catalyst B are surfactants. It was shown that, because molecules A and B are concentrated in surface layers of the emulsion, these layers act as nanoreactors ensuring a significant acceleration of the catalytic reaction within a certain range of emulsion droplet sizes. The reaction rate depends significantly on the emulsion droplet’s size and there exists an optimal droplet size at which the reaction acceleration is maximal. If the product of the reaction is not surface-active, the reaction rate can remain practically unchanged up to virtually complete substrate conversion. Besides, it was shown that the Michaelis–Menten-type dependence of the reaction rate on the substrate concentration (i.e., the increase in the rate with subsequent saturation) can be observed in the system under consideration.     

Radiation Graft Copolymerization of n-Butyl Acrylate on Natural Rubber Latex

Abstract

A method of radiation graft copolymerization of n-butyl acrylate (NBA) on natural rubber (NR) latex has been studied. The rate of conversion increases with the increase of NBA in latex. An irradiation dose of about 12 kGy is needed to obtain 90% conversion with 40 phr of NBA in latex. Tensile strength, tear strength, and elongation at break of grafted NR are found to decrease with increasing degree of grafting. The physical strength of a vulcanizate prepared from a mixture of NR and ply-NBA was found to be better than that of NBA-NR graft copolymer vulcanizate. The graft copolymerization reaction takes place in the outer layer of NR particles, and because the secondary bonds between poly-NBA molecules may be weaker than those between NR molecules, the existence of a poly-NBA layer in NR particles will decrease its physical strength.     

Copolymerisation von Vinylacetat und Divinyladipat Reaktivität der anhängenden Doppelbindungen

The weight average degree of polymerization of vinyl acetate-divinyl adipate copolymers (low content of divinyl adipate) is measured at conversions up to 22%. From thisk ₁₃/k ₁₂ (rate constants of reaction of growing radicals with pendent double bonds and divinyl adipate double bonds, respectively) is calculated to be 0.28. A quantitative relation for the gel point (critical conversion) is given.     

Polymerization of allyl esters of unsaturated acids. V. Cyclopolymerization of methyl allyl fumarate

The kinetics of radical polymerization of methyl allyl fumarate (MAF) is discussed in terms of cyclopolymerization and compared with the polymerization results of methyl allyl maleate (MAM) as a cis isomer. In the polymerization of MAF, the rate and degree of polymerization were quite enhanced compared with MAM, and gelation occurred at low conversion. The content of the unreacted allylic double bonds of the MAF polymer was quite large; whereas those of the unreacted fumaric double bonds and the cyclic structural units showed reverse tendencies. Only a slight presence of a five-membered ring was observed in the MAF polymer. The cyclization constants K_A and K_V, the ratios of the rate constants of the unimolecular cyclization reaction to those of the bimolecular propagation reaction of the uncyclized allylic and fumaric radicals, were estimated to be 2.73 and 1.48 mole/liter, respectively. These values suggest the great difference in the cyclopolymerization behavior between two isomeric monomers. These results are discussed in detail in connection with the high reactivity of the fumaric double bond compared to the maleic double bond. In addition, the formation mode and the sequence distribution of the structural units of the polymer produced are discussed on the basis of these analytical results. Thus, for the MAF polymer obtained in the bulk polymerization, about 60% of the cyclic structure can be formed via the intramolecular attack of the uncyclized fumaric radical on the allylic double bond, as opposed to the case of MAM via the predominant intramolecular attack (ca. 90%) of the uncyclized allylic radical on the maleic double bond; these results and the low probability for the succession of cyclic structures and the rather high probability of a vinyl-to-vinyl addition are presented.     

Enzymatic epoxidation of soybean oil in the presence of perbutyric acid

Enzymatic epoxidation of vegetable oils using a long chain fatty acid as an active oxygen carrier could produce a desirable epoxy oxygen group content (EOC); however, the acid value (AV) of final epoxidized oil is too high. The present study was to investigate the effect of different fatty acids with varying length of carbon chain on EOC and AV of the final epoxidized soybean oil (ESO); finding butyric acid was the choice of active oxygen carrier when hydrogen peroxide was used as an oxygen donor in the presence of lipase Novozyme 435. And in situ IR was used to monitor the epoxidation process, which revealed that the formation of perbutyric acid was the key step in the whole reaction. The epoxidation process was optimized as follows: molar ratio of butyric acid/C=C bonds of 0.19:1, 8% of immobilized lipase Novozyme 435 load (relative to the weight of soybean oil) and molar ratio of H₂O₂/C=C bonds of 3.5:1, reaction time of 4 h and reaction temperature of 45 °C. Under these conditions, ESO with a high EOC (7.62 ± 0.20%) and a lower AV value (8.53 ± 0.18 mgKOH/g) was obtained. The oxriane conversion degree was up to 97.94%.     

Synthesis and characterization of allyl functionalized poly(α-hydroxy)acids and their further dihydroxylation and epoxidation

In this study, the synthesis of an allyl functionalized aliphatic polyester and the subsequent oxidation of the double bonds was investigated. Allylglycolide (3-allyl-1,4-dioxane-2,5-dione) was synthesized and its homopolymer and copolymers with l-lactide were prepared by ring opening polymerization in the melt using benzyl alcohol and SnOct₂ as initiator and catalyst, respectively. The polymerizations proceeded with high yields and conversions and good control over molecular weights and copolymer composition. The obtained polymers were amorphous materials and their T_g increased with increasing lactide content. Dihydroxylation of the double bonds in poly(allylglycolide) and copolymers with lactide was attempted with osmiumtetroxide/4-methylmorpholine-4-oxide (OsO₄/NMO). However, particularly the polymers rich in allylglycolide could not be isolated after dihydroxylation because they likely underwent degradation during workup. Optimizing the reaction conditions gave partially dihydroxylated copolymers only for copolymers with high lactide content (50 and 75 mol%) with a conversion of the double bonds of only ∼60%. GPC analysis showed that chain scission had occurred during the dihydroxylation reaction and/or workup.The allyl groups of poly(allylglycolide) homopolymers and copolymers with lactide were oxidized using m-chloroperoxy benzoic acid (mCPBA) to yield the corresponding epoxidated polymers in high yield. NMR analysis showed that conversion of the double bonds to epoxides was quantitative, whereas GPC analysis showed that the epoxidation was not associated with chain scission. All epoxidated polymers were amorphous materials with a T_g depending on the composition.     

Two 3D supramolecules with helices constructed by hydrogen bonds based on p-thioacetatebenzoic acid ligand

Abstract  

p-Thioacetatebenzoic acid (H₂L) and a combination of N-donor ligands such as 4,4′-bipyridine (4,4′-bipy) and 1,3-bi(4-pyridyl)propane (bpp) with metal ions Mn(II) and Ni(II) give rise to two coordination polymers, namely, [Mn(HL)₂(bpp)₂(H₂O)₂] _n (1), [NiL (4,4′-bipy)(H₂O)₃] _n ·nH₂O (2). 1 features an unusual “8” shaped double layer by hydrogen bonds and two different types of helical chains are arrayed alternatively in the 2D double layer framework, which further extends into a 3D supramolecular structure through C–H···O hydrogen bonds. 2 consists of 1D chains which further connect with each other via hydrogen bonds to form the final 3D framework including two different types of helical structure. Photoluminescence study reveals that 1 displays intense structure-related fluorescent emission bands (λ _ex = 369 nm) at 414 nm in the solid state at room temperature. Electrochemical property of 2 reveals that the process of the redox is irreversible.     

Enhancing CO2 Electroreduction to Methane with a Cobalt Phthalocyanine and Zinc–Nitrogen–Carbon Tandem Catalyst

Developing copper-free catalysts for CO₂ conversion into hydrocarbons and oxygenates is highly desirable for electrochemical CO₂ reduction reaction (CO₂RR). Herein, we report a cobalt phthalocyanine (CoPc) and zinc–nitrogen–carbon (Zn-N-C) tandem catalyst for CO₂RR to CH₄. This tandem catalyst shows a more than 100 times enhancement of the CH₄/CO production rate ratio compared with CoPc or Zn-N-C alone. Density functional theory (DFT) calculations and electrochemical CO reduction reaction results suggest that CO₂ is first reduced into CO over CoPc and then CO diffuses onto Zn-N-C for further conversion into CH₄ over Zn-N₄ site, decoupling complicated CO₂RR pathway on single active site into a two-step tandem reaction. Moreover, mechanistic analysis indicates that CoPc not only generates CO but also enhances the availability of *H over adjacent N sites in Zn-N₄, which is the key to achieve the high CH₄ production rate and understand the intriguing electrocatalytic behavior which is distinctive to copper-based tandem catalysts.     

Electronic structure, geometry, and reactivity of arylsilenes: a quantum-chemical study

The geometry and electronic structure of 1,4-di(silaethen-1-yl)benzene (2), itsmeta- andbrtho-isomers (3 and4, respectively), and its carbon analog, 1,4-divinylbenzene (5), were studied by the semiempirical MNDO-PM3 method. Unlike5, two pairs of the frontier MOs in isomers2–4 are mainly π-orbitals of Si=C bonds, while the structure of the lowest occupied π-MO indicates delocalization of π-electrons of the entire system. The main characteristic features of the double Si=C bonds (the high polarity and narrow HOMO-LUMO energy gap, which favors the [2+2]-cycloaddition reaction) remain in arylsilenes2–4. The interaction between π-electrons of benzene fragment and the double Si=C bonds results in violation of the benzene ring symmetry, which is most pronounced in structure5. Weakening of the C−H bonds in theortho-positions of the aromatic nucleus in the compounds under study is observed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 256–260, February, 1999.     

Emulsion Polymerization of Acrylic Monomers Stabilized by Poly(Ethylene Oxide)

Abstract

The stability of acrylic latices stabilized by poly(ethylene oxide) (PEO) is governed by the bridging flocculation process during polymerization. The final latex particle size increases with increasing concentration of initiator, PEO, or NaCl. The total scrap formed during the reaction increases rapidly with increasing NaCl concentration due to the ionic strength effect. It is shown that the final latex particle size decreases rapidly with an increase in the agitation speed. The amount of total scrap formed during polymerization is generally greater at a higher agitation speed. These results suggest that the fraction of the particle surface covered by PEO and the ratio of the thickness of the PEO adsorption layer to that of the electric double layer of the latex particles should play an important role in determining the final latex particle size and colloidal stability.     

Calorimetric investigation of polymerization reactions. IV. Curing reaction of polyester fumarate with styrene

The curing reaction of polyester fumarate with styrene was investigated with a differential scanning calorimeter (DSC) operated isothermally. The change in rate of cure was followed over the whole range of conversion. The rate of cure is accelerated by the gel effect to about ten to fifty times the rate of model copolymerization of diethyl fumarate with styrene. This autoacceleration is much enhanced for systems with higher crosslinking densities and at lower temperatures. The results confirm that both termination and propagation steps of the curing reaction are controlled by diffusion of polymeric segments and monomer molecules over almost the whole range of conversion. The final extent of conversion is short of completion for isothermal cure and even for postcure of polyester fumarate with styrene because of crosslink formation. The final conversion of isothermal cure decreases with increasing crosslinking density and shows a maximum with increasing reaction temperature. This temperature dependency of the final conversion is caused by the difference in the activation energies for two propagation rate constants k_pf and k_ps, which were evaluated to be 7–10 and 5–8 kcal/mole, respectively, for the intermediate stage of the curing reaction.     

The influence of N‐vinyl pyrrolidone on polymerization kinetics and thermo‐mechanical properties of crosslinked acrylate polymers

Polymerization of multifunctional acrylate monomers generates crosslinked polymers that are noted for their mechanical strength, thermal stability, and chemical resistance. A common reactive diluent to photopolymerizable formulations is N‐vinyl pyrrolidone (NVP), which is known to reduce the inhibition of free radical photopolymerization by atmospheric oxygen. In this work, the copolymerization behavior of NVP was examined in acrylate monomers with two to five functional groups. At concentrations as low as 2 wt %, NVP increases the polymerization rate in copolymerization with multifunctional acrylate monomer. The relative rate enhancement associated with adding NVP increases dramatically as the number of acrylate double bonds changes from two to five. The influence of NVP on polymerization kinetics is related to synergistic cross‐propagation between NVP and acrylate monomer, which becomes increasingly favorable with diffusion limitations. This synergy extends bimolecular termination into higher double bond conversion through reaction diffusion controlled termination. Copolymerizing concentrations of 5–30 DB% NVP with diacrylate or pentaacrylate monomer also increases Young's modulus and the glass transition temperature (T_g) in comparison to neat acrylate polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4062–4073, 2007