Conventional Free Radical Initiation Research Articles

Peroxide free radical grafting of maleic anhydride onto polyethylene under high melt temperature: A comprehensive approach

“Flash Reactive Extrusion” under very high temperatures (T=200–420°C) allowed to graft maleic anhydride onto polyethylene (PE) without the use of conventional free radical initiators such as peroxides. Herein, the use of unusual temperatures induced the formation of radicals through degradation/chain scission of the PE backbone and the latter radicals were able to react with maleic anhydride (MA) and yielding a MA grafting degree close to 1.4 wt% at 390°C. Moreover, the MA grafting mechanistic scheme was deeply discussed with the help of 1H NMR and 13C NMR analysis and it was pointed out that β-scissions reactions are favoured in comparison with the cross-linking ones at very high temperatures for the thermal initiation step with a MA grafting mainly onto the PE side chain.

Combining isosorbide and lignin-related benzoic acids for high-Tg polymethacrylates

The insertion of rigid aliphatic or aromatic building blocks in polymer structures is an efficient synthetic strategy towards high-Tg polymer materials. In the present work, we have functionalized isosorbide 5-methacrylate with various aromatic lignin-inspired esters to yield a series of isosorbide-2-aryl carboxylate-5-methacrylate monomers (ArIMAs) as single regioisomers. The selection of phenyl carboxylate side groups included benzoate, p-cyanobenzoate, meta/para-methoxybenzoates, and different vanillic acid esters. This afforded a range of seven bio-based monomers with different size, polarity, and substitutions. Polymerizations were carried out by conventional free radical initiation to obtain the corresponding high molecular-weight poly(aryl carboxylate isosorbide methacrylate)s (PArIMAs). The polymerizations were evaluated in different solvents, including toluene, EtOAc, γ-valerolactone, 2-MeTHF, and DMSO to identify the most suitable conditions. The polymers exhibited high glass transition temperatures in a broad range, from 80 to 168 °C, and were thermally stable up to 268 °C. Furthermore, dynamic rheology experiments indicated that the polymers were sufficiently stable for melt processing. The availability of the building blocks combined with the high stability of the polymers make these materials attractive for use as high-performance plastics and coatings.

Enhanced melt free radical grafting efficiency of polyethylene using a novel redox initiation method

In this work, for the first time, redox initiation was employed in the melt free radical grafting of glycidyl methacrylate (GMA) and maleic anhydride (MA) onto polyethylene (PE) by reactive extrusion. Since it is very challenging to obtain high grafting degrees of GMA and MA onto polymer backbones through conventional free radical initiation, especially in the extrusion process, the strategy of using a peroxide/reducing agent initiator was introduced. The redox initiation system was composed of dicumyl peroxide (DCP) and Tin(II) 2-ethylhexanoate (Sn(Oct)2). The grafting reaction was monitored by torque rheometry and graft products were analyzed by Fourier Transform Infrared Spectroscopy. Effects of monomer concentration and DCP/Sn(Oct)2 ratio on the degree of grafting were studied. The redox initiation proved to be very effective in improving the grafting degree of GMA and MA onto PE. It is reasoned that, in the presence of Sn(Oct)2, the free radicals produced were not subject to a cage effect, yielding high initiator efficiency.

Synthesis of block and graft copolymers of styrene by raft polymerization, using dodecyl‐based trithiocarbonates as initiators and chain transfer agents

AbstractA series of dodecyl‐based monofunctional trithiocarbonate chain transfer agents (CTAs) were successfully synthesized, toward the reversible addition‐fragmentations chain transfer (RAFT) polymerization of styrene. The CTAs were used as initiators for RAFT polymerization, in the absence of the conventional free radical initiator, at higher temperature. Polystyrene (PS) of narrow polydispersity index (PDI) is synthesized. Subsequently, poly(styrene‐b‐benzyl methacrylate) diblock and poly(styrene‐b‐benzyl methacrylate‐b‐2‐vinyl pyridine) triblock copolymers were synthesized from the PS macro‐RAFT agent by simply heating with the second and third monomer, respectively. These experiments suggest that it should be possible to control the RAFT polymerization initiated by a CTA through the adjustment of the temperature of polymerization in such manner that initiation is tailored to proceed at faster rate (at higher temperature) in comparison to propagation (lower temperature). For the specific CTAs studied in this work, the polymerization rate of styrene was high in the case of the reinitiating cyano (CN)‐substituted group (R group) compared to the other groups studied. The results further show that 4‐cyano pentanoic acid group is superior to the other R groups used for the RAFT polymerization of styrene, especially based on the polydispersity at a given conversion as well as the variation in the expected and experimental number‐average‐molecular weights. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Nanoparticle-Based and Organic-Phase-Based AGET ATRP PolyHIPE Synthesis within Pickering HIPEs and Surfactant-Stabilized HIPEs

PolyHIPEs are porous polymers synthesized within high internal phase emulsions (HIPEs). Recent research has shown that the locus of conventional free radical initiation (at the oil–water interface or in the organic phase) can affect the polyHIPE’s structure and properties. This paper is the first investigation of polyHIPE synthesis using activators generated by electron transfer for atom-transfer radical polymerization (AGET ATRP). Two different types of initiators (organic-phase-based and nanoparticle-based) were used to synthesize polyHIPEs within both surfactant-stabilized HIPEs and nanoparticle-stabilized Pickering HIPEs. The type and locus of initiation affected the macromolecular structure for all the polyHIPEs. Furthermore, the initiator also affected both the porous structure and the wall structure of polyHIPEs synthesized within Pickering HIPEs. Depending upon the initiator, either a polyhedral void shape or a nanoparticle assembly at the interface could become “locked in” at the very beginning of the polymerization.

Synthesis of Highly Branched Poly(methyl methacrylate)s Using the “Strathclyde Methodology” in Aqueous Emulsion

High-conversion copolymerizations of methyl methacrylate (MMA) and divinylbenzene (DVB) in aqueous emulsion have been carried out using sodium dodecyl sulfate as the emulsifier and potassium persulfate as the conventional free radical initiator. Various thiols have been investigated in order to inhibit cross-linking and hence favor the formation of branched products. Hexanethiol (HT) and benzylthiol (BT) have been found to be particularly effective. Use of appropriate levels of BT allow mole feed ratios of MMA/DVB up to 100/20 to be employed in producing highly branched products without cross-linking. Typically DVB/BT mole ratios of ≤1 ensure that cross-linking is avoided. Perhaps most remarkably of all no organic solvent is required in producing these branched products, whereas analogous polymerization feed compositions under bulk or aqueous suspension polymerization conditions lead inevitably to cross-linked products irrespective of the level of chain transfer agent used. The molar mass and branching ar...

Initiation mechanism of ultrasonically irradiated emulsion polymerization

AbstractThe emulsion polymerization of some monomers can occur without the conventional free radical initiators under ultrasonic irradiation. However, the initiation mechanism is still under controversy. In this paper, the sources of free radicals arising from ultrasonically initiated emulsion polymerization were investigated. Experimental results show that ionic surfactants play a very important role in obtaining a high polymer yield. While monomer conversion is very low in the absence of surfactants or in the presence of nonionic surfactants, it increases significantly upon addition of a little amount of ionic surfactant. FTIR and a radical trapping experiment confirm that the free radicals involved in the irradiation process originate from the decomposition of the ionic surfactants. Under ultrasonic irradiation, ionic surfactants undergo bond scission between the alkyl and ionic group, where the bond is the weakest along the chain. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2617–2624, 2005

Physically Controlled Radical Polymerization of Vaporized Vinyl Monomers on Surfaces. Synthesis of Block Copolymers of Methyl Methacrylate and Styrene with a Conventional Free Radical Initiator

Physically controlled radical polymerization of vinyl monomers with conventional free radical initiators was achieved in the presence of monomer vapors on solid substrate surfaces. The polymerizati...

Antinociceptive and antipyretic properties of a new conjugated ibuprofen-methacrylic polymeric controlled delivery system

Antinociceptive and antipyretic properties of a biocompatible polymer, poly( N-(4-(2-(4-isobutylphenyl) propionyloxy) phenyl) methacrylamide (PolyMIA) (Fig. 1) as a carrier drug has been investigated. This new polyacrylic compound and its monomeric form (MIA) were prepared by coupling methacrylic acid, p-aminophenol amd 2-(4-isobutyl phenyl)propionic acid (Ibuprofen, IB) by means of well-known esterification and amidation reactions and finally by free radical polymerization of the monomeric acrylic derivative in solution, using a conventional free-radical initiator. Plasma levels were determined by gas-liquid chromatography (GLC) after the administration of IB, MIA and PolyMIA in mice. The results obtained indicate that PolyMIA acts as a controlled IB delivery system. Practically constant plasma levels of IB, about 20 μg/ml, were obtained at least for 6 h after the intraperitoneal administration of PolyMIA. Antinociceptive in vivo tests confirm the existence of a prolonged release of IB from the macromolecular systems. PolyMIA shows a time-extended activity and equal or even higher intensity of antinociceptive effects than that of free IB. In antipyretic test, PolyMIA was the only compound which increased its activity beyond the sixth hour of treatment (84.8%, p < 0.01 vs. control and vs. IB), doubling the traditional IB activity. The appearance of pharmacological activity of PolyMIA in a very short time after administration, seems to indicate that the system could be active in its polymeric form, since the hydrolytic behaviour followed in vitro in alkaline medium, showed a rate of cleavage of the side IB residue much lower than that indicated by the effects observed in vivo.

Investigation of Acrylamide-Styrene Copolymerization System in Cyclohexanone in the Absence of a Conventional Radical Initiator

Abstract Copolymerization of the acrylamide-styrene system has been studied at 75°C in the absence of any conventional radical in dioxane, cyclo-hexanone, cyclohexanone/dioxane, and cyclohexanone/benzene mixtures. The reactivity coefficient values have been evaluated by YBR, KT, and EVM methods. The polarity and strong hydrogen bonding ability of the reaction medium with the amide group of the acrylamide affects the reactivity ratio values by disturbing the keto/enol equilibrium of the acrylamide. A comprehensive comparative study of the r-parameters achieved for these copolymer systems with the reactivity coefficient values of the same monomer system initiated by the conventional free-radical initiator has been presented. From the r-product values obtained by the EVM method, the following order of alternating tendency of the monomers in copolymers prepared in different media was found: cyclohexanone/benzene mixture < dioxane < cyclohexanone/dioxane mixture < cyclohexanone. The kinetic results obtained co...

Polymer degradation modified by blending with polymers chosen on the basis of their Φ-factors

It is difficult to modify the rate of pyrolysis of a polymer by the addition of conventional free radical initiators, inhibitors or retarders, because these would generally be decomposed or volatilised below the elevated temperatures of interest in degradation work. The objective in the present work is to modify the thermal degradation rate of a polymer by blending it with another polymer to either enhance or reduce the degradation rate. With respect to the latter, it was thought possible that the selection of the blend polymer could be based on the assumption that high Φ-factors for monomer pairs in copolymerisation would imply that the long chain radicals in a degrading blend of the corresponding polymers would preferentially interact, causing mutual termination. To test this idea in the present work, poly(styrene) was used as the host polymer, and poly(methyl acrylate) or poly(butyl acrylate) were chosen on the above basis as the blend polymers. All degradation rates were monitored by pyrolysis—gas chromatography. The results for these systems show that the degradation rates of the added blend polymers are reduced by a factor of about eight, supporting the Φ-factor proposal, and indicating homogeneous kinetics at the pyrolysis temperature (430°C). However, the results also show that the rate of degradation of the host poly(styrene) is enhanced, so cross-termination is not the principal factor influencing the rate in this case. Hence, the Φ-factor proposal alone is too simplistic. It is proposed that chain transfer processes also have an effect, and play a more important role in determining the degradation rate of the polystyrene in the blends. Further experiments performed with poly(α-methyl styrene) as the host polymer provided supporting evidence for this proposal. Consideration is also given to the effect of other factors, such as heterogeneity, on the pyrolysis rates of the blends.

Surface characterization of 2-hydroxyethyl methacrylate/styrene copolymers by angle-dependent X-ray photoelectron spectroscopy and static secondary ion mass spectrometry.

The surface composition and structure of three structurally distinct amphiphilic copolymers of 2-hydroxyethyl methacrylate (HEMA) and styrene have been examined with angle-dependent X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectrometry (SIMS). The phase-separated block copolymer made by anionic living polymerization, HSH-A50, showed significant surface enrichment of styrene. The outermost 2-3 A appeared to be approximately 100% styrene, with the styrene concentration decreasing to its bulk value at a depth of approximately 50 A from the surface. However, HEMA was detected in the outer 20 A of this copolymer. The presence of HEMA in the surface region implies this copolymer may undergo significant restructuring when hydrated in a hydrophilic environment (as opposed to the hydrophobic environment in which the sample was prepared and analyzed). The phase-separated block copolymer made by telechelic coupling of free radical polymerized functionalized oligomers, HSH-B60, showed only slight styrene enrichment at the surface. Both HEMA and styrene were detected at all sampling depths, including the outermost surface layer, consistent with the presence of discrete HEMA and styrene domains at the copolymer surface. Since both components are already present at the surface under hydrophobic conditions, the degree of restructuring this copolymer may undergo upon hydration should be minor. The random HEMA--styrene copolymer made by conventional free radical initiation techniques, HS-RAN50, had a surface composition that was similar to the bulk composition and independent of depth, as expected for a homogeneously mixed copolymer film.

Grafting of Poly(Methyl Methacrylate) onto Poly(Vinyl Chloride) through Benzodithioate Groups

Abstract The synthesis and the characterization of poly (methyl methacrylate) (PMMA) grafted to poly(vinyl chloride) (PVC) through benzodithioate groups are studied. Unlike results generally obtained with conventional free-radical initiators for systems involving PVC and MMA, high conversions, and grafting efficiencies are achieved with azobis-isobutyronitrile. The paper describes the synthesis of p-vinylbenzodithioate-containing PVC and the dependence of the characteristics of PVC-g-PMMA on the composition of the reaction mixture. Characterization of the graft copolymers includes UV and IR spectroscopy, GPC, and microstructure analysis by removal of PMMA side chains by aminolysis of dithioesters groups. Intrinsic viscosity, glass transition temperature, and thermal sensitivity were investigated to confirm the grafted nature of the copolymer.

Block polymer preparation via both anionic and free radical polymerization

AbstractA new method of block polymer preparation using combined anionic and free radical polymerization was investigated. In the method the first monomer was polymerized anionically. The resulting polymeric anions were then reacted with an episulfide to form a polymer with mercaptan end‐groups. This mercapto—polymer was mixed with a second monomer(s) in an inert solvent for the free radical polymerization. Conventional free radical initiation methods were used to initiate the polymerization of the second monomer but because of the high chain transfer constant of the mercaptan groups, a large number of the free radical chains would grow from the first polymer to form a block polymer. Block polymers difficult or impossible to make by direct anionic polymerization can thus be prepared. Several block polymers, including the new thermoplastic elastomers, poly[(styrene‐co‐acrylonitrile)‐b‐butadiene‐b‐(styrene‐co‐acrylonitrile)] and poly(bromostyrene‐b‐butadiene‐b‐bromostyrene) were prepared by this method.

Co(II) acetylacetonate, a radical initiator

AbstractThe bulk polymerisation of methyl methacrylate and the studies of the resulting polymers have revealed the radical mechanism of initiation with Co(II) acetylacetonate. Copolymerisation studies of styrene/methyl methacrylate, styrene/acrylonitrile, and methyl methacrylate/acrylonitrile under similar conditions, using azoisobutyronitrile, benzoyl peroxide, Mn(III) acetylacetonate, and Co(II) acetylacetonate, have also shown the identical nature of the mechanism of initiation of these initiators. The tacticity of the polymer, deduced from NMR spectra, is identical with that of a polymer prepared with a conventional free radical initiator.

Time dependence of viscosity and streaming birefringence of solutions of high molecular weight polymethylmethacrylate in Aroclor

I t was previously reported that solutions of linear polymers of very high molecular weight in extremely viscous solvents show an increase in viscosity with increasing shear gradient (1, 2). The effect was observed with polymethylmcthacrylate (Mw = 4.2 X 106) in Aroclor, polystyrene (My ~-~ 7 X 106) in Aroclor, and polyisobutene (My ~ 6 X 106) in polybutene oligomers (3). The solutions were dilute (0.125 2.0 • .3 g/ec) and the solvents were extremely viscous (~2 = 1 to l0 S P). The purpose of this note is to report that in experiments, so far confined to polymethylmethacrylate in Aroclor, changes in viscosity may be observed by prolonged shearing at a constant gradient. Moreover, related time dependent effects are also observed in streaming birefringence experiments. Polymethylmethacrylate, polymerized by conventional free radical initiation, of molecular weight Mw = 4.2 • 106, was dissolved in chloroform and the solution mixed with Aroclor 1248 (a chlorinated diphenyl). The chloroform was subsequently pumped off completely at about 70 ~ in vacuo with occasional gentle stirring, to yield a solution containing 1.0 • 10 -~ g/cc of polymer. t~igs. 1 a und 1 b show plots of extinction angle, Z, and streaming birefringence, An, against velocity gradient. The behaviour of the solvent is normal and also changes of Z and An for a first run of the solution are, qualitatively, typical of results obtained with long flexible macromolecules. However, in two consecutive runs at 25 ~ unusual changes were observed for the solution in that there was a trend towards lower values of Z accompanied by higher values of An. Immediately before a fourth run the solution was sheared for some minutes at 60 see -1 with the result tha t the upward displacement of the curve for An versus gradient became even more pronounced. Similar behaviour also was observed with a portion of the solution which had been passed through a "medium" glass filter. Further experiments were run at a lower temperature of 15.5~ and, consequently, at a higher viscosity (for Aroclor 1248~/ = 1.97P at 15~ 7.93P at 15.5~ At gradients of less than 10 see -i the solution behaved normally. By contrast, its behaviour at a gradient of 20 see -i was found to be influenced in a striking and complex manner by the time of shearing. An increased