Onium Salts of Amino Acids as Co‐Initiators in Photoinduced Free‐Radical Polymerization

Summary: The onium salts of selected aliphatic and aromatic amino acids were investigated as electron donors in photoinduced free radical polymerization, in conjunction with either DIBF or BP as sensitizer. The laser flash photolysis experiments unmistakably documented that the free radical formation occurs via an electron transfer reaction from the amino acid salt to the chromophore triplet state. The kinetic studies clearly showed that either the DIBF or BP onium salts of selected aliphatic and aromatic sulfur‐containing amino acids exhibit a significant increase in the efficiency of free‐radical polymerization of TMPTA as compared to non‐sulfur‐containing co‐initiators and that the efficiency of all tested electron donors is only slightly dependent on the cation type coupled with co‐initiators tested.

Possible mechanism for the free radical photoinitiated polymerization.     

Photoinitiating free‐radical polymerization electron‐transfer pairs applying amino acids and sulfur‐containing amino acids as electron donors

A series of free‐radical polymerization initiation systems, based on xanthene dyes as the absorbing chromophores [Rose bengal derivative, 3‐(3‐methylbutoxy)‐5,7‐diiodo‐6‐fluorone and 3‐acetoxy‐2,4,5,7‐tetraiodo‐6‐fluorone] and sulfur‐containing amino acids as the electron donors, were investigated. The photoredox pair xanthene dye/sulfur‐containing amino acid was effectively used for photoinitiation of free‐radical polymerization of the mixture composed of poly(ethylene glycol)diacrylate–1% NH₄OH (3 : 1). The highest initiating efficiencies were observed for the system composed of methionine derivatives as the electron donor. The mechanism of photoinduced electron transfer between sulfur‐containing amino acids and triplet state of xanthene dye was investigated using laser‐flash and steady‐state photolysis techniques. Based on photochemistry of xanthene dyes, photochemistry of sulfur‐containing amino acids, and obtained results, the mechanism describing the major processes occurring during the photoinitiated polymerization by a photoinduced intermolecular electron‐transfer process was postulated. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 358–365, 2005     

Applicability of hemicyanine phenyltrialkylborate salts as free‐radical photoinitiators in the visible‐light polymerization of acrylate

The photoinitiation ability of photoredox pairs composed of a hemicyanine dye cation and different borate anions for the radical polymerization of 2‐ethyl‐2‐(hydroxymethyl)‐1,3‐propanediol triacrylate was investigated. In such a system, the excited dye chromophore is reduced by different tetraorganylborate anions. Upon irradiation at 488 nm, reductive carbon–boron bond cleavage occurs, producing reactive radicals, which start the chain reaction. The efficiency of bond‐breaking processes was found to be dependent on the nature of both the acceptors and the donors. The experimental results show that the photoinitiating ability of the tested photoredox pairs were controlled by both the driving force of the electron‐transfer process between the electron donor and the electron acceptor and the reactivity of the free radical that resulted from the secondary reactions occurring after the photoinduced electron‐transfer process. Using the nanosecond flash photolysis method, we studied the spectral and kinetic characteristics of the triplet state of cyanine dye and determined the rate constants of the triplet quenching by phenyltrialkylborate salts. The results obtained show that the tetramethylammonium phenyl‐tri‐n‐butylborate (TB7) has a faster electron‐transfer rate than the tetramethylammonium n‐butyltriphenylborate (TB2) salt, which bore only one butyl group attached to the boron. The relative initiator efficiency of the triphenylbutylborate salts, as compared to the corresponding phenyltrialkylborate salts with a common chromophore, was determined. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The first report on the atom transfer radical polymerization of an optically active acidic monomer based on L‐phenylalanine

For the first time, acidic monomer chiral N‐acryloyl‐L ‐phenylalanine was polymerized directly by atom transfer radical polymerization under mild conditions. Controlled polymerization was carried out in pure water, methanol/water mixture, or pure methanol using water‐soluble initiators, such as 2‐hydroxyethyl‐2′‐methyl‐2′‐bromopropionate and sodium‐4‐(bromomethyl)benzoate at room temperature. The corresponding optically active biocompatible amino acid‐based homopolymers were obtained in good yields with narrow molecular weight distributions. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The Effect of Initiators and Reaction Conditions on the Polymer Syntheses by Atom Transfer Radical Polymerization

The controlled/“living” radical polymerization of methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and styrene by atom transfer radical polymerization (ATRP) is reported. The effect of initiators and reaction conditions on the ATRP results was investigated. Controlled polymerizations with predictable molecular weights were performed on MMA at 40^○C and 80^○C using a CuCl/bipyridine (bipy) catalyst system in conjunction with 1-bromoethyl benzene as the initiator. The addition of a polar solvent was necessary to decrease the polymerization rate and afford low polydispersity materials. The ATRP processes followed a first-order kinetics with respect to the monomer concentration. The molecular weights of the resulting polymers were very close to their calculated values and increased with the conversion. The ATRP results of styrene showed a similar trend and revealed that CuBr/bipy or CuBr/PMDETA was a more suitable catalyst system than CuCl/bipy. In addition, it was found that controlled polymerizations could be readily carried out both in a nonpolar solvent or in bulk. Furthermore, by using the bromine-terminated polymer as the macroinitiator, diblock copolymers of PSt-b-PMMA, PSt-b-PHEMA, PMMA-b-PSt, and PMMA-b-PHEMA could be obtained. Thermal analysis and X-ray diffraction studies confirmed the amorphous structures of the resulting polymers.     

Hyperbranched polyacrylates prepared by self‐condensing vinyl copolymerization in the presence of a tetrafunctional initiator

Hyperbranched copolymers were synthesized by self‐condensing vinyl copolymerization (SCVCP) of 2‐[(2‐bromopropionyl)oxy]ethyl acrylate (BPEA) and methyl acrylate (MA) in the presence of a tetrafunctional initiator, 6,6‐bis[5‐(α‐bromoisobutyryloxy)‐2‐oxapentyl]‐4,8‐dioxaundecane‐yl‐1,11 dibromoisobutyrate (THABI). The structures of the polymers obtained were characterized by NMR and size exclusion chromatography/right‐angle laser‐light scattering/differential viscometry/differential refractometry. Molecular weight, molecular weight distribution and degree of branching were influenced by conversion and initial feed molar ratio of BPEA, MA and THABI. The addition of THABI can narrow the polydispersity of the hyperbranched copolymers obtained. The results are consistent with our previous simulation work. © 2003 Society of Chemical Industry     

The Effect of Clay Loading on the Morphology and Properties of Poly(styrene‐co‐butyl acrylate)/Clay Nanocomposites

This paper investigates the effect of both the clay loading and the monomer feed rate on the morphology and properties of poly(styrene‐co‐butyl acrylate)‐clay nanocomposites prepared in emulsion polymerization. Analysis by X‐ray diffraction (XRD) and transmission electron microscopy (TEM) of the nanocomposites prepared by batch polymerization showed that the polymer clay nanocomposites (PCNs) with 1–3 wt.‐% clay loading resulted in intercalated structures, while exfoliated structures were obtained at 10 wt.‐% clay loading. The polymerization was also carried out with semi‐batch polymerization. The morphology, thermal stability, and mechanical properties of nanocomposites obtained were found to be more strongly dependent on the clay/polymer ratio than the monomer feed rate.

     

Effect of Silica‐Coated Poly(butyl methacrylate)‐block‐poly(methyl methacrylate) Double‐Shell Particles on the Mechanical Properties of PMMA Composites

Silica nanoparticles with an average diameter of 12 nm are grafted with PBMA‐b‐PMMA double shells through typical sequential ATRP from bromoisobutyrate initiators anchored at the silica surface using an epoxysilane. A commercially available PMMA homopolymer is used for the preparation of composites with unmodified, silane‐modified and double‐shell‐modified silica particles. Good mechanical properties are obtained for silica double shell containing systems. The silica content in double shell particle systems is varied from 0 to 2.5 wt%. A significant improvement in impact properties is observed. The surface‐modified silica particles are characterized by ATR‐FTIR, NMR, GPC, and thermal analyses. TEM analysis is used to analyze the nature of dispersion of particles in the composites.