Alex M. van Herk

Polymer Encapsulated Gibbsite Nanoparticles: Efficient Preparation of Anisotropic Composite Latex Particles by RAFT-Based Starved Feed Emulsion Polymerization

Anisotropic polymer-inorganic composite latex particles were synthesized by using a RAFT-based encapsulation approach on cationic gibbsite platelets. By using the RAFT agent dibenzyl trithiocarbonate, a series of amphipatic living random RAFT copolymers with different combinations of acrylic acid and butyl acrylate units were synthesized. These RAFT copolymers were used as living stabilizers for the gibbsite platelets and chain extended to form a polymeric shell by starved feed emulsion polymerization. Cryo-TEM characterization of the resulting composite latexes demonstrates the formation of anisotropic composite latex particles with mostly one platelet per particle. Monomer feed composition, chain length, and hydrophilic-lipophilic balance of the RAFT copolymer were found to be important factors for the overall efficiency of the encapsulation. Good control over platelet orientation and high encapsulation efficiency were achieved via this route.

Photooxidative degradation of poly(alkyl methacrylate)s

Photooxidative degradation of four different poly(alkyl methacrylate)s: poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate), poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) have been investigated using FTIR, UV–Vis spectroscopy and gel permeation chromatography. The influence of the ester group size on the course of photochemical reactions in poly(alkyl methacrylayte)s has been estimated. It has been found that PMMA undergoes slower photooxidation but faster photodegradation than those in higher poly(alkyl methacrylate)s. The different behavior of the polymers studied is caused by the different reactivity of the macroradicals influenced by the different flexibility and mobility of macrochains at room temperature.

Pulsed initiation polymerization as a means of obtaining propagation rate coefficients in free‐radical polymerizations. II Review up to 2000

The application of the pulsed initiation polymerization technique (PIP) to determine propagation rate coefficients has evolved into a valuable tool. The number of monomers studied has rapidly increased since 1987. Besides homopropagation rate coefficients also copolymerizations are studied extensively. The calibration of the size exclusion chromatography is still a limiting factor in studying delicate differences in reactivities, for example in families of monomers like that of the methacrylates. In studying solvent effects in propagation, PIP is capable of disclosing even small solvent effects, typically in the order of magnitude of plus or minus 20%. For some families of monomers PIP does not result in reliable parameters. The reasons for the breakdown of the PIP technique with the acrylates are discussed.

Critically evaluated rate coefficients for free-radical polymerization. Part 6: Propagation rate coefficient of methacrylic acid in aqueous solution

Critically evaluated propagation rate coefficients, kp, for free-radical polymerization of methacrylic acid, MAA, in aqueous solution are presented. The underlying kp values are from two independent sources, which both used the IUPAC-recommended technique of pulsed-laser-initiated polymerization (PLP) in conjunction with molar mass distribution (MMD) analysis of the resulting polymer by size-exclusion chromatography (SEC). Different methods of measuring the MMD of the poly(MAA) samples have, however, been used: (i) direct analysis via aqueous-phase SEC and (ii) standard SEC with tetrahydrofuran as the eluent carried out on poly(methyl methacrylate) samples obtained by methylation of the poly(MAA) samples from PLP. Benchmark kp values for aqueous solutions containing 15 mass % MAA are presented for temperatures between 18 and 89 °C. The Arrhenius pre-exponential and activation energy of kp at 15 mass % MAA are 1.54 × 106 L mol-1 s-1 and 15.0 kJ mol-1, respectively. Also reported are critically evaluated kp values for 25 °C over the entire MAA concentration range from dilute aqueous solution to bulk polymerization.

Critically evaluated rate coefficients in radical polymerization – 7. Secondary-radical propagation rate coefficients for methyl acrylate in the bulk

Propagation rate coefficient (kp) data for radical polymerization of methyl acrylate (MA) in the bulk are critically evaluated and a benchmark dataset is put forward by a task-group of the IUPAC Subcommittee on Modeling of Polymerization Kinetics and Processes. This dataset comprises previously published results from three laboratories as well as new data from a fourth laboratory. Not only do all these values of kp fulfill the recommended consistency checks for reliability, they are also all in excellent agreement with each other. Data have been obtained employing the technique of pulsed-laser polymerization (PLP) coupled with molar-mass determination by size-exclusion chromatography (SEC), where PLP has been carried out at pulse-repetition rates of up to 500 Hz, enabling reliable kp to be obtained through to 60 °C. The best-fit – and therefore recommended – Arrhenius parameters are activation energy EA = 17.3 kJ mol−1 and pre-exponential (frequency) factor A = 1.41 × 107 L mol−1 s−1. These hold for secondary-radical propagation of MA, and may be used to calculate effective propagation rate coefficients for MA in situations where there is a significant population of mid-chain radicals resulting from backbiting, as will be the case at technically relevant temperatures. The benchmark dataset reveals that kp values for MA obtained using PLP in conjunction with MALDI-ToF mass spectrometry are accurate. They also confirm, through comparison with previously obtained benchmark kp values for n-butyl acrylate, methyl methacrylate and n-butyl methacrylate, that there seems to be identical family-type behavior in n-alkyl acrylates as in n-alkyl methacrylates. Specifically, kp for the n-butyl member of each family is about 20% higher than for the corresponding methyl member, an effect that appears to be entropic in origin. Furthermore, each family is characterized by an approximately constant EA, where the value is 5 kJ mol−1 lower for acrylates.

Vesicle-templated pH-responsive polymeric nanocapsules

We report the synthesis of pH responsive polymeric nanocapsules by templating unilamellar vesicles of dimethyldioctadecylammonium bromide (DODAB) using a RAFT-based templating approach. A short-chain living anionic copolymer containing randomly distributed acrylic acid and butyl acrylate units was first synthesized by RAFT in solution using dibenzyl trithiocarbonate (DBTTC) as the RAFT agent. The anionic copolymer chains were subsequently adsorbed onto the surface of cationic DODAB vesicles and were further chain extended to form a thick polymeric shell by feeding a monomer mixture comprising methyl methacrylate (MMA) and tertiary butyl acrylate (t-BA) in combination with the divinyl crosslinker ethylene glycol dimethacrylate (EGDMA) under starved feed conditions. CryoTEM characterization demonstrated successful formation of a thick crosslinked polymeric shell around the vesicles. Subsequent acid hydrolysis of the tertiary butyl ester groups of the crosslinked polymeric shell resulted in the formation of pH-responsive nanocapsules.

Effects of ionenes on catalytic activity and structure of cobalt phthalocyanine: Part 2. Kinetics as a function of thiol and oxygen concentrations

Abstract The catalytic oxidation of 2-mercaptoethanol was investigated kinetically for the system cobalt(II) phthalocyanine-tetra(sodium sulfonate) in the presence of poly(quaternary ammonium salts). The kinetics follow the two-substrate Michaelis-Menten rate law, in which 2-mercaptoethanol is one substrate and oxygen the other. At low thiol concentrations, the decrease in the rate of oxygen consumption during a catalytic reaction can be described by an exponential decay curve. At higher thiol concentrations, the complete two-substrate Michaelis-Menten rate law must be used. A very high turnover number of 4300 ± 400 s −1 was found. Furthermore, the equilibrium constant for the addition of thiol was found to be 46 ± 10 M −1 .

Pulsed Initiation Polymerization Applied to Acrylate Monomers: Sources of Experimental Failure

The reasons for the breakdown of the pulsed initiation polymn. technique in the case of acrylates seem to be transfer-to-polymer followed by slow re-initiation and b-scission. Also a contribution of transfer-to-monomer is possible. The only way to circumvent these problems is in going to low temp. and high frequencies. A nonlinear Arrhenius plot seems to be realistic because the actual propagation rate is decreased by the occurrence of mid-chain radicals at higher temps. In order to apply kp values to predict the kinetics of acrylate polymns., one can not extrapolate pulsed laser polymn. data from low temps., but transfer-to-polymer reactions must be taken into account. [on SciFinder (R)]

An Emulsifier‐Free RAFT‐Mediated Process for the Efficient Synthesis of Cerium Oxide/Polymer Hybrid Latexes

Hybrid latexes based on cerium oxide nanoparticles are synthesized via an emulsifier-free process of emulsion polymerization employing amphiphatic macro-RAFT agents. Poly(butyl acrylate-co-acrylic acid) random oligomers of various compositions and chain lengths are first obtained by RAFT copolymerization in the presence of a trithiocarbonate as controlling agent. In a second step, the seeded emulsion copolymerization of styrene and methyl acrylate is carried out in the presence of nanoceria with macro-RAFT agents adsorbed at their surface, resulting in a high incorporation efficiency of cerium oxide nanoparticles in the final hybrid latexes, as evidenced by cryo-transmission electron microscopy.

Controlled synthesis of polymeric nanocapsules by RAFT-based vesicle templating

Polymeric nanocapsules were synthesized by encapsulating extruded vesicles of dimethyldioctadecyl ammonium bromide (DODAB) using a reversible addition-fragmentation chain transfer (RAFT)-based encapsulation approach. Random copolymers containing acrylic acid and butyl acrylate units were first synthesized by RAFT in solution using dibenzyl trithiocarbonate (DBTTC) as the RAFT agent. These anionic copolymer chains were subsequently adsorbed onto the surface of cationic DODAB vesicles and then chain extended to form a polymeric shell by starved feed emulsion polymerization. Cryogenic transmission electron microscopy (cryo-TEM) characterizations demonstrate the successful formation of nanocapsules.