Jpa Johan Heuts

RAFT-mediated polymerization - A story of incompatible data?

The mechanistic interpretation of kinetic anomalies in reversible addition-fragmentation chain transfer (RAFT)-mediated polymerization is critically reviewed. The main conclusion of this exercise is that available data do not allow model discrimination between the two prevailing mechanistic schemes, i.e., the slow fragmentation model and the intermediate radical termination model. However, assessment of the rate parameters reveals that the incompatibilities may not be as large as previously reported in literature. Dedicated kinetic studies on model compounds should be performed to shed further light on the seemingly incompatible data that currently exists in literature.

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.

Selective partial hydrolysis of amphiphilic copoly(2-oxazoline)s as basis for temperature and pH responsive micelles

The acidic and basic hydrolyses of gradient and diblock copolymers based on 2-methyl-2-oxazoline (MeOx) and 2-phenyl-2-oxazoline (PhOx) were investigated. Various reaction times were examined revealing polymers with varying ratios of PMeOx, PPhOx and poly(ethylene imine) (PEI). It could be shown that under acidic conditions, PMeOx as well as PPhOx are readily cleaved while under basic conditions PPhOx was almost not hydrolyzed leading to higher selectivity. However, partial degradation of the polymers occurred under basic conditions as evidenced by SEC. Thermal investigations of the polymers cleaved under acidic conditions revealed that most obtained copolymers exhibited a melting temperature due to the large PEI content. Self-assembly studies revealed that the partially hydrolyzed copolymers formed micelles at both ambient and elevated temperatures in acidic medium due to protonation of the ethylene imine units leading to good solubility. In contrast, the copolymers were insoluble at ambient temperature in water or basic medium, but self-assembled into spherical micelles at elevated temperatures as evidenced by transmission electron microscopy and dynamic light scattering. As such, these novel PEI–PPhOx copolymers exhibit thermoresponsive micellization behavior based on the crystallization induced phase separation of linear PEI at lower temperatures. Moreover, the copolymer consisting of 84 ethylene imine repeat units, 16 PPhOx groups also revealed pH responsive micellization due to increased solubility of the ethylene imine units upon protonation in acidic solution.

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.

Encapsulation of non-chemically modified montmorillonite clay platelets via emulsion polymerization

This paper demonstrates that a covalent bonding between clay platelets and polymer chains is not necessary for a successful encapsulation of the inorganic compounds through emulsion polymerization. In the work described in this paper, the chemical modification of clay was performed using two kinds of titanate coupling agents: titanium IV, (2-propanolato)tris(2-propanoata-O), 2-(2-methoxyethoxy) ethanol and titanium IV, 2-propanolato,tris(isooctadecanoato-O), where the former is saturated and the second has unsaturated alkyl groups. The hydrolytic stability of the organoclays thus synthesized was thereafter investigated. It was found that the titanate modifiers were highly sensitive towards hydrolysis as evidenced by Fourier transform infrared and thermal gravimetry analyses. The effect of a chemical modification step on clay encapsulation was studied by comparing the results obtained between reactions using the synthesized organoclays and the ones using native clay platelets. It was found that the incorporation of polymerizable double bonds on clay platelets was unnecessary to achieve successful encapsulation and that, surprisingly, the chemical modification step could be omitted for the synthesis of clay-containing latex particles. The effects of monomer feed composition, i.e., monomer mixtures consisting of different weight ratios methyl methacrylate/butyl acrylate, and process types on clay encapsulation were also investigated. Both parameters were found to have a strong influence on clay encapsulation. Finally, the surfactant concentration and the surfactant type were statistically found to have a significant effect on the clay/polymer interaction as evidenced by the results of glass transition temperatures of dried latex/clay nanocomposites powders.

Further effects of chain-length-dependent reactivities on radical polymerization kinetics

In the present paper, we finalize some threads in our investigations into the effects of chain-length-dependent propagation (CLDP) on radical polymerization kinetics, confirming all our previous conclusions. Additionally, and more significantly, we uncover some unexpected and striking effects of chain-length-dependent chain transfer (CLDTr). It is found that the observed overall rate coefficients for propagation and termination (and therefore the rate of polymerization) are not significantly affected by whether or not chain transfer is chain-length dependent. However, this situation is different when considering the molecular weight distributions of the resulting polymers. In the case of chain-length-independent chain transfer, CLDP results in a considerable narrowing of the distribution at the low molecular weight side of the distribution in a chain-transfer controlled system. However, the inclusion of both CLDP and CLDTr yields identical results to classical kinetics – in these latter two cases, the molecular weight distribution is governed by the same chain-length-independent chain transfer constant, whereas in the case of CLDP only, it is governed by a chain-length-dependent chain transfer constant that decreases with decreasing chain length, thus enhancing the probability of propagation for short radicals. Furthermore, it is shown that the inclusion of a very slow first addition step has tremendous effects on the observed kinetics, increasing the primary radical concentration and thereby the overall termination rate coefficient dramatically. However, including possible penultimate unit effects does not significantly affect the overall picture and can be ignored for the time being. Lastly, we explore the prospects of using molecular weight distributions to probe the phenomena of CLDP and CLDTr. Again, some interesting insights follow.

An ATRP-based approach towards water-borne anisotropic polymer–Gibbsite nanocomposites

Polymer–Gibbsite composite latex particles were synthesised via an atom transfer radical polymerisation (ATRP) based approach. A random ATRP cooligomer, consisting of acrylic acid and butyl acrylate units, was synthesized using ATRP. This cooligomer was used as a stabiliser for the Gibbsite platelets and served as a macroinitiator for copper-mediated starved-feed emulsion polymerisation. Using a hydrophobic ligand for Cu2+ and optimising the feeding profile of ascorbic acid and the [ascorbic acid]/[Cu2+] ratio, successful Activator ReGenerated by Electron Transfer (ARGET) ATRP emulsion polymerisation was conducted in a controlled way, using only the anionic ATRP cooligomer as a surfactant. Cryo-TEM characterisation revealed a “muffin-like” morphology of the resulting composite latex particles, which was not affected by monomer feed composition and feeding profile.

Facile and selective synthesis of aldehyde end-functionalized polymers using a combination of catalytic chain transfer and rhodium catalyzed hydroformylation

A novel synthetic pathway towards aldehyde end-functionalized polymers is presented from a combination of catalytic chain transfer polymerization (CCTP) and rhodium catalyzed hydroformylation in supercritical carbon dioxide. CCTP allows for the synthesis of well-defined macromonomers in terms of the average molecular weight and the terminal unit carrying the unsaturated bond. The rhodium catalyzed hydroformylation allows for a high selectivity towards aldehyde end-group functionalized polymers. The introduction of the synthetically versatile aldehyde end-group opens up a broad range of possible applications.

Encapsulation of unmodified Gibbsite via conventional emulsion polymerisation using charged co-oligomers

Gibbsite platelets were successfully encapsulated via starved-feed conventional emulsion polymerisation using anionic co-oligomers without the need for any surface modification of the platelets. Charged co-oligomers, consisting of butyl acrylate and acrylic acid units, were synthesized using atom transfer radical polymerisation (ATRP) and used as stabilisers for the initial Gibbsite platelets and the formed latex particles. Optimisation of co-oligomer concentration resulted in efficient encapsulation where every latex particle contained a Gibbsite platelet. Cryo-TEM characterisation showed the Gibbsite platelet completely covered with a polymer layer and this morphology was not affected by the investigated co-oligomer composition or chain length.

The effect of clay on the morphology of multiphase latex particles

The effect of montmorrilonite clay (MMT) platelets on the morphology of polystyrene/poly(methyl methacrylate) (PMMA) composite latex particles prepared via PMMA-seeded (semi-) batch emulsion polymerization of styrene was studied. It was found that the particle morphology obtained greatly depended on the ability of the clay platelets to diffuse through the polymer particle. Indeed, when the reactions were strictly under kinetic control, i.e., where the clay platelets were unable to diffuse during polymerization, anisotropic core-shell-like morphologies with split core were observed. A better mobility of the clay platelets could more or less restrict the diffusion of the second-stage polymers within the host polymer, leading to original kinetically controlled morphologies. In the case of a full migration of the clay platelets to the particle surface, the penetration of the second-stage polymer species in the seed latex was found to be more limited, enhancing the formation of secondary particles.