Carlos Guerrero-Sanchez

Water uptake of hydrophilic polymers determined by a thermal gravimetric analyzer with a controlled humidity chamber

The moisture uptake of several water-soluble polymers at different humidities was investigated with a thermal gravimetric analyzer equipped with a controlled humidity chamber. The water sorption of poly(acrylic acid) sodium salt, poly(ethylene glycol) and silica, which are known as super absorbers, were examined. In addition, various hydrophilic polymeric materials were selected according to their structural features. These included hydroxyl functions on the side chains (e.g. poly(2-hydroxyethyl methacrylate)), as well as acidic or basic functionalities (e.g. poly (dimethylaminoethyl methacrylate) or poly(vinylimidazole)). In addition, poly(2-methyl-2-oxazoline) (P(MeOx)) and poly(2-ethyl-2-oxazoline) (P(EtOx)), which are well-known hydrophilic polymers, were also investigated in this context. More significant weight percent changes were obtained for P(MeOx) (60% at 90% relative humidity (RH)) in comparison to P(EtOx) (35% at 90% RH) as a result of the slight difference in hydrophilicity of the structures. The effect of the chain length on the ability for water uptake was also investigated for both poly(oxazolines). Finally, thermoresponsive polymers with a lower critical solution temperature (LCST) behavior (e.g. poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate)) were also examined. The measurements for the latter polymers were performed below and above the LCST of each polymer whereby the humidities are varied from 0 to 90% with steps of 10%. Upon increasing humidity, the results revealed relatively high water uptake values (8% and 22% for P(NIPAM) and for P(DMAEMA), respectively) below the LCSTs of the polymers and, contrastingly, a small weight loss above their LCSTs. The present results allow a deeper insight into important structure–property relationships (e.g. the influence of the polymer backbone, functional groups, LCST behavior, etc. on the water-uptake properties), and will in subsequent steps permit the directed design of tailor-made polymers for selected applications.

Fast and “green” living cationic ring opening polymerization of 2-ethyl-2-oxazoline in ionic liquids under microwave irradiation

The living cationic ring opening polymerization of 2-ethyl-2-oxazoline performed in an ionic liquid under microwave irradiation showed an enhanced polymerization rate in comparison to the reaction in common organic solvents; the ionic liquid was efficiently recovered and reused in new reaction cycles, completely avoiding the use of organic volatile compounds.

Solubility and Thermoresponsiveness of PMMA in Alcohol-Water Solvent Mixtures

To reduce the environmental burden of polymer processing, the use of non-toxic solvents is desirable. In this regard, the improved solubility of poly(methyl methacrylate) (PMMA) in ethanol/water solvent mixtures is very appealing. In this contribution, detailed investigations on the solubility of PMMA in alcohol/water solvent mixtures are reported based on turbidimetry measurements. PMMA revealed upper critical solution temperature transitions in pure ethanol and ethanol/water mixtures. However, around 80 wt-% ethanol content a solubility maximum was observed for PMMA as indicated by a decrease in the transition temperature. Moreover, the transition temperatures increased with increasing PMMA molar mass as well as increasing polymer concentration. Careful analysis of both heating and cooling turbidity curves revealed a peculiar hysteresis behaviour with a higher precipitation temperature compared with dissolution with less than 60 wt-% or more than 90 wt-% ethanol in water and a reverse hysteresis behaviour at intermediate ethanol fractions. Finally, the transfer of poly(styrene)-block-poly(methyl methacrylate) block copolymer micelles from the optimal solvent, i.e. aqueous 80 wt-% ethanol, to almost pure water and ethanol is demonstrated.

One pot synthesis of higher order quasi-block copolymer libraries via sequential RAFT polymerization in an automated synthesizer

Recently developed sequential reversible addition–fragmentation chain transfer (RAFT) polymerization protocols allow the rapid, fully unattended preparation of quasi-block copolymer libraries that cover a wide range of copolymer compositions in an automated synthesizer. This contribution explores the scope and limitations of this sequential approach for the synthesis of higher order quasi-multiblock copolymers (including copolymer sequences of BAB, CBABC, ABC and ABCD). These syntheses illustrate the utility of this high-throughput approach for the one pot synthesis of functional polymers of increased complexity. Additionally, the use of this experimental technique for method development is highlighted.

Aqueous gelation of ionic liquids: reverse thermoresponsive ion gels

The aqueous gelation of a quaternary ammonium oligo(propylene oxide)-based ionic liquid yields ion gels with a reverse thermoresponsive behavior (i.e., mechanical moduli and viscosity increase with temperature) and enhanced ionic conductivities.

The effect of RAFT-derived cationic block copolymer structure on gene silencing efficiency

In this work a series of ABA tri-block copolymers was prepared from oligo(ethylene glycol) methyl ether methacrylate (OEGMA(475)) and N,N-dimethylaminoethyl methacrylate (DMAEMA) to investigate the effect of polymer composition on cell viability, siRNA uptake, serum stability and gene silencing. Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization was used as the method of polymer synthesis as this technique allows the preparation of well-defined block copolymers with low polydispersity. Eight block copolymers were prepared by systematically varying the central cationic block (DMAEMA) length from 38 to 192 monomer units and the outer hydrophilic block (OEGMA(475)) from 7 to 69 units. The polymers were characterized using size exclusion chromatography and (1)H NMR. Chinese Hamster Ovary-GFP and Human Embryonic Kidney 293 cells were used to assay cell viability while the efficiency of block copolymers to complex with siRNA was evaluated by agarose gel electrophoresis. The ability of the polymer-siRNA complexes to enter into cells and to silence the targeted reporter gene enhanced green fluorescent protein (EGFP) was measured by using a CHO-GFP silencing assay. The length of the central cationic block appears to be the key structural parameter that has a significant effect on cell viability and gene silencing efficiency with block lengths of 110-120 monomer units being the optimum. The ABA block copolymer architecture is also critical with the outer hydrophilic blocks contributing to serum stability and overall efficiency of the polymer as a delivery system.

Rapid and systematic access to quasi-diblock copolymer libraries covering a comprehensive composition range by sequential RAFT polymerization in an automated synthesizer

A versatile, cost-effective approach to the rapid, fully unattended preparation of systematic quasi-diblock copolymer libraries via sequential RAFT polymerization in an automated synthesizer is reported. The procedure is demonstrated with the synthesis of a 23 member library of low dispersity poly(butyl methacrylate)-quasiblock-poly(methyl methacrylate) covering a wide (fivefold) range of molar mass for the second block in a one-pot, sequential, RAFT polymerization.

Quasi-block copolymer libraries on demand via sequential RAFT polymerization in an automated parallel synthesizer

A convenient synthetic method for the systematic preparation of quasi-diblock copolymer libraries utilizing a sequential RAFT polymerization strategy is described. This method utilizes a parallel synthesizer and allows the unattended and fully automated synthesis of this type of library in a short period of time. The materials obtained in this investigation have shown properties very similar to those expected in “pure” diblock copolymers as determined by differential scanning calorimetry. The described method can be a useful and less expensive alternative for the rapid preparation and screening of block copolymer libraries.

The reactivity of N-vinylcarbazole in RAFT polymerization: trithiocarbonates deliver optimal control for the synthesis of homopolymers and block copolymers

The use of various RAFT agents (ZC(S)SR) including dithiobenzoates (Z = Ph), trithiocarbonates (Z = SR′), xanthates (Z = OR′), and conventional and switchable N-aryldithiocarbamates (Z = NR′Ar) in RAFT polymerization of N-vinylcarbazole (NVC) has been explored with a view to establishing which is most effective. Consistent with earlier work, we find that xanthates and N-aryldithiocarbamates give adequate control (dispersities (Đ) < 1.3) while dithiobenzoates give marked retardation. However, contrary to popular belief, we find that the polymerization of NVC is best controlled with trithiocarbonate RAFT agents, which provide both good molecular weight control, very narrow dispersities (Đ < 1.1), and high end-group fidelity. The results demonstrate that NVC has intermediate reactivity, i.e. between that of the traditional more activated (MAMs; styrene, acrylates) and less activated monomers (LAMs; vinyl acetate, N-vinylpyrrolidone). A further key to good control is the selection of RAFT agent R substituent to be both a good leaving group and a good initiating radical. The cyanomethyl group meets these criteria whereas phenylethyl is a poor initiating radical for NVC polymerization. A further demonstration of the intermediate reactivity of NVC and the derived propagating radical was the successful preparation of both poly(n-butyl acrylate)-block-poly(N-vinylcarbazole) and poly(N-vinylcarbazole)-block-poly(n-butyl acrylate) with a trithiocarbonate RAFT agent (the sequence of block synthesis is not important). Two-dimensional, liquid chromatography near critical conditions-gel permeation chromatography (LCCC-GPC) has been applied to demonstrate block purity. The corresponding styrene-based blocks can also be successfully synthesized, however, the reinitiation of NVC polymerization by the polystyryl radical proved to be a constraint on the purity of polystyrene-block-poly(N-vinylcarbazole).

Automated parallel freeze-evacuate-thaw degassing method for oxygen-sensitive reactions: RAFT polymerization.

An automated and parallel freeze-evacuate-thaw degassing method in a commercially available synthesizer is disclosed and tested for its applicability to reversible addition-fragmentation chain transfer (RAFT) polymerization. The effectiveness of this method to eliminate oxygen in polymerization reactions is demonstrated by directly comparing it against experiments performed using conventional laboratory techniques. Apart from the demonstrated accuracy, the proposed method has also shown significant precision when performing RAFT polymerizations. The reported experimental technique can be easily adapted to other chemical systems where the removal of oxygen is mandatory. This new high-throughput method has the potential to significantly increase the productivity and/or research outcomes in laboratories where oxygen-sensitive reactions are carried out.