José Luis de la Fuente

The importance of solvent polar character on the synthesis of PMMA-b-PBA block copolymers by atom transfer radical polymerization

Abstract The synthesis of diblock copolymers using atom transfer radical polymerization (ATRP), of methyl methacrylate (MMA), and butyl acrylate (BA), is reported. These copolymers were prepared from bromo-terminated macroinitiators of poly(MMA) and poly(BA), using copper chloride, CuCl,/N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA), as catalyst system, at 100°C in bulk and in benzonitrile solution. The block copolymers were characterized by means of size exclusion chromatography (SEC), and 1H-NMR spectroscopy. The SEC analysis of the synthesized diblock copolymers confirmed the importance of solvent on the molecular weight control. In addition, differential scanning calorimetry (DSC), measurements were performed, showing for all the copolymers a phase separation.

Solvent effects on the synthesis of poly(methyl methacrylate) by atom-transfer radical polymerization (ATRP)

Methyl methacrylate has been polymerized by ATRP at 100°C in bulk, in toluene (50%, v/v) and benzonitrile (50% and 25%, v/v), using methyl 2-halopropionate as initiator (MeXPr, X = Cl or Br), copper halide (CuX) as catalyst and 2,2′-bipyridine (bpy) as ligand. The mixed halogen system MeBrPr/CuCl has also been used. For all initiator systems used, the bulk polymerizations showed linear first-order rate plots, a linear increase of the number-average molecular weight with conversion, and relatively low polydispersities, but low initiator efficiency. The polymerizations performed in toluene do not significantly increase the polymerization efficiency, but the molecular weight distribution is broadened. When the polymerizations are performed in benzonitrile, the polymerization efficiency increases considerably, but is independent of monomer dilution. The molecular weight distributions are narrowed when the polymerizations are performed in 25% (v/v) benzonitrile solution. Experimental number-average molecular weights match the theoretical values when the polymerizations are carried out using the mixed halogen initiator system in benzonitrile solution.

A Comparative Study of Methyl Methacrylate/Butyl Acrylate Copolymerization Kinetics by Atom‐Transfer and Conventional Radical Polymerization

Methyl methacrylate and butyl acrylate monomers are copolymerized by atom-transfer radical polymerization, affording polymers with well-controlled molecular weight and low polydispersity. A kinetic analysis of this system is compared with the corresponding free-radical polymerization system. The copolymerization rate follows an opposite trend to that observed in conventional copolymerization. This fact is attributed to a smaller population of radicals generated in the reaction, since the relative fraction of propagating radicals is the same as that in classical copolymerization.

Solvent effects on the free-radical copolymerization of butyl acrylate with methyl methacrylate

Free-radical copolymerization of butyl acrylate with methyl methacrylate was carried out at 50°C in a 3 mol/L benzonitrile solution. Differences between the apparent reactivity ratios determined in this work with those previously reported in bulk, toluene and benzene indicate noticeable solvent effects. This fact can be explained through different models, but independent of the chemistry of the system, to obtain the same copolymer composition in bulk or in solution, the monomer feed that should be used is determined by solvent polarity.

An Analysis of the Solvent Effects on the Monomer Reactivity Ratios Using the Copolymer Glass Transition Temperatures

The glass transition temperatures (T g ) of styrene-butyl acrylate copolymers obtained by free radical polymerization in bulk, and in 3 mol.L -1 benzene and benzonitrile solutions, have been measured using differential scanning calorimetry (DSC) to verify the boot-strap effect previously reported for this system. The corresponding values have been correlated with copolymer composition and microstructure using the Johnston equation.

Polypyrrole-Modified-Carbon-Supported Ru-Pt Nanoparticles as Highly Methanol-Tolerant Electrocatalysts for the Oxygen-Reduction Reaction

The modification of mesoporous carbon black with polypyrrole (C‐PP) facilitates the incorporation of homogeneously dispersed nanosized Ru‐Pt particles (1–2 nm) onto the support. Polypyrrole‐modified carbon supports were prepared by the addition of different amounts of pyrrole, followed by in situ polymerization through oxidative treatments. The Ru‐Pt/C‐PP electrocatalysts were tested in the oxygen‐reduction reaction in methanol‐containing acidic electrolytes by using both conventional electrochemical techniques and in a direct methanol single cell. The performance of Ru‐Pt/C‐PP was far superior to that of Pt/C for the oxygen‐reduction reaction in the direct methanol fuel cell. Under severe methanol‐crossover conditions, the maximum power density that was delivered by Ru‐Pt/C‐PP after degradation tests was decreased by 20 %, whereas that of Pt/C decreased by 75 %. Thus, the presence of Ru oxides in the cathode led to more tolerant electrocatalysts towards the presence of methanol in the reaction medium during the oxygen‐reduction reaction.

Characterization and thermal properties of poly(n-butyl acrylate-g-styrene) graft copolymers

Poly(butyl acrylate-g-styrene) graft copolymers were prepared by free-radical polymerization using a polystyrene macromonomer carrying a methacryloyloxy group at the chain end and they were characterized by size-exclusion chromatography, and Fourier transform infrared spectroscopy. Glass transition temperatures and degradation behavior were determined by thermal analysis. Only a single glass transition temperature was observed for the resulting graft copolymers, indicating the miscibility between the poly(styrene) phase and poly(butyl acrylate) (pBA) phase in the graft copolymer. The incorporation of polystyrene segments in the graft copolymer improved the thermal stability of pBA and enhanced the apparent activation energy for the thermal degradation of pBA.

Copolymerization of methyl methacrylate and butyl acrylate in the presence of a chain transfer agent

Copolymerizations of methyl methacrylate (MMA) and butyl acrylate (BA) have been performed using different monomer feed compositions in the presence of the chain-transfer agent dodecanethiol (DDT) at 60°C in benzene solution. The average chain-transfer constants as a function of the monomer feed composition were determined by using the conventional Mayo method and by the chain length distribution (CLD) procedure. The average chain-transfer constants obtained from both methods are compared to model predictions based upon both the terminal and penultimate unit models of free-radical copolymerization, and they are in satisfactory aggreement with each other.

Homopolymerization of methyl methacrylate and styrene : Determination of the chain-transfer constant from the Mayo equation and the number distribution for n-dodecanethiol

Chain-transfer constants were evaluated for n-dodecanethiol in the homopolymerization of styrene (S) and methyl methacrylate (MMA). The polymerizations were carried out in benzene at 50 °C with different amounts of 2,2′-azobisisobutyronitrile as the initiator. The new chain length distribution (CLD) analytical method was used and compared to the traditional Mayo method. The chain-transfer-constant values were independent of the initiator concentration and slightly higher (by a factor of 1.1 for MMA and 1.2 for S) when obtained according to the CLD method compared to the Mayo method. The chain-transfer constant for S was 20 times higher than for MMA.

Determination of chain transfer constant to dodecanethiol in styrene/methyl methacrylate and butyl acrylate/methyl methacrylate copolymerization and effect of chain length on composition and stereochemical configuration of copolymers

Free radical copolymerization of styrene/methyl methacrylate (S/MMA) and butyl acrylate/methyl methacrylate (BA/MMA) in the presence of n-dodecanthiol (DDT) has been studied at 60°C in a 3 mol/L benzene solution using 2,2′-azobis(isobutyronitrile) (AIBN) as initiator. Overall chain transfer constant to DDT has been determined for both copolymerization systems, as a function of monomer feed composition using complete molecular weight distribution and the Mayo method. Overall transfer coefficients have values which are dependent on both monomer feed composition and individual comonomer transfer values. Composition, sequence distribution, and stereoregularity of copolymers obtained are, in our experimental conditions, independent of copolymer molecular weight.