Arménio C. Serra

The quest for sustainable polyesters – insights into the future

Polyesters from renewable resources are an expanding area with a burgeoning scientific activity, nevertheless little has been reviewed about this particular class of polymers. The present appraisal intends to contribute to fill this literature gap by reviewing recent aspects related to the most promising renewable-based polyesters. Emphasis will be placed on bio-based polyesters that, given their comparable properties, may replace polymers derived from fossil fuel feedstock, and on bio-based polyesters with completely innovative properties for novel applications. Furthermore, the sources of renewable monomers will also be reviewed, together with the most relevant eco-friendly synthetic approaches used in polycondensation reactions leading to polyesters.

Heavy-atom effects on metalloporphyrins and polyhalogenated porphyrins

The photophysics of halogenated and metallated tetrakisphenylporphyrins is investigated using single-photon counting, photoacoustic calorimetry and luminescence techniques. The radiationless transition rates in these and related molecules are interpreted with a quantum-mechanical tunnelling model modified to include the effect of spin–orbit coupling in the intersystem crossing rates. It is shown that tetrakisphenylporphyrins with halogens in the ortho positions have long-lived triplet states that are formed in high yields. 2002 Elsevier Science B.V. All rights reserved.

Halogen atom effect on photophysical and photodynamic characteristics of derivatives of 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin.

Brominated and iodinated derivatives of 5,10,15,20-tetrakis(3-hydroxyphenyl)porphyrin were synthesised directly from the corresponding aldehydes. Photophysical and photochemical properties, singlet oxygen formation quantum yields, photobleaching and log P were measured. Cellular uptake measurements and cytotoxicity assays on WiDr and A375 tumour cell lines were performed. 5,10,15,20-Tetrakis(2-bromo-5-hydroxyphenyl)porphyrin showed the best cytotoxicity with values of IC(50) of 113 nM over WiDr cells and 52nM over A375 cells.

Ambient temperature rapid SARA ATRP of acrylates and methacrylates in alcohol–water solutions mediated by a mixed sulfite/Cu(II)Br2 catalytic system

The new generation of catalytic systems for Controlled/“Living” Radical Polymerization (CLRP) of vinyl monomers should be non-toxic, inexpensive and provide fast polymerizations in environmentally friendly media. Herein, we report the successful ambient temperature ATRP of several vinyl monomers (MA, n-BA, MMA and DMAEMA) catalyzed by inorganic sulfites (Na2S2O4 and Na2S2O5) and small amounts of a Cu(II)Br2/Me6TREN system in alcohol–water mixtures. The controlled character of ATRP of acrylates and methacrylates was confirmed by the linear increase of molecular weights with monomer conversion, narrow molecular weight distributions (Mw/Mn ∼ 1.05) and by reinitiation experiments (copolymerization and chain extension). 1H NMR and MALDI-TOF analyses confirmed the molecular structure and chain-end functionality of the obtained polymers. ATRP of MA using this novel catalytic system in alcohol–water mixtures with multifunctional Br-based initiators provides 4 and 6 arm star polyacrylates in a controlled manner without any observable gel formation. The data presented open up the possibility of using fast ATRP catalyzed by inorganic sulfites (approved by FDA as food and beverage additives) in solvents that are inexpensive, eco-friendly and widely used in chemical industrial processes.

Synthesis of well-defined poly(2-(dimethylamino)ethyl methacrylate) under mild conditions and its co-polymers with cholesterol and PEG using Fe(0)/Cu(II) based SARA ATRP

Atom transfer radical polymerization (ATRP) of 2-(dimethylamino)ethyl methacrylate (DMAEMA) with a mixed transition metal catalytic system consisting of Fe(0) and small amounts of CuBr2/PMDETA (N,N,N′,N′′,N′′-pentamethyldiethylenetriamine) in water–isopropanol mixtures at 60 °C and 25 °C is reported and compared with commonly used CuBr/PMDETA-mediated ATRP systems. The obtained kinetics of polymerization showed that, in addition to the environmental attractiveness of this catalytic system, Fe/Cu mixed transition metal mediated ATRP of a DMAEMA monomer provides a significantly improved control of polymer chain growth, forming PDMAEMA of low dispersity values at all conversions. The controlled character of ATRP of DMAEMA was confirmed by the linear increase of molecular weights with monomer conversion, narrow molecular weight distributions, and re-initiation/chain extension experiments. The molecular structure of the obtained polymer(s) was confirmed by 1H nuclear magnetic resonance spectroscopy and matrix-assisted laser desorption ionization time-of-flight mass spectroscopy. ATRP of DMAEMA was also extended to synthesis of functional bio-relevant polymers with cholesterol and poly(ethylene glycol) segments.

Singlet oxygen in antimicrobial photodynamic therapy: Photosensitizer-dependent production and decay in E. coli

Several families of photosensitizers are currently being scrutinized for antimicrobial photodynamic therapy applications. Differences in physical and photochemical properties can lead to different localization patterns as well as differences in singlet oxygen production and decay when the photosensitizers are taken up by bacterial cells. We have examined the production and fate of singlet oxygen in Escherichia coli upon photosensitization with three structurally-different cationic photosensitizers, namely New Methylene Blue N (NMB), a member of the phenothiazine family, ACS268, a hydrophobic porphyrin with a single cationic alkyl chain, and zinc(II)-tetramethyltetrapyridinoporphyrazinium salt, a phthalocyanine-like photosensitizer with four positive charges on the macrocycle core. The kinetics of singlet oxygen production and decay indicate different localization for the three photosensitizers, whereby NMB appears to localize in an aqueous-like microenvironment, whereas ACS268 localizes in an oxygen-shielded site, highly reactive towards singlet oxygen. The tetracationic zinc(II) tetrapyridinoporphyrazine is extensively aggregated in the bacteria and fails to produce any detectable singlet oxygen.

On the mechanism of carboxylic acid co-catalyst assisted metalloporphyrin oxidations

The role of the carboxylic acid as co-catalyst in metalloporphyrin catalytic hydrogen peroxide oxidations is discussed, taking into account its dependence on the ratio relatively to the catalyst. The catalytic efficiency and stability of the catalyst in the presence of an excess of carboxylic acid suggests that a metallo-acylperoxo complex can be the effective oxidation intermediate.

New unsaturated copolyesters based on 2,5-furandicarboxylic acid and their crosslinked derivatives

The synthesis and characterisation of a novel family of unsaturated polyesters (UPs) and their crosslinked resins (UPRs) based on 2,5-furandicarboxylic acid (FDCA) are reported. Their original features stem from the use of FDCA as the aromatic monomer, and also from the fact that UPs are entirely based on renewable resources, oppositely to most reported materials which are typically based on petrochemicals or instead they are derived from both petrochemicals and a percentage of renewables. Additionally, instead of styrene, 2-hydroxyethylmethacrylate (HEMA) was used as the reactive solvent to obtain the UPRs. These novel resins showed adequate thermal and mechanical behavioural tendencies similar to petrochemical ones, namely high glass transition temperature (up to 104 °C) and good thermal stability (up to 230 °C). These characteristics enhance their prospects of being a successful renewable-based material.

Improvement of the control over SARA ATRP of 2-(diisopropylamino)ethyl methacrylate by slow and continuous addition of sodium dithionite

The kinetics and detailed mechanism of SARA ATRP of 2-(diisopropylamino)ethyl methacrylate (DPA) were investigated. Supplemental activator and reducing agent (SARA) atom transfer radical polymerization (ATRP) using sodium dithionite (Na2S2O4) was used to create well controlled polymers of PDPA. The influence of the initiator, solvent, structure and concentration of the catalyst was studied, and the ratios of Na2S2O4 were adjusted to optimize the polymerization. Well controlled polymers required Na2S2O4 to be slowly and continuously fed to the reaction mixture, with 500 parts per million (ppm) of CuBr2 with tris(2-dimethyamino)amine (Me6TREN) as a ligand. The initial content of Na2S2O4 in the reaction mixture, the feeding rate and the Cu catalyst concentration were optimized to provide polymers with narrow molecular weight distribution (Mw/Mn < 1.15) at high monomer conversion (∼90%). Interestingly, the results revealed that when tris(2-pyridylmethyl)-amine (TPMA) was used as a ligand, the amount of copper required to achieve similar control of the polymerization could be decreased 5 times. This system was successfully extended to the polymerization of oligo(ethylene oxide) methyl ether methacrylate (OEOMA). The high conversion and preservation of the chain-end functionality allows the direct synthesis of POEOMA-b-PDPA block copolymers. The low catalyst concentrations and benign nature of Na2S2O4 make this SARA ATRP method attractive for the synthesis of well controlled water soluble polymers for biomedical applications.

Synthesis of cationic poly((3-acrylamidopropyl)trimethylammonium chloride) by SARA ATRP in ecofriendly solvent mixtures

Supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) of the cationic monomer (3-acrylamidopropyl)trimethylammonium chloride (AMPTMA) was successfully performed for the first time. The polymerizations were performed in water or ethanol–water mixtures at room temperature in the presence of Cu(0), using relatively low concentrations of soluble copper catalyst and an excess of ligand (Me6TREN). The reaction conditions were optimized to give the best control over the polymerization under environmentally friendly conditions. The polymerization data showed good control over the molecular weights with narrow molecular weight distributions for the entire polymerization. The preservation of the chain-end functionality was confirmed by self-chain extension and the synthesis of a block copolymer containing AMPTMA and oligo(ethylene oxide) methyl ether acrylate (OEOA). SARA ATRP was also extended to the synthesis of alkyne-terminated poly-AMPTMA (PAMPTMA), which was subsequently functionalized, using copper(I) catalyzed azide–alkyne cycloaddition, with an azido-functionalized coumarin derivative.