Zinc‐Catalyzed Depolymerization of End‐of‐Life Polysiloxanes

Polymers occupy an important role in our current society. Besides their great success, an issue is the accumulation of huge amounts of end‐of‐life polymers. Currently, the waste management is based primarily on landfills, thermal recycling, and downcycling. Notably, only a small portion of end‐of‐life materials is recycled by depolymerization, which refers to the creation of synthetic precursors that can be polymerized to new polymers to close the cycle. Widely used polymers in modern times are silicones (polysiloxanes), the intrinsic properties of which make their depolymerization demanding; only a few high‐temperature or less environmentally friendly processes have been reported. In this regard, we have established an efficient low‐temperature protocol for the depolymerization of silicones with benzoyl fluoride in the presence of cheap zinc salts as precatalysts to yield defined products. Notably, the products can be useful synthetic precursors for the preparation of new polymers, so that an overall recycling process is feasible.     

Heterogeneous Aromatic Amination of Aryl Halides with Arylamines in Water with PS‐PEG Resin‐Supported Palladium Complexes

Catalytic aromatic amination is achieved in water under heterogeneous conditions by the use of immobilized palladium complexes coordinated with the amphiphilic polystyrene‐poly(ethylene glycol) resin‐supported di(tert‐butyl)phosphine ligand. Aromatic amination of aryl halides with diphenylamine and N,N‐double arylation of anilines with bromobenzene were found to proceed in water with broad substrate tolerance to give the triarylamines in high yield with high recyclability of the polymeric catalyst beads. Very little palladium leached from the polymeric catalyst under the water‐based reaction conditions to provide a green and clean (metal‐uncontaminated) protocol for the preparation of triarylamines, including the optoelectronically active N,N,N′,N′‐tetraaryl‐1,1′‐biphenyl‐4,4′‐diamines (TPDs).     

Depolymerization of poly(2,6-dimethyl-1,4-phenylene oxide) under oxidative conditions

Depolymerization of an engineering plastic, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), was accomplished by using 2,6-dimethylphenol (DMP) under oxidative conditions. The addition of an excess amount of DMP to a solution of PPO in the presence of a CuCl/pyridine catalyst yielded oligomeric products. When PPO (M(n)=1.0x10(4), M(w)/M(n)=1.2) was allowed to react with a sufficient amount of DMP, the molecular weight of the product decreased to M(n)=4.9x10(2) (M(w)/M(n)=1.5). By a prolonged reaction with the oxidant, the oligomeric product was repolymerized to produce PPO essentially identical to the starting material, making the oligomer useful as a reusable resource. During the depolymerization reaction, an intermediate phenoxyl radical was observed by ESR spectroscopy. Kinetic analysis showed that the rate of the oxidation of PPO was about 10 times higher than that of DMP. These results show that a monomeric phenoxyl radical attacks the polymeric phenoxyl to induce the redistribution via a quinone ketal intermediate, leading to the substantial decrease in the molecular weight of PPO, which is much faster than the chain growth.     

Green chemistry: state of the art through an analysis of the literature

Abstract

The literature of green chemistry has undergone a dramatic increase in the new millennium. Besides that, in ad hoc journals, papers of this type are published in journals of general, organic, and catalytic chemistry. The high proportion of communications within this area indicates that this is a hot topic. These reports mainly concern more environment-friendly synthetic methods, based on better catalytic systems, less harmful solvents and, more rarely, “alternative” physical techniques. Although the compliance with the green chemistry postulates is still partial, a trend in this direction is recognizable. For example, the number of preparative papers that introduce an environmental assessment is rapidly increasing.     

The effect of light intensity,film thickness,and monomer composition in styrene-based bioinspired polymers

Abstract

The effect of irradiation light intensity, film thickness, and polymer composition upon photo-irradiation of water-soluble polymers containing thymine was studied by ultraviolet (UV)-vis spectroscopy. Coatings of aqueous solution of the polymer on PET substrates were exposed to UV light at 254 nm through a standard mask. The effect of irradiation dose is similar to the typical behavior of photo-resists: the degree of crosslink increases over the time until it reaches saturation. The polymer composition effect shows an expected trend on the curing process, the more vinylbenzylthymine (VBT) content in the copolymer the higher the degree of crosslinking, a situation that agrees with the fact the VBT is the monomer playing the main role in the photo-reaction. It was observed that the effect of film thickness was as expected, the thicker the film the slower the immobilization of the polymer on the substrate. It was demonstrated that by varying these parameters one could control the crosslinking rate of the polymer.     

Synthesis and Thermoreversible Gelation Properties of Main‐Chain Poly(pyridine‐2,6‐dicarboxamide‐triazole)s

A series of main‐chain poly(amide‐triazole)s were prepared by copper(I)‐catalyzed alkyne–azide AABB‐type copolymerizatons between five structurally similar diacetylenes 1 – 5 with the same diazide 6 . The acetylene units in monomers 1 – 5 possessed different degrees of conformational flexibility due to the different number of intramolecular hydrogen bonds built inside the monomer architecture. Our study showed that the conformational freedom of the monomer had a profound effect on the polymerization efficiency and the thermoreversible gelation properties of the resulting copolymers. Among all five diacetylene monomers, only the one, that is, 1 ‐Py(NH)₂ which possesses the pyridine‐2,6‐dicarboxamide unit with two built‐in intramolecular H bonds could produce the corresponding poly(amide‐triazole) Poly‐(PyNH)₂ with a significantly higher degree of polymerization (DP) than other monomers with a lesser number of intramolecular H bonds. In addition, it was found that only this polymer exhibited excellent thermoreversible gelation ability in aromatic solvents. A self‐assembling model of the organogelating polymer Poly‐(PyNH)₂ was proposed based on FTIR spectroscopy, XRD, and SEM analyses, in which H bonding, π–π aromatic stacking, hydrophobic interactions, and the structural rigidity of the polymer backbone were identified as the main driving forces for the polymer self‐assembly process.     

Organocatalytic depolymerization of poly(ethylene terephthalate)

We describe the organocatalytic depolymerization of poly(ethylene terephthalate) (PET), using a commercially available guanidine catalyst, 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD). Postconsumer PET beverage bottles were used and processed with 1.0 mol % (0.7 wt %) of TBD and excess amount of ethylene glycol (EG) at 190 °C for 3.5 hours under atmospheric pressure to give bis(2‐hydroxyethyl) terephthalate (BHET) in 78% isolated yield. The catalyst efficiency was comparable to other metal acetate/alkoxide catalysts that are commonly used for depolymerization of PET. The BHET content in the glycolysis product was subject to the reagent loading. This catalyst influenced the rate of the depolymerization as well as the effective process temperature. We also demonstrated the recycling of the catalyst and the excess EG for more than 5 cycles. Computational and experimental studies showed that both TBD and EG activate PET through hydrogen bond formation/activation to facilitate this reaction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011