Like most of the materials used by humans, polymeric materials are proposed in the literature and occasionally exploited clinically, as such, as devices or as part of devices, by surgeons, dentists, and pharmacists to treat traumata and diseases. Applications have in common the fact that polymers function in contact with animal and human cells, tissues, and/or organs. More recently, people have realized that polymers that are used as plastics in packaging, as colloidal suspension in paints, and under many other forms in the environment, are also in contact with living systems and raise problems related to sustainability, delivery of chemicals or pollutants, and elimination of wastes. These problems are basically comparable to those found in therapy. Last but not least, biotechnology and renewable resources are regarded as attractive sources of polymers. In all cases, water, ions, biopolymers, cells, and tissues are involved. Polymer scientists, therapists, biologists, and ecologists should thus use the same terminology to reflect similar properties, phenomena, and mechanisms. Of particular interest is the domain of the so-called "degradable or biodegradable polymers" that are aimed at providing materials with specific time-limited applications in medicine and in the environment where the respect of living systems, the elimination, and/or the bio-recycling are mandatory, at least ideally.

No abstract

The use of polymer-supported reactants in organic synthesis is currently of considerable interest, especially in the context of combinatorial syntheses. To carry out successfully reactions using polymer-supported reactants it is important to be aware of what takes place inside the beads. Examples are presented in this article which show that compared to the analogous homogeneous reaction systems, polymer-supported reactions can show substrate selectivity, be slower or faster, follow a different reaction course, or give a significantly different stereochemical result.

No abstract

Styrene copolymers containing various amounts of a novel comonomer bearing a pendant furan ring were synthesized and characterized before being submitted to Diels−Alder reactions with either a monomaleimide or a bismaleimide. Spectroscopic evidence, supported by data from model compounds, indicated that the resulting linear and cross-linked products contained extensive percentages of adduct structures formed from the furan moieties. Both types of materials were then heated in a solvent containing a large excess of 2-methylfuran in order to induce the retro-Diels−Alder and the coupling of the released maleimides with the furanic additive. The reaction proceeded as expected and the original copolymers could be recovered from the treatment. The interest in the general strategy reported here resides in the possibility of recycling cross-linked polymers by a simple thermal treatment conducted in the presence of a suitable trap.

The ring-opening-metathesis polymerization (ROMP) of strained cyclic olefins has been studied extensively, [1,2] especially since Grubbs' catalyst 1 [3] and the more recently introduced "second-generation" Grubbs' catalyst 2, [4] both of which are tolerant of many functional groups, became commercially available. The ROMP of strained cyclic olefins is mainly enthalpy-driven.A relatively new type of ring-opening polymerization (ROP) exploits the well-known equilibria between cyclic oligomers and polymers [5][6][7][8][9][10] (Scheme 1). At high dilutions the equilibria lie heavily in favor of the cyclic oligomers, whereas at high concentrations they lie heavily in favor of the polymers. Thus, if one or more cyclic oligomers are taken neat as starting materials, and equilibrium is established, polymer synthesis results. The cyclic oligomers used as the feedstock are generally not strained, so the enthalpy change on polymerization is minimal. This type of polymerization is, therefore, mainly entropically driven, and so the process can be abbreviated ED-ROP. As a neat mixture the cyclic oligomers have relatively little translational entropy and the rings occupy limited conformations; conformational flexibility increases greatly upon conversion into polymers. Since ED-ROP is an equilibration process the polydispersity of the polymer is expected to have a value of 2.0. ED-ROP has been investigated for various types of macrocyclic oligomers, [11,12] but not for macrocyclic olefins.Examples in which large, unstrained macrocycles (> 13 ring atoms) have been subjected to ROMP are rare, [2,[13][14][15] and there only appear to be two examples in which the polymer has been formed in high yield, isolated, and characterized. The first was the ROMP of the 14-membered cyclic ether 3 in the presence of the catalyst 1. This gave a polymer with M n 65 900 (M n = number-average molar mass).[2] The second example was the ROMP of ambrettolide, an unsaturated macrolide with 17 ring atoms.[13] A neat sample of this compound was polymerized in the presence of a catalyst prepared from tungsten hexachloride and tetramethyltin to give a polymer in 95 % yield with M n 95 000. Herein we show that when appropriate reaction conditions are used, unstrained macrocyclic olefins with up to 84, and possibly even more, ring atoms readily undergo entropically driven ROMP (ED-ROMP). Given that Grubbs and co-workers have recently reported an efficient method for the synthesis of very large macrocyclic olefins, [16] and that olefin-containing polymers are easily hydrogenated in the presence of decomposed metathesis catalysts, [17,18] ED-ROMP of large macrocyclic olefins is of more than theoretical interest.In the present work the monomers 4, 5, and 6, which have 21-, 28-, and 38-membered rings, respectively, were prepared by ring-closing metathesis (RCM). [2,[19][20][21] The monomer 4 had been synthesized previously by RCM in 70 % yield.[20] In a similar procedure, the a,w-diolefinic ester 7 and Grubbs' catalyst 1 (3 mol %) were slowly added over 24 h to ...

The useful dynamic range of an image in the diffraction limited regime is usually limited by speckles caused by residual phase errors in the optical system forming the image. The technique of speckle decorrelation involves introducing many independent realizations of additional phase error into a wavefront during one speckle lifetime, changing the instantaneous speckle pattern. A commonly held assumption is that this results in the speckles being 'moved around' at the rate at which the additional phase screens are applied. The intention of this exercise is to smooth the speckles out into a more uniform background distribution during their persistence time, thereby enabling companion detection around bright stars to be photon noise limited rather than speckle-limited. We demonstrate analytically why this does not occur, and confirm this result with numerical simulations. We show that the original speckles must persist, and that the technique of speckle decorrelation merely adds more noise to the original speckle noise, thereby degrading the dynamic range of the image.

No abstract