Block Oligomers Research Articles

Temperature dependence of the thermophysical characteristics of oligomers and cross-linked polymers based on them

The results of an analysis of the experimental values of the thermophysical characteristics of oligomers which are distinguished by a long and flexible oligomer block and cross-linked polymers based on them in the temperature range 90–373°K are presented.

Investigation of the Structure and Properties of Crosslinked Polymers Based on Oligocarbonate Methacrylates in Film Formation

Abstract The effect of structure and flexibility of an oligomeric block of oligocarbonate methacrylates (OCM) as well as of various modifiers on the coating formation, the structure, and properties of crosslinked polymers formed under conditions of adhesive interaction has been studied. Reactivity and conversion of OCM during polymerization increase in coatings formed from oligomers with short and rigid blocks, capable of forming on the substrate a network from the ordered anisodiametric structural elements with high adhesive properties, as distinct from the globular structures appearing in the formation of the network from the oligomers with more flexible blocks. The tendencies observed have been explained with the aid of a mechanism of network formation relating the morphology of the resultant crosslinked polymers to the flexibility (conformation) of the chains of the branched polymer obtained in the initial stage of polymerization and representing a matrix of the corresponding crosslinked structures. The mode of action of modifiers is markedly dependent on the nature and flexibility of the oligomeric block.

Biodegradable Cellulose Block Copolymers

Abstract Biodegradable polymers can be prepared by the incorporation of cellulose oligomeric blocks into the polymer backbone. Block copolymers prepared by reacting equimolar amounts of depolymerized cellulose triacetate (CTA) with MDI or TDI and block terpolymers prepared by reacting the CTA oligomer with poly (propylene glycol) capped with MDI or TDI were deacetylated and incubated with cellulysin at pH 5 and 50 °C. The intrinsic viscosity of each copolymer decreased substantially even after 10 hr incubation time. There was negligible acid hydrolysis. Cellulose showed a smaller decrease at the same incubation time-probably due to its lower accessibility. Methods for incorporating cellulose oligomeric blocks and other biodegradable blocks into different types of polymers are discussed.

Synthese d'oligomeres a sequences perfluorees

The synthesis of block oligomers containing a perfluoroalkyl sequence has been achieved by an anionic technique. For this purpose, a monolithiated oligomer (e.g. oligoisoprenyl or oligostyryl) is deactivated by a perfluoro compound “R 1X”. The optimisation of the coupling over the metalhalogen exchange reaction is studied. 19F NMR analyses of the block oligomers allow identification of the reactive site in the “R 1X” compound.

Dielectric relaxation processes in three-dimensional polymers formed from oligo-ester acrylates

Two regions of dipole group losses are characteristic of polymers formed from oligo-ester acrylates (OEA) with small oligomeric blocks; a region of dipole segmental losses is not observed because of the onset of thermal degradation processes and because of the sharp increase in losses due to electrical conductivity with a further increase in temperature. A region of dipole segmental losses is observed in OEA polymers as the dimensions of the oligomeric blocks are increased, the region then being shifted towards lower temperatures.

Temperature dependence of the mechanical properties of network polymers based on oligo-oxyethylene-urethane methacrylates

The glass transition temperature T g was determined on the basis of the thermomechanical curves for amorphous network polymers based on oligo(oxyethylene-urethane methacrylates) of different molecular weight. It was found that T g is proportional to the reciprocal of the molecular weight of the original oligomer and ceases to be a function of block length if the oligomer has a molecular weight of >2000. The limiting values obtained for the stress and the elastic modulus of the network crystalline polymers rise as the length of the oligomeric block increases, and in the case of n⩾60 these values become practically constant, which is due to the formation of physical crosslinks increasing the overall network density. For the same polymers in the high elastic state the limiting stress values fall as the block length increases, and at n⩾60 become virtually independent of n.

A study of the dynamic, mechanical properties of three-dimensional polymers of oligoester acrylates over a wide temperature range

The effect of the chemical structure of three-dimensional polymers of oligoester acrylates on the dynamic, mechanical properties is investigated over a wide temperature range. It is observed that the properties of oligoester acrylates depend on the nature and dimensions of the oligomer block and on the nature of the main chains. The observed relaxational transitions can be explained by the movement of various molecular groups.

Effect of structurization in oligomers on the temperature characteristics of their thermophysical properties

Results are shown of a study concerning the effect of oligomer block length and flexibility on the temperature characteristics of thermophysical properties, on the internal stresses, and on the structural transformations of oligomer systems.

Some new trends in the chemistry of polysaccharides.

The data on the fine structure of polysaccharides should form the basis for understanding on a molecular level the biological role played by carbohydrate systems, but the methods of structural chemistry are not always sufficient. The most general approach for determination of the primary structure of polysaccharides might be the development of standard methods, which involve specific cleavage to oligosaccharidic blocks and their subsequent analysis. The possibilities of application of some reaction for selective modification of monosaccharide units in a polymer chain with subsequent hydrolysis of glycosidie linkages in the modified unit will be discussed. The application of mass spectrometry for the determination of monosaccharide units in oligosaccharides provides possibilities for effective structural analysis of oligomeric blocks using very small amounts of material. Finally, the development of methods for the synthesis of polysaccharides is discussed.

The radiothermoluminescence study of the low-temperature molecular vibrations in steric oligo-acrylate ester polymers

Radiothermoluminescence (RTL) was used to study the low-temperature molecular vibrations in steric oligo-acrylate ester polymers. These processes are dependent on the length and chemical composition of the oligomeric blocks, and also the type of main chains present. The RTL data correlated with those obtained by NMR. The activation energies of the individual molecular motions were calculated by various methods. It was found possible to evaluate the distribution parameters of the relaxation periods from the RTL data.

Cooperative non-enzymatic base recognition and the stability of the G-U wobble pair.

INVESTIGATIONS of paired complementary polynucleotides containing occasional mismatched bases have indicated that these bases are looped out of the double helical regions thereby making the helix unstable. But in a system containing mismatched G and U bases, no such definite conclusion could be drawn1–4—stoichiometry and Tm values of such complexes can be interpreted in terms of the formation of a G-U wobble pair4. Recently the complexes formed by self-complementary block oligomers interrupted by mismatched bases, for example, AnXUn (X=G, C and U), have been studied in detail5·6. It seems that stacking interactions play a much more dominant role in the stabilization of double helices than was previously thought. Thus the extent of looping out and of non-Watson-Crick base pairing can hardly be assessed independently.

Polymerization kinetics of oligoesteracrylates at high degrees of conversion

IF [1, 2] IT was reported tha t the polymerization kinetics of oligoesteracrylates t (OEA) is a diffusion-controlled process even at low degrees of conversion (G< 1%). At these G the influence of the physical properties (viscosity) of the reaction system on the apparent reactivity of OEA consists mainly in the recombination of polymer radicals, the reaction which has the smallest true activation energy. This was confirmed in [3] where the growth rate and chain termination constants of OEA were studied as functions of G. I t is very interesting to s tudy the relation between the apparent reactivity and structure of OEA at high G, where the polymerization product reaches the consistency of a solid (viscosity above 10 la poise). Under these conditions not only the chain termination but also the growth is a diffusion-controlled process. Even for small molecules in media approximating the consistency of a solid, D, the diffusion coefficient is 10 -11 cm2/sec [4]. This means tha t any bimolecular process will be restricted by diffusion if the rate constant of this process exceeds 10 -3 1./mole. sec. For methacrylates at G 0 (3,5-7), kp~--2-7×10 2 1./mole.sec and ko~_10v 1./mole.sec. Of course, these processes are completely controlled by diffusion at high G. Under these conditions the apparent reactivity of an oligomer will depend, not on the chemical features of the structure, but on the mobility and steric accessibility of the reaction eentres (radicals and double bonds). Thus the -C-C-chain is identical in all OEA. I t can be assumed that the mobility and steric accessibility of the reaction centres of different OEA will be determined by the structure of the transverse (oligomeric) block. The aim of the present work was to establish the relation between the OEA structure and kinetic features of the polymerization process at high G. The experiments used the thermometric procedure already described in [8,14].