Acta Polymerica最新文献

The degree of branching (DB) as well as the ANB (average number of branches) of polymers obtained from polycondensation of AB₂ and AB_m-type monomers in general is analyzed in detail on the basis of kinetic considerations. Various possibilities for the enhancement of the DB of hyperbranched polymers beyond the limit for a random AB_m polycondensation (0.5 for AB₂, 0.44 for AB₃) are considered, i.e., (i) enhanced reactivity of linear vs. terminal units; (ii) polymerization of prefabricated, perfect dendritic units and (iii) the slow addition technique. For AB₂ monomers, the formation of terminal (T), linear (L) and perfect dendritic (D) units in the course of the random one-pot polymerization is calculated. Furthermore, the evolution of the DB and ANB with conversion is calculated for AB_m monomers. It is shown that the definition of the DB for AB₂ systems (DB = 2D/(2D+L)) derived in the first paper on the DB (Acta Polym. 1997, 48, 30) in the case of a random one-pot polycondensation exhibits the same conversion dependence as the branching parameter α = 1/2P_A, defined by Flory for AB₂ polycondensations. The dilution principle is introduced, which is based on slow addition of AB_m monomers to a core molecule B_f or an AB_m-type hyperbranched molecule with an arbitrary DB. It is shown that the maximum DB value obtainable from such a dilution/slow addition process is 0.67 for AB₂ systems. Thus, the DB is considerably higher than in the case of a random one-step polymerization (0.5). The DB for AB_m-type hyperbranched polymers prepared according to the dilution principle has been calculated to be m/(2m–1).

Liquid crystal poly[oxy(1,2-butene)oxycarbonyl-1,4-phenyleneoxycarbonyl-1,4-phenylenecarbonyloxy-1,4-phenylenecarbonyl], PTOBEE, has been synthesized in a different way than the previously reported nematic and it has been obtained as cholesteric, although synthesized from a racemic mixture of the 1,2-butanediol involved. Stereoselective solubilization of one enantiomer during its decantation in toluene, at the reaction temperature, explains why this chiral polymer phase appears. Evidence of this could be precisely obtained when a white solid recrystallized within the same toluene after the filtration of the polymer, being identified as (—) PTOBEE with a value of [α] = −2.33 [0.0056 mol/1, toluene].

The uniform structure of cellulose, combined with its hydrophilicity, biocompatibility, chirality, and structure-forming capacity, as well as its broad synthetic potential, make the design of cellulose-based advanced materials a challenge for polysac-charide research. The synthesis of new cellulose products with regiocontrolled pattern of functionalization and of well defined supramolecular nanoscale architectures is a highlight in recent cellulose chemistry.

The present feature will discuss regioselective functionalization of cellulose and cellulose derivatives with respect to reactivity of the polymer depending on molecular and supramolecular structure. Experimental examples of functionalization via ester intermediates of controlled stability as well as via protecting or activating ether groups are described and the efficiency of the reaction routes and procedures is evaluated.

Moreover, problems and progress in techniques of product analysis and structure–property relationships as well as potential applications of regioselectively functionalized cellulosics are presented together with a discussion of possible future developments in cellulose chemistry.

Steady-state photolysis at 365 nm has been employed to carry out a structure—reactivity investigation of the 2,6-dihalogen derivatives of p-nitroaniline (pNA). Detailed studies of the spectroscopy of these molecules were accomplished. Photoreduction behavior of the 2,6-dihalogen derivatives of p-nitroaniline in the presence of N,N-dimethylaniline (DMA) has been analyzed. The efficiency of these compounds as photoinitiators was studied by following the polymerization kinetics of the lauryl acrylate (LA) monomer by differential scanning photo-calorimetry (Photo-DSC) under aerobic and anaerobic conditions. Using size exclusion chromatography (SEC) analysis of the polymers their number-average molecular weights, the number-average degree of polymerization, and the length of the kinetic chain were determined and information on the nature of the end-chain groups and the chain-transfer behavior of these photoinitiators systems were obtained. The polymerization activity of the 2,6-dihalogen derivatives of pNA proved to be higher than those obtained with conventional aromatic ketone photoinitiators.

A soluble copolymer formed by thiophene and 3-octylthiophene has been obtained by using nickel catalyst polymerization. The nearly alternated structure allows the formation of a highly conjugated system although the copolymer microstructure is not regioregular. ¹H-NMR, GPC characterization and pyrolysis experiments support the alternated structure of the copolymer. Moreover, theoretical calculations indicate that the average angle between adjacent monomeric units has been reduced, in agreement with the observed shift towards lower energy of the electronic spectra of this alternated structure.

In situ conductivity measurements on the low band-gap polymer, poly-4-dicyanomethylene-4H-cyclopenta[2,1-b;3,4,b′]dithiophene have provided both p- and n-type conductivities as a function of the potential. A minimum in the conductivity, at a potential between the formal potentials for p- and n-doping, yields an intrinsic conductivity of 1.0 × 10⁻⁸ S cm⁻¹, consistent with the optical band-gap of ∼0.8 eV.

Important advances have been made in controlled radical polymerization, particularly in the application of the new technique for the preparation of novel block copolymers. CO-copolymers are becoming increasingly popular. Increasing research efforts are also being devoted to discrete supermolecular structures as well as the preparation of nanometer-size noble metal clusters in polymer matrices. MALDI-TOF continues its triumph in synthetic polymer chemistry.