Monte Carlo investigations of dense copolymer systems, 1. Pivot algorithm and pair distribution function

ABA triblock copolymers (the solvent being a θ-solvent for B and an athermal one for A) as well as their constituent homopolymers were investigated by means of Monte Carlo simulations in a 5-choice cubic lattice. The polymer concentration (represented by the fraction ϕ of sites occupied by the polymer) varied from 0 (isolated chain) to 0.80. An investigation of the usefulness of the pivot algorithm in dense systems resulted in reasonable acceptance fractions up to a volume fraction of 0.12 (athermal chain and ABA copolymer) and 0.05 (θ-chain). For larger volume fractions only the chain-ends remain mobile. The overlap concentration of the polymers defined by several quantities was approximately 0.07 (athermal chain), 0.12 (θ-chain) and 0.09 (ABA copolymer). At a polymer fraction of 0.32, the chains had the same number of inter- and intramolecular contacts on average. At higher concentrations, the behaviour of the chains was primarily determined by intermolecular interactions. Contrary to isolated pairs, the pair distribution function g(r) of two athermal chains in a (cubic) box exceeded unity at intermediate distances because of the influence of the finite size of the box. The larger the size of the box, the smaller the (positive) deviation was. In the limit of r → 0 the pair distribution function g(r) – being smaller than unity – increased with increasing concentration while the maximum at intermediate distances simultaneously decreased. Ultimately, at the highest concentration, the pair distribution function resembled that of isolated θ-chains. The concentration dependence of the θ-pair distribution function itself, however, is negligible. At low concentrations the pair distribution function of the ABA triblock copolymer behaved like that of an athermal chain (up to ϕ ≈ 0.40) where characteristic oscillations around 1 developed. This might be taken as indicative of the formation of a lamellar phase.     

New polyesters with photosensitive backbones based on the triazene unit

The synthesis of photosensitive polymers that contain the triazeno group (Ph NN NR₂) as a sensitive unit in their backbone is reported. The synthesis pathway consists of difunctional alcohols incorporating the photosensitive unit that react with diacyl derivatives to give the respective polyesters. Upon irradiation, the photosensitive chromophores both in monomeric and polymeric surroundings are cleaved fast and irreversibly, as shown in photolytic studies and their kinetic evaluations. Thermogravimetrical investigations indicate the loss of nitrogen being the initial thermal decomposition step and exhibit stabilities sufficient for the requirements in applications as photoresists. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 3017–3025, 1997     

Polyfunctional polyisobutenes as building blocks for amphiphilic graft polymers

Polyfunctional polyisobutenes (PIB) have been synthesized by cationic copolymerization of isobutene and chloromethylstyrene. Their potential applications with focus on their use as macroinitiators (MI) for oxazoline polymerization were discussed. The “grafting from” reaction led to tailor-made graft copolymers with various backbones and adjustable graft arm length. Graft copolymers with hydrophobic PIB backbone and poly(2-methyloxazoline) graft arms have unusual viscosity properties due to their amphiphilic character and show aggregate formation.     

Pseudostationary Pulsed Laser Polymerization: A Simple Method for the Direct Determination of Axial Dispersion as Occurring in Size Exclusion Chromatography

Theoretical distribution curves were calculated for several values of pulse separation t₀ and concentrations of initiating radicals ρ formed by each pulse for both types of termination (disproportionation and combination). The absolute and relative peak widths of the additional peaks for the number, molar mass, and hyperdistribution were determined and compared. In all cases, the peak widths of these different distributions became the same with increasing values of C = k_tρt₀ and/or L₀ = k_p[M]t₀, with [M] = monomer concentration and k_t and k_p are the rate constants of termination and propagation, respectively. This is similar to the behavior of Poisson distributions if only the degrees of polymerization are fairly high (P̄_n ≥ 50). The analogy is further supported by the finding that under the same conditions the peak widths themselves approach the theoretical ones for Poisson distributions. Thus, the fulfilment of these two criteria suggests that the various peaks in multimodal distributions should be treated in the same formal way as Poissonian peaks although they clearly originate from a superposition of adjacent Poissonian peaks of different amplitude and, on the whole have a peak width somewhat in excess of the theoretical one of true Poisson peaks of the same P̄_n. The point of inflection can be used as a measure of L₀ without reservation only if the first criterion is not fulfilled. The influence of axial dispersion on the location of the extrema was calculated by use of standard deviations σ_ad,k (0.05 and 0.025) to give “experimental curves” for which a pronounced increase in the absolute and relative peak widths was observed. Based on an assumed additivity of the peak variances, a simple procedure was tested for the direct determination of σ_ad,k. The accuracy increased with increasing peak chain lengths and C values for those values determined from the first peak. The values determined from the second peak in the chain length region between 400–700 were closest to the input value.

The points of inflection on the low and high molecular weight side of the additional first three peaks are given as a function of the peak maximum. The lower and upper lines were calculated with Equation ( 11 ) and ( 12 ), respectively.     

Oxetane photopolymerization—A system with low volume shrinkage

Several mono-, bi- and trifunctional oxetanes were synthesized and polymerized in bulk and in solution cationically. Selected photoinitiators have been applied. It was found that sulfonium salts are very efficient due to good solubility, almost no discolouration of the product and storage stability in the monomer in the absence of light. The conversion was determined by quantitative IR-spectroscopy. Conversion between 75% and 85% was found in all cases. The shrinkage during polymerization was much lower than for vinyl monomers. No inhibition by oxygen was recognized. Monomer layers thicker than 5.5 mm could be polymerized. The products are transparent and almost colourless. The glass transition temperature of the crosslinked polymers was above the temperature of a human body.     

Fourier-transformation of model chain pair configurations and their analysis in fourier-space

Light scattering experiments may be used to determine a lot of quantities characteristic of a polymer. Numeric analysis of calculated scattering intensities of ensembles of polymer model chains in dilute solution (taking into account intramolecular as well as intermolecular interferences) according to the familiar methods and comparison of the results to the respective quantities evaluated directly in real space allows a check of the liability of data extracted from light scattering experiments. It turned out that the mean-square radius of gyration as well as the excluded volume (which may be regarded as the second moment of the pair distribution function) are nearly perfectly reproduced. Higher moments, however, are subject to some error which is due to the characteristic changes in size, shape and orientation of polymers caused by intermolecular interaction which are not accounted for in the theories forming the basis of the analysis of light scattering data.     

Biochemometry-Based Discovery of Phenylpropanoids from Azadirachta indica Fruits as Inhibitors of In Vitro Osteoclast Formation

(1) Background: Inhibition of osteoclast differentiation is the key approach in treating osteoporosis. However, using state-of-the-art treatments such as bisphosphonates and estrogen-based therapy is usually accompanied by many side effects. As opposed to this, the use of natural products as an osteoporotic remedy delivers promising outcomes with minimal side effects. (2) Methods: In the present study, we implemented a biochemometric workflow comprising (i) chemometric approaches using NMR and mass spectrometry and (ii) cell biological approaches using an osteoclast cytochemical marker (TRAP). The workflow serves as a screening tool to pursue potential in vitro osteoclast inhibitors. (3) Results: The workflow allowed for the selective isolation of two phenylpropanoids (coniferyl alcohol and sinapyl alcohol) from the fruits of neem tree (Azadirachta indica). These two isolated phenylpropanoids showed a very promising dose-dependent inhibition of osteoclast differentiation with negligible effects in terms of cell viability. (4) Conclusion: The presented workflow is an effective tool in the discovery of potential candidates for osteoclast inhibition from complex extracts. The used biochemometric approach saves time, effort and costs while delivering precise hints to selectively isolate bioactive constituents.     

Syntheses and reactions of azo-group containing networks

Azo monomers of the general structure have been terpolymerized with styrene and butadiene (I) to form azo-group containing rubber (SBR). Copolymerization of butadiene and acrylonitrile with 3 yields azo group containing nitrile rubber (II). Networks are formed, if the copolymerization of butadiene and acrylonitrile is carried out in the presence of divinylbenzene (DVB) (III) or in the presence of DVB and 3 (IV). Thermolysis or photolysis of the polymeric azo initiators (I, II) yields networks without addition of a low molar-mass crosslinking agent. Heating or irradiation of the azo-containing pre-network (IV) increases the network density. Heating a mixture of (II) or (IV) with natural rubber yields polymer blends which are stabilized by covalent links between the different polymers. The network characterization was carried out by swelling experiments. The incorporation of the azo monomers into the azo copolymers is almost ideal under “cold rubber” conditions, meaning that the composition of the products does not change with conversion.