Ch.B. Tsvetanov

Ultraviolet-induced crosslinking of solid poly(ethylene oxide)

Poly(ethylene oxide) (PEO) of molecular weight from 200,000 to 2,000,000 was crosslinked by exposure to ultraviolet radiation with a high-pressure 150 W mercury lamp. Photochemical crosslinking of PEO proceeds in the presence of photoinitiators such as benzophenone and acetophenone which act as a hydrogen-abstracting agent. Gel fraction yield exceeds 90%, and the number-average molecular weight between crosslinks determined by equilibrium swelling in chloroform varies from 3,000 to 100,000. The degree of crosslinking can be controlled by changing the irradiation temperature.

Thermoassociative block copolymers of poly(N-isopropylacrylamide) and poly(propylene oxide)

Abstract At the present time, the thermoassociative block copolymers usually consist of a favorable soluble fragment and a temperature‐responsive one. In this study, the ceric ion redox method and the atom transfer radical polymerization (ATRP) were applied to synthesize novel thermally sensitive ABA block copolymers of poly(N‐isopropylacrylamide) (PNIPAAm) and polypropylene glycol (PPO) by using PPO with molecular weight of 2000 g/mol as a precursor. Due to the different lower critical solution temperatures (LCST) of PNIPAAm and PPO, we were able to obtain thermoassociative block copolymers, which have PNIPAAm segments with higher transition temperature and PPO segments of lower transition temperature. As a result, our copolymers exhibited a two‐stage phase transition in aqueous solution. This attractive behavior offers an opportunity to obtain nanoparticles or microaggregates. The aggregation of the block copolymers in aqueous solutions was monitored by using the dye technique. The size of the copolymer nanoaggregates was determined with the aid of the dynamic light scattering (DLS). The particles obtained from aqueous solution were visualized by means of scanning force microscopy (SFM) and scanning electron microscopy (SEM) methods.

Synthesis and polymerization of novel oxirane bearing an aliphatic double chain moiety

Abstract A novel epoxide monomer, 1,3-didodecyloxy-2-glycidyl-glycerol (DDGG), bearing an aliphatic double chain was prepared. DDGG was anionically homopolymerized and block copolymerized with ethylene oxide (EO). The polymers were characterized by 1 H nuclear magnetic resonance spectroscopy, gel permeation chromatography, vapor pressure osmometry, and differential scanning calorimetry. The decrease in transmittance of the aqueous solutions of the copolymer, denoted EO 115 DDGG 2 , with raising temperature and/or concentration was attributed to the presence of aggregates. An anomalous increase in the transmittance at 35°C was noted as well. A dye solubilization method was used to demonstrate the ability of EO 115 DDGG 2 to associate in aqueous solution. The associative interactions gave rise to a sharp increase in viscosity.

Preparation of pH-sensitive networks by ultraviolet induced crosslinking of poly(ethylene oxide)-poly(2-vinylpyridine) blends

Blends of poly(ethylene oxide) of molecular weight 1 000 000 and poly(2-vinylpyridine) of molecular weight 3 00 000 were crossinleked by exposure to ultraviolet radiation at 25, 75 and 115°C in the presence of benzophenone as a hydrogen-abstracting agent. The crosslinking efficiency as well as the pH sensitivitystrongly depend on the blend composition, the concentration of benzophenone and the irradiation temperature. The equilibrium swelling behavior of the mixed networks in aqueous solution was studied as a function of pH at constant ionic strength. A more pronounced swelling transition upon pH changes from 2 to 7 is observed at greater content of P2VP in the bnetworks. The addition of poly(propylene oxide) to the P2VP-PEO networks leads to enchanced mechanical strength and to much greater swelling dependence on pH, due to the hydrophobic and elastic nature of PPO.

Investigation of oligomethylmethacrylate-lithium and its donor-acceptor complexes with organometal compounds by means of infrared spectroscopy☆

Abstract Anionic polymerization of methyl methacrylate in THF and toluene at −70° was studied by means of i.r. spectroscopy. The reaction mixtures show characteristic absorption at 1600–1750 cm−1 caused by the stretching vibrations of the propagating centre as well as by the groups obtained in side reactions (metal splitting, addition to a carbonyl group, cyclization). Sec-butyllithium is more effective in THF than n-butyllithium, ethyllithium and fluorenyllithium. When polymerizing in toluene, small amounts of a donor (THF or HMPT) cause formation of active centres similar to those in pure THF. The “living” ends of oligo-MMA form 1:1 complexes with electron acceptors such as trialkylaluminium and triethylboron.

DNA encapsulation via nanotemplates from cationic block copolymer micelles

We report on a method for the encapsulation of DNA into polymeric nanocapsules. The encapsulation procedure involves three steps: formation of polyplexes between DNA and cationic polymeric micelles; coating of polyplexes with a cross-linked shell; dissociation of polyplexes. In the first step copolymer micelles were obtained by self-assembly of an amphiphilic polystyrene-b-poly(quaternized 2-vinylpyridine) (PS-b-PQ2VP) block copolymer in aqueous solution. Nanosized polyplexes (Dh = 190 nm and ζ = −21 mV) were formed upon the addition of DNA to the micellar solution at a phosphate/amine group ratio of 3 : 1. In the second step the complexes were coated with a cross-linked shell formed by seeded radical polymerization of N-isopropylacrylamide which resulted in a slight shift of ζ potential of the particles to a less negative value. The dissociation of polyplexes and removal of PS-b-PQ2VP were achieved by addition of salt and solvent exchange. Following rehydration viadialysis against water, polymeric nanocapsules with entrapped DNA were obtained. The nanocapsules were visualized by transmission electron microscopy; they exhibited smaller dimensions compared to the initial polyplexes and ζ potential very close to that of the pristine DNA.

Investigation by means of I.R. spectroscopy of styrene oligomers containing acrylonitrile active ends

Abstract The successive addition of acrylonitrile (AN) portions to oligostyrene anionic ends was studied by means of i.r. spectroscopy. The attachment of AN to the oligostyrene active centre took place mainly across the double bond of the monomer accompanied by formation of the carbanion CHCN with characteristic absorption band at 2030–2040 cm−1. Comparatively high temperatures of the experiments (above −40°) provided conditions for side reactions in the polymerization. The latter were checked spectrophotometrically: cyclization according to Ziegler-Thorpe and formation of anionic centre and proton transfer reaction between this centre and AN causing the formation of enaminonitrile structure. By study of model compounds, the band at 2110–2130 cm−1 was assigned to the group.

Graft copolymers composed of high molecular weight poly(ethylene oxide) backbone and poly(N-isopropylacrlylamide) side chains and their thermoassociating properties

Novel, water-soluble thermoassociative graft copolymers based on high molecular weight (HMW) poly(ethylene oxide-co-glycidol) backbone and relatively short grafts of poly-N-isopropyl acrylamide (NIPAAm) were prepared. The copolymer precursors with two architectures (block and graft) were synthesized using Ca-amide-alkoxide initiators. The OH groups in the copolymer precursors have been utilized for grafting NIPAAm using ceric ion (Ce 4+ ) redox initiation. The idea was to imprint the smart properties of PNIPAAm grafts into common HMW poly(ethylene oxide). The sensitive moieties undergo reversible association transitions by changing the temperature of dilute and semidilute aqueous solutions of the copolymers. Associative properties were studied by viscosity and rheology measurements. Two types of interactions, induced by heating, depending on the copolymer concentration namely intra- and intermolecular association were observed.

Interactions of ligands with lithium picrate in dioxane

The interaction of several cyclic ethers, poly(ethylene glycol dimethyl ether)s, nitriles, esters and ketones with lithium picrate in dioxane at 25°C was investigated by means of a competition method using cross-linked poly(ethylene oxide) (PEO) as the insoluble ligand. The Langmuir-Klotz isotherm of LiPi binding to the PEO gel gave an apparent binding constant, K′, as a function of the concentration of added ligand. Plots of 1/K′ vs. the ligand concentration then yielded the formation constant, KL, of the ligand complex with LiPi. The results demonstrate that in dioxane one ligand molecule binds to LiPi. While there is some dependence on the Gutmann donicity number of the respective ligands, steric factors often play a dominant role in determining the value of KL. The results were compared with complexation data obtained on similar systems but by different methods.

Influence of the polyether chain on the dissociation of “living” polymers obtained in the anionic polymerization of ethylene oxide☆

Abstract Conductivity studies were carried out on block copolymers of composition K + , −(EO) n - α MeSt-(St) x - α MeSt-(EO) n − , K + , where n = 1–5 or 20, in THF. The conductivity of the solutions exhibits a strong tendency to increase as n is raised up to 3, and to change only slightly on further extension of the polyether chain. An equilibrium between ion pairs and free ions is established in the solution only when n is 1 or 2; for n ⩾ 3, ion triplets are formed also. The dissociation constants for the ion pairs and the ion triplets and their temperature dependence were determined. The formation of ion triplets, as well as the changes in dissociation of the ion species on extending the polyether block, are explained by solvation of the counter-ion by the polyether chain.