Search results for "polymers"
showing 10 items of 3567 documents
Cyclodextrins in polymer synthesis: polymerization of methyl methacrylate under atom-transfer conditions (ATRP) in aqueous solution
2000
Host guest complexes of methyl methacrylate (MMA) and randomly methylated β-cyclodextrin (m-β-CD, 1 a) were polymerized in aqueous medium using atom-transfer radical polymerization. Ethyl 2-bromoisobutyrate (EBIB) was used as an initiator, copper(I) bromide as the catalyst, and bipyridine (bipy) or 4,4′-di-(5-nonyl)-2,2´-bipyridine (dNbipy) as ligands. The unthreading of m-β-CD during the polymerization led to water-insoluble poly(methyl methacrylate) (PMMA). It was found that using dNbipy resulted in higher monomer conversion than using bipy as the ligand under similar conditions. Furthermore, it is shown that the polymerization of MMA under these conditions has a living character. The pol…
Group transfer and anionic polymerization: A critical comparison
1990
The mechanism of group transfer polymerization (GTP) of methacrylates in THF is investigated by using data on kinetics of homo- and copolymerization, polymer microstructure and molecular weight distribution. By comparison with corresponding data on anionic polymerization it is concluded that the mechanisms of monomer addition to the active chain end is very similar for both anionic and group transfer polymerization and that GTP is ionic in character. On the other hand, GTP uniquely is characterized by the existence of a catalyst exchange equilibrium. The position of this equilibrium determines the rates of polymerization, and the dynamics determine the molecular weight distribution.
Preparation of Hyperbranched Polyacrylates by Atom Transfer Radical Polymerization. 2. Kinetics and Mechanism of Chain Growth for the Self-Condensing…
1997
The self-condensing vinyl polymerization (SCVP) of 2-((2-bromopropionyl)oxy)ethyl acrylate (BPEA) has resulted in the formation of hyperbranched polyacrylates. The polymerization mechanism used to polymerize the BPEA was atom transfer radical polymerization (ATRP), a “living”/controlled radical polymerization. This paper details the study of the kinetics of polymerization and the growth of the macromolecule during the polymerization. The results obtained in the polymerization were compared to the theoretical predictions for SCVP. It was determined that the polymerization deviated from the ideal case, as a consequence of the establishment of a dynamic equilibrium in ATRP resulting in the add…
Polymerization of 3-alkylthiophenes with FeCl3
1992
We report on the mechanism of direct oxidation of 3-alkylthiophenes using ferric chloride (FeCl3) as the polymerization oxidant/catalyst to produce high molecular weight poly(3-alkylthiophenes) (P3ATs), conjugated polymers that have potential as electrically and optically active polymers. This study shows that the FeCl3 must exist in the solid state in the reaction mixture to be active as an oxidant in the polymerization of P3AT. A feasible polymerization mechanism for 3-alkylthiophene was developed on the basis of the crystal structure of FeCl3 and quantum chemical computations of thiophene derivatives. The polymerization is hypothesized to proceed through a radical mechanism rather than a…
Influence of hydrogen bonding on the viscoelastic properties of thermoreversible networks: analysis of the local complex dynamics
1995
Abstract The viscoelastic properties of thermoreversible polybutadiene networks in which junctions are formed by binary contacts between polar stickers (phenylurazole) are investigated by a dynamic mechanical spectroscopy within the frequency range 0.0079–79.5 Hz (0.05–500 rads−1). Time-temperature superposition is applicable in the terminal flow region and the glass transition regime, whereas thermorheologically complex behaviour is observed within the rubbery plateau region. For the terminal relaxation zone the polar stickers enhance the relaxation time and broaden the relaxation time spectrum. The thermorheologically complex behaviour within the rubbery plateau region results from the oc…
Preparative fractionation and characterization of polycarbonate/eugenol-siloxane copolymers
2002
Bisphenol-A polycarbonate/eugenol-siloxane copolymers were fractionated at the preparative scale by the continuous polymer fractionation (CPF) technique. It is the first example of copolymer fractionation by CPF. The distribution of siloxane species across the fractions was assessed for copolymers differing in initial siloxane concentration and block length. On- and off-line combinations of size exclusion chromatography and infrared spectroscopy were used to analyze chemical composition (CC) of the unfractionated samples across the molecular weight distribution enabling comparison with the fractions. A polycarbonate-siloxane copolymer containing 10 wt% of very short siloxane blocks (dp = 2)…
Ring-opening polymerization of lactides using heterobimetallic yttrocene complexes
1999
Structurally characterized, chiral heterobimetallic yttrocene derivatives Li[Y(η 5 :η 1 -C 5 R 4 Si-Me 2 NCH 2 CH 2 OMe) 2 ] (R = Me, H) have been shown to be active in the controlled ring-opening polymerization of L-lactide to give poly(L-lactide)s with high molecular weights and moderately narrow molecular weight distributions (M w /M n < 1.50). Both transesterification and racemization appear to be less prominent. 'H NMR spectroscopic tetrad analysis of copolymers prepared using a mixture of L- and D-lactide demonstrates the absence of any preference for one enantiomer during the polymerization.
Copolymerization of ethene with styrene using different methylalumoxane activated half-sandwich complexes
1997
Ethene was copolymerized with styrene using five different methylalumoxane (MAO) activated half-sandwich complexes of the general formula Me 2 Si(Cp)(N-R)MCl 2 , varying the substituents on the cyclopentadienyl ring and the substituent on the amide (Cp = tetramethylcyclopentadiene CBT, 1-indenyl IBT, 3-trimethylsilyl-1-indenyl SIBT, or fluorenyl FBZ, R = tert-butyl (complexes CBT,IBT, SIBT, FBZ ) or benzyl CAT), as well as the metal center ( M = Ti, except FBZ: M = Zr). Polymerization behavior was analyzed with respect to catalyst activity and polymerization kinetics, styrene incorporation, copolymer microstructure, and molecular weight. All complexes produced random poly(ethene-co-styrene)…
Synthesis of Polymer Colloids Using Polymerizable Surfactants
2001
A series of new anionic surfactants were prepared by reacting a polymerizable alcohol with either maleic or succinic anhydride, as well as sulfosuccinic anhydride, resulting in either bireactive or monoreactive surfactants, either carboxylated or sulfonated. The synthesis presented and the critical micelle concentration (CMC) determined. These compounds were engaged in the emulsion polymerization of styrene in batch and also in semi-batch seeded copolymerization producing core-shell latexes. The core was a polystyrene seed prepared using sodium dodecyl sulfate (SDS), thoroughly purified through ion exchange resins. The seed was then swollen overnight with a small amount of a 1 : 1 mixture o…
Cure monitoring of catalysed cyanate ester resins
2000
The cure behaviour of two bisphenol A-based cyanate ester resins, AroCy B10 and B30, catalysed by copper acetylacetonate and nonylphenol was studied. For this purpose, differential scanning calorimetric (DSC) and rheological measurements were carried out at temperatures between 130 and 170 °C. The cyanate conversion profiles are fitted with a second-order rate law in the kinetically controlled regime where a good time–temperature superposition is attained. However, it is necessary to add an empirical kinetic term to give a good description of the entire range of curing. Simultaneously, times to gelation and vitrification have been determined by dynamic rheological measurements over the same…