Search results for "polymerization"
showing 10 items of 1689 documents
Third virial coefficient for 4-arm and 6-arm star polymers
2008
We discuss the computation of the third virial coefficient in polymer systems, focusing on an additional contribution absent in the case of monoatomic fluids. We determine the interpenetration ratio and several quantities that involve the third virial coefficient for star polymers with 4 and 6 arms in the good-solvent regime, in the limit of a large degree of polymerization.
One-pot synthesis of poly(l-lactide) multi-arm star copolymers based on a polyester polyol macroinitiator
2013
Abstract Using a hyperbranched poly(glycolide) ( hb PGA) macroinitiator the synthesis of poly( l -lactide) (PLLA) multi-arm star polyesters has been achieved via a core-first approach. The star-shaped copolymers were prepared in a one-pot two-step process via Sn(Oct) 2 -catalyzed ring-opening polymerization (ROP) conducted in the melt. Complete conversion of the end groups of the hb polyglycolide polyester polyols is ensured by the reactive primary hydroxyl termini. By adjusting the monomer/initiator ratio a series of star copolymers with varying PLLA arm length has been obtained with molecular weights in the range of 1500 to 10,000 g/mol (SEC). The successful coupling of the PLLA arms to t…
PolyAT chemical denaturation in w/o microemulsion
2004
CD and UV spectroscopies have been used to investigate the effects caused by the addition of either strong acid-or base-containing microemulsions on the behaviour of the synthetic polynucleotide polyAT entrapped in the aqueous core of a cationic quaternary water-in-oil microemulsion (μE). The titrations were performed in the presence of variable concentrations of NaCl, in the range 0.00 to 0.60 M. In both cases, the primary effect was the reversible transition from B-double helix to random coil of the guest polynucleotide. However, in the microemulsive medium, the number of moles of protons (RH) and hydroxide ions (ROH) per mole of titrable sites are independent of the salt concentration bu…
"Dormant" secondary metal-alkyl complexes are not omnipresent.
2006
This theoretical study was inspired by the perpetual debate over the so-called “dormancy” of the active sites in propylene polymerization, i.e., a drop in their activity after a regioerror (2,1-insertion), which was reported to occur in many (although not all) catalytic systems. To explore the range of possible situations, we have selected two homogeneous systems of fundamentally different structure: an octahedral system of C 2 symmetry with a tetradentate O N N O ligand and a bridged indenyl catalyst. This choice was not accidental; it is in these two systems where the experimentalists cannot reach a consensus about dormancy. Our density-functional theory calculations explain why in certa…
Polyreaktionen in orientierten systemen. II.. Polymerisation von 4-vinylpyridinium-salzen in micellar geordneten lösungen
1972
Die Kinetik der „spontanen” Polymerisation von 4-Vinylpyridiniumperchlorat in Wasser zeigt, das oberhalb einer „kritischen” Konzentration ein Wechsel des Polymerisationsmechanismus eintritt entsprechend einer Anderung in der Struktur des sich bildenden Polymeren. Untersuchungen der Viskositat und der Oberflachenspannung an Modellsubstanzen zeigen, das dieses Phanomen der Bildung von Micellen und der darin ablaufenden Polymerisation zuzuschreiben ist. Der alkalische Abbau der Polymeren wurde untersucht. The kinetics of the “spontaneous” polymerization of 4-vinylpyridiniumperchlorate in water proves that above a “critical” concentration the polymerization mechanism is changed. Correspondingly…
Poly(S-ethylsulfonyl-l-homocysteine): An α-Helical Polypeptide for Chemoselective Disulfide Formation
2018
Homocysteine and cysteine are the only natural occurring amino acids that are capable of disulfide bond formations in peptides and proteins. The chemoselective formation of asymmetric disulfide bonds, however, is chemically challenging and requires an activating group combining stability against hard nucleophiles, e.g., amines, with reactivity toward thiols and soft nucleophiles. In light of these considerations, we introduced the S-alkylsulfonyl cysteines in our previous work. Here, we present the synthesis and ring-opening polymerization of S-ethylsulfonyl-l-homocysteine N-carboxyanhydrides. We demonstrate that the polymerization leads to narrowly distributed polypeptides (Đ = 1.1–1.3) wi…
Ferrocene-Containing Multifunctional Polyethers: Monomer Sequence Monitoring via Quantitative 13C NMR Spectroscopy in Bulk
2014
Ferrocenyl glycidyl ether (fcGE) and allyl glycidyl ether (AGE) are copolymerized via living anionic ring-opening polymerization to generate polyfunctional copolymers with molecular weights up to 40 300 g/mol and low molecular weight dispersities (Mw/Mn < 1.18). Copolymerizations were carried out in bulk at 100 °C and unexpectedly found to proceed without any isomerization of the allyl double bonds. The copolymerization behavior of fcGE and AGE was monitored by in situ quantitative 13C NMR kinetic measurements in bulk, evidencing the formation of random copolymers under these conditions, showing no gradient of comonomer incorporation. The redox-active behavior of the copolymers and homopoly…
Evidence for chain transfer in the atom transfer radical polymerization of butyl acrylate
2000
Poly(butyl acrylate) (PBuA) of high molecular weight was synthesized by atom transfer radical polymerization (ATRP) in ethyl acetate. Whereas for low molecular weight polymers, a linear increase of the number-average molecular weight. M n , versus conversion and narrow molecular weight distributions indicate the suppression of side reactions, a downward curvature in the plot of M n versus conversion was observed for high molecular weights (M n > 50 000). This effect is explained by chain transfer reactions, leading to branched polymers. GPC measurements with a viscosity detector give evidence for the branched structure of high molecular weight polymers obtained in ATRP. In addition, transfe…
Patterning of a Surface Immobilized ATRP Initiator with an Inkjet Printer
2010
A new technique for patterning polymer brushes on the micrometer scale has been developed in which an inkjet printer was used to deposit droplets of acid on a surface-immobilized initiator for atom transfer radical polymerization (ATRP). The acid cleaved an ester bond in the ATRP initiator in a saponification reaction. As a result, the ATRP initiator was rendered inactive. To control the degree of defunctionalization, a new initiator containing a weak ester bond was derived from a tertiary alcohol. Comparison to an established ATRP initiator, derived from a primary alcohol, showed that the novel initiator was defunctionalized with a higher efficiency. Control of the reaction time allowed to…
Molecular Weight Distribution of Hyperbranched Polymers Generated by Self-Condensing Vinyl Polymerization in Presence of a Multifunctional Initiator
1999
The molecular weight distribution (MWD) is derived for polymers generated by self-condensing vinyl polymerization (SCVP) of a monomer having a vinyl and an initiator group (“inimer”) in the presence of a multifunctional initiator. If the monomer is added slowly to the initiator solution (semi-batch process), this leads to hyperbranched polymers with a multifunctional core. If monomer and initiator are mixed simultaneously (batch process), even at vinyl group conversions as high as 99%, the total MWD consists of polymers which have grown via reactions between inimer molecules (i.e., the normal SCVP process) and those which have reacted with the initiator. Consequently, the weight distributio…