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PubMed2000

Carbohydrate/monomer complexes in aqueous polymerizations: methylated-beta-cyclodextrin mediated aqueous polymerization of hydrophobic methacrylic monomers.

Hydrophobic methacrylic monomers were polymerized in aqueous media using methylated (1.8)-beta-cyclodextrin (MeCD) additives. Hydrophobic monomers tert-butyl methacrylate (tBuMA), cyclohexyl methacrylate (CMA), and 2-ethylhexyl methacrylate (2EHMA) were each dissolved in chloroform with MeCD. Chloroform was then evaporated to yield solid monomer/cyclodextrin complexes. Complexes were shown by 1H NMR and thermogravimetric analysis (TGA) to have molar ratios of monomer to MeCD as high as 0.72/1.00. The water-soluble complexes were readily polymerized in aqueous media using free radical initiation. During polymerization, hydrophobic methacrylic polymers precipitated and the majority of MeCD remained in solution. Poly(alkyl methacrylates) synthesized via this method exhibited number-average molecular weights ranging from 50,000 to 150,000 with polydispersities from 3.2 to 5.5 depending on monomer structure, and isolated yields were as high as 86%. Additionally, corresponding methacrylic/carbohydrate films were prepared and examined. High molecular weight poly(tBuMA), poly(CMA), and poly(2EHMA) were blended with MeCD to produce optically clear films with as high as 20 wt % MeCD. Differential scanning calorimetry (DSC) characterization indicated that the glass transition temperatures of these novel carbohydrate blends were controllable over a 20 degrees C range depending on the relative concentration of each component.

Carbohydrates↗
PubMed2004

Marked counteranion effects on single-site olefin polymerization processes. Correlations of ion pair structure and dynamics with polymerization activity, chain transfer, and syndioselectivity.

Counteranion effects on the rate and stereochemistry of syndiotactic propylene enchainment by the archetypal C(s)-symmetric precatalyst [Me(2)C(Cp)(Flu)]ZrMe(2) (1; Cp = C(5)H(4); Flu = C(13)H(8), fluorenyl) are probed using the cocatalysts MAO (2), B(C(6)F(5))(3) (3)(,) B(2-C(6)F(5)C(6)F(4))(3) (4)(,) Ph(3)C(+)B(C(6)F(5))(4)(-) (5), and Ph(3)C(+)FAl(2-C(6)F(5)C(6)F(4))(3)(-) (6), offering greatly different structural and ion pairing characteristics. Reaction of 1 with 3 affords [Me(2)C(Cp)(Flu)]ZrMe(+) MeB(C(6)F(5))(3)(-) (7). In the case of 4, this reaction leads to formation the micro-methyl dinuclear diastereomers [([Me(2)C(Cp)(Flu)]ZrMe)(2)(micro-Me)](+) MeB(2-C(6)F(5)C(6)F(4))(3)(-) (8). A similar reaction with 6 results in diastereomeric [Me(2)C(Cp)(Flu)]ZrMe(+) FAl(2-C(6)F(5)C(6)F(4))(3)(-) (10) ion pairs. The molecular structures of 7 and 10 have been determined by single-crystal X-ray diffraction. Reorganization pathways available to these species have been examined using EXSY and dynamic NMR, revealing that the cation-MeB(C(6)F(5))(3)(-) interaction is considerably weaker/more mobile than in the FAl(2-C(6)F(5)C(6)F(4))(3)(-)-derived analogue. Polymerizations mediated by 1 in toluene over the temperature range of -10 degrees to +60 degrees C and at 1.0-5.0 atm propylene pressure (at 60 degrees C) reveal that activity, product syndiotacticity, m and mm stereodefect generation, and chain transfer processes are highly sensitive to the nature of the ion pairing. Thus, the complexes activated with 4 and 5, having the weakest ion pairing, yield the highest estimated propagation rates, while with 6, having the strongest pairing, yields the lowest. The strongly coordinating, immobile FAl(2-C(6)F(5)C(6)F(4))(3)(-) anion produces the highest/least temperature-dependent product syndiotacticity, lowest/least temperature-dependent m stereodefect abundance, and highest product molecular weight. These polypropylene microstructural parameters, and also M(w), are least sensitive to increased propylene pressure for FAl(2-C(6)F(5)C(6)F(4))(3)(-), but highest with MeB(C(6)F(5))(3)(-). In general, mm stereodefect production is only modestly anion-sensitive; [propylene] dependence studies reveal enantiofacial propylene misinsertion to be the prevailing mm-generating process in all systems at 60 degrees C, being most dominant with 6, where mm stereodefect abundance is lowest. For 1,3-dichlorobenzene as the polymerization solvent, product syndiotacticity, as well as m and mm stereodefects, become indistinguishable for all cocatalysts. These observations are consistent with a scenario in which ion pairing modulates the rates of stereodefect generating processes relative to monomer enchainment, hence net enchainment syndioselectivity, and also dictates the rate of termination relative to propagation and the preferred termination pathway. In comparison to 3-6, propylene polymerization mediated by MAO (2) + 1 in toluene reveals an estimated ordering in site epimerization rates as 5 > 4 > 2 > 3 > 6, while product syndiotacticities rank as 6 > 2 > 5 approximately 4 > 3.

Journal Article↗
PubMed1981

Fibrin Polymerization. 1. Alkylating peptide inhibitors of fibrin polymerization.

A series of analogues relating to the NH2-terminal region of the fibrin alpha chain, i.e., Gly-Pro-Arg-Pro, were prepared by stepwise solid-phase synthesis, and their abilities to inhibit fibrin polymerization and to prolong thrombin-initiated clotting time were evaluated. Among the analogues systematically modified at different positions, replacement of the NH2-terminal three residues of Gly-Pro-Arg-Pro by either chlorambucil, p-nitrophenyl-L-alanine, or p-aminophenyl-L-alanine gave inactive compounds in the thrombin time assay, whereas similar substitution or extension of the COOH terminus produced the highly active analogues Gly-Pro-Arg-Phe(4-NH2), 22%; Gly-Pro-Arg-Pro-Phe(4-NO2), and Gly-Pro-Arg-Pro-Phe(4-NH2), 105%; relative to Gly-Pro-Arg-Pro = 100% in the fibrin polymerization inhibitory assay. As potential photoaffinity labeling probes, analogues containing a nitrophenylalanine residue in position 4 or 5 underwent photolysis under the experimental photoactivation conditions. As a potential alkylating probe, Chl-Pro-Arg-Pro was selectively effective in inhibiting thrombin amidolysis and fibrin polymerization. In the latter assay, Chl-Pro-Arg-Pro was approximately 20 times more potent than Gly-Pro-Arg-Pro in inhibiting fibrin aggregation.

Alkylation↗
PubMed1998

Semicrystalline Polymers via Ring-Opening Polymerization: Preparation and Polymerization of Alkylene Phthalate Cyclic Oligomers.

Preparation of cyclic oligomeric alkylene phthalates via pseudo-high dilution condensation of alkylene diols with iso- and terephthaloyl chlorides and conversion to high molecular weight polyesters via ring-opening polymerization is described. Sterically unhindered amines such as quinuclidine or 1,4-diazabicyclo[2.2.2]octane (DABCO) catalyze the condensation significantly faster than other tertiary amines and are useful for carrying out this conversion in high yield, in the first direct reaction of diol and diacid chloride to form cyclic polyesters. The mixtures of oligomeric cyclics melt at 150-200 degrees C, providing liquids of low viscosity. Ring-opening polymerization using tin or titanate catalysts affords high molecular weight polymers within minutes. Complete polymerization of PBT oligomeric cyclics can be achieved at 180-200 degreesC, significantly below the polymer's melting point of 225 degreesC, and with molecular weights as high as 445 x 10(3). Polymers formed via such a process are more crystalline than conventionally prepared polyesters.

Journal Article↗
PubMed1983

Network formation by neurofilament-induced polymerization of tubulin: 200K subunit of neurofilament triplet promotes nucleation of tubulin polymerization and enhances microtubule assembly.

The previous paper (Minami, Y., et al. (1982) J. Biochem. 92, 889-898) demonstrated that the neurofilament preparation obtained from porcine brain promotes tubulin polymerization, thereby leading to network formation in vitro. Since this preparation fractionated by gel filtration and centrifugation was contaminated mainly by tubulin, the neurofilaments thus prepared were solubilized in a 6 M urea solution, further purified by hydroxyapatite column chromatography, and reconstituted into neurofilaments. This highly purified neurofilament was found to retain the ability to stimulate microtubule assembly and to cause gelation. Next we separated the purified neurofilament into the individual triplet subunits, referred to as 200K, 150K, and 70K proteins, by DEAE-cellulose (DE-52) column chromatography in the presence of 6 M urea. By measuring viscosity and turbidity changes, it has been found that the activity to stimulate polymerization of tubulin is due to the 200K polypeptide, while the 150K protein was less active to promote tubulin polymerization and the 70K protein was totally inactive.

Animals↗
PubMed1998

Mechanochemical solid-state polymerization. IX. Theoretical analysis of rate of drug release from powdered polymeric prodrugs in a heterogeneous system.

We theoretically derived the rate equation of drug release from a simple model in a heterogeneous system. Four assumptions were used to simplify the model. Two kinds of rate equations for drug release derived from two possible limiting cases, that the rate-determining step is a diffusion or hydrolysis controlled process, can predict the experimental results up to 50% hydrolysis. However, the predictions at the later stage by these equations are insufficient. These results suggest that the process of drug release from powdered polymeric prodrugs in a heterogeneous system must be described by both diffusion and hydrolysis. The rate equation derived from a model considering both the diffusion and hydrolysis processes can successfully predict the experimental results for several kinds of polymeric prodrugs. It is also shown that the diffusion coefficient and rate constant for hydrolysis calculated from this equation thoroughly express the character of the comonomer. The rate equation derived from the model that considers both diffusion and hydrolysis is very useful to analyze drug release from various kinds of polymeric prodrugs in a heterogeneous system.

Hydrolysis↗
PubMed2002

[Molecularly imprinted polymeric microspheres prepared by seed swelling and suspension polymerization].

A series of molecularly imprinted polymeric microspheres(MIPMs) were prepared by seed swelling and suspension polymerization method in aqueous system using tyrosine as printing molecules, methacrylic acid as functional monomers and trimethylolpropane trimethacrylate (TRIM) as cross-linkers. The morphology including the size, size distribution, pore and pore distribution of the polymer beads was analyzed by scanning electron microscope(SEM). The major factors that influence these properties of the beads are discussed. The molecule selecting property of the MIPMs was detected through liquid chromatography. The results showed that uniform-sized spherical MIPMs had been prepared in aqueous system by seed swelling and suspension polymerization method, and its surface was distributed with pores between 1 micron to 10 microns. The average bead size was adjusted from about 50 microns to 400 microns by changing the stirring speed and the amount of seeds used during the preparation. The prepared MIPMs exhibited a considerable tendency to adsorb tyrosine, the printing molecules selectively, and the separation factor was up to 1.82 when phenylalanine was chosen as the competitive molecules.

Binding, Competitive↗
PubMed2005

Polymerization and polymerization shrinkage stress: fast cure versus conventional cure.

Dentists nowadays have a choice of conventional halogen lights, halogen lights with more sophisticated curing cycles (step-cure, rapid-cure, ramp-cure & pulse-cure), fast halogen lights, laser lights, plasma arc lights (PAC) and, lately, LED lights. While the manufacturers of some of the curing units try to improve on the operational reliability of their lights with a slower initial rate of cure, other manufacturers simply wish to offer as fast a curing time as possible. The conventional approach to cure accepts that sufficient light intensity of at least 400 mW/cm2 at a wavelength of 400-500 nm, and an exposure time of at least 40 seconds is needed to cure a 2-mm layer of composite. When a halogen light with higher or very high intensity is used, alternative curing strategies provide for an initial slower cure to allow flow, and after that a higher-intensity cure to improve the degree of cure. In contrast, in the fast-cure or rapid-cure approach it is suggested that a layer of composite can be cured for only 5- 10 seconds at >2000 mW/cm2. Some go so far as to say that an exposure time of 3 seconds per layer may be enough. This contradictory approach is compounded by the fact that this support for fast cure does not seem to consider the negative consequences. Therefore, to address these concerns, this review discusses the possible effects of a fast cure approach compared to a more conventional approach in polymerization and polymerization shrinkage, and the consequences there-off. Other factors that play an influencing role in polymerization shrinkage stress are also included in the discussion.

Composite Resins↗
PubMed1984

Actin polymerization. The effect of brevin on filament size and rate of polymerization.

Fluorescent probes covalently bound to actin or to the actin binding protein, brevin, have been utilized to provide information about actin filaments formed in the presence of brevin as well as about the effect of brevin on the rate of polymerization. At actin to brevin ratios of 10:1 to 100:1, the observed diffusion coefficients of filaments, as measured by fluorescence photobleaching recovery using rhodamine-labeled actin or fluorescein-labeled brevin, are similar to those calculated from theoretical considerations for rigid rods. At lower brevin concentrations, the observed diffusion coefficients for actin filaments are lower than predicted, indicating that the filament structure is closer to that observed in the absence of brevin where filaments are immobilized due to interactions between them. The fluorescein-labeled brevin was found to be about as effective in influencing actin polymerization as unlabeled brevin. Using pyrene-labeled actin, we show that brevin binds 2 mol of monomeric actin. We conclude that at sufficiently high brevin concentration there is one brevin molecule per actin filament. From measurements of the initial rate of polymerization at 5.9 microM actin in the presence of brevin, we calculate both the apparent elongation rate constant and dissociation rate constant from one end (presumably the slow-growing end) of the actin filament. The former is highly dependent on Mg2+ concentration while the latter is not.

Actins↗
PubMed1998

In vivo stimulation of polymeric Ig receptor transcytosis by circulating polymeric IgA in rat liver.

Binding of human polymeric IgA ligand to its epithelial cell polymeric Ig receptor, pIgR, has been shown to stimulate pIgR apical transcytosis in an in vitro system, based on polarized confluent MDCK cells expressing rabbit pIgR. The present study aimed at testing whether such a stimulation also occurs in vivo. Transcytosis of pIgR was monitored by rat liver output of total secretory component (SC) into bile, measured by radial immunodiffusion as the sum of free SC and pIgA-bound SC. Whereas in the perfused rat liver system addition of pIgA to the perfusate showed no effect, i.v. injection of human and rat pIgA, but not of monomeric IgA nor PBS, in living rats significantly increased total bile SC output for more than 1 h. Furthermore, depletion of the normal pIgA level circulating in the liver before injecting more pIgA was not required to show the stimulation. Our data thus strongly suggest that stimulation of liver pIgR transcytosis by pIgA ligand binding is physiologically relevant, helping to quickly adjust pIgA transport into bile to increase circulating pIgA levels, without need for increased SC/pIgR synthesis.

Animals↗
PubMed2004

Synthesis of AB diblock copolymers by atom-transfer radical polymerization (ATRP) and living polymerization of alpha-amino acid-N-carboxyanhydrides.

The synthesis of poly(methyl acrylate)-block-poly(gamma-benzyl-L-glutamate) (PMA-b-PBLG) diblock copolymers, using atom-transfer radical polymerization (ATRP) of methyl acrylate and living polymerization of gamma-benzyl-L-glutamate-N-carboxyanhydride (Glu-NCA) is described. Amido-amidate nickelacycle end groups were incorporated onto amino-terminated poly(methyl acrylates), and the resulting complexes were successfully used as macroinitiators for the growth of polypeptide segments. This method allows the controlled preparation of polypeptide-block-poly(methyl acrylate) diblock architectures with control over polypeptide chain length and without the formation of homopolypeptide contaminants.

Amino Acids↗
PubMed2002

In situ polymerized polyaniline films. 4. Film formation in dispersion polymerization of aniline.

Polyaniline films were grown on glass supports during dispersion polymerizations of aniline using poly(N-vinylpyrrolidone) and hydroxypropylcellulose as stabilizers. The initiation of polyaniline chains is proposed to be heterogeneously catalyzed by the surfaces immersed in the reaction mixture. Film formations in dispersion and precipitation polymerizations are compared. Surfometry and optical absorption were used to assess the submicrometer film thickness, and FTIR spectroscopy was used to analyze the chemical structure of films and prove the absence of stabilizer. The film thickness was proportional to the dimensions of simultaneously produced colloidal polyaniline particles. The conductivity of films increased with increasing film thickness.

Aniline Compounds↗
PubMed2003

Synthesis and characterization of C13 to C18 stationary phases by monomeric, solution polymerized, and surface polymerized approaches.

A series of bonded phases were synthesized from consecutive length alkylsilanes ranging from C13 through C18, with three different bonding chemistries (monomeric, solution polymerized, and surface polymerized) at each phase length. The phases were characterized in terms of methylene selectivity, shape selectivity, and band broadening. No significant chromatographic differences were found to result from the synthetic routes, except that the different bonding chemistries provide a different range of bonding densities. For bonding densities ranging from 2 to 8 micromol/m2, a linear increase in methylene selectivity was observed with increasing bonding density. Over the narrow range of bonded phase lengths investigated, shape selectivity is more dependent on density than phase length.

Polymers↗
PubMed1997

Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives.

OBJECTIVES: This paper is intended to contribute to the recognition and understanding of problems related to polymerization shrinkage. DATA SOURCES: Scientific publications of relevance with regard to this subject were critically reviewed. STUDY SELECTION: The dimensional changes which develop during the curing of resin composites and glass polyalkenoate cements are studied, with special reference to methods of determining shrinkage, shrinkage stress and stress relief. CONCLUSIONS: As no method for handling the adhesive restorative materials has yet been described which guarantees a leakproof restoration, the practitioner has to accept the problem of polymerization shrinkage and destructive shrinkage stress. Only a proper understanding of the mechanisms that cause these problems and the techniques that may reduce their effects will enable the practitioner to derive maximum benefit from the application of resin composites and glass polyalkenoate cements in restorative dentistry.

Composite Resins↗
PubMed2000

Synthesis, characterization, and polymerization activity of [bis(4,4'-bis(neophyldimethylsilylmethyl)-2,2'-bipyridyl)copper(I)]+CuBr2- and Implications for copper(I) catalyst structures in atom transfer radical polymerization.

A series of 4,4'-disilyl-substituted-2,2'-bipyridine ligands were prepared using a metathesis reaction of the dianion of 4,4'-dimethyl-2,2'-bipyridine with several trialkylsilyl chlorides: 4,4'bis(tert-butyldimethylsilylmethyl)-2,2'-bipyridine (dTBDMSbipy), 4,4'-bis(dimethylthexylsilylmethyl)-2,2'-bipyridine (dTHEXbipy), and 4,4'-bis(neophyldimethylsilylmethyl)-2-2'-bipyridine (dNEObipy). It was observed that the side chain length correlated with the ability of the ligand to form hydrocarbon soluble complexes of copper(I) bromide, with dNEObipy forming the most soluble and easily crystallized complexes. The atom transfer radical polymerization (ATRP) of styrene using dNEObipy as the ligand displayed molecular weight control equivalent to other ATRP systems in which solubilizing ligands, such as 4,4'-di-5-nonyl-2,2'-bipyridine or 4,4'-di-n-heptyl-2,2'-bipyridine, were used. The one-to-one complex of dNEObipy with CuBr was prepared and its crystal structure was determined. The resulting complex had the ionic formulation [(dNEObipy)2Cu]+[CuBr2]- and displayed similar activities in styrene ATRP as the standard 2 dNEObipy/CuBr catalyst system. These and other polymerization results in addition to NMR experiments suggest that the predominant copper(I) species formed in ATRP solutions is the 2-to-1 ligand-to-copper(I) cation, [(dNEObipy)2Cu]+, with either a dihalocuprate or halide counteranion, depending upon the conditions.

Journal Article↗
PubMed2002

First example of N-heterocyclic carbenes as catalysts for living polymerization: organocatalytic ring-opening polymerization of cyclic esters.

A novel metal-free, organocatalytic approach to living polymerization is presented. N-heterocyclic carbenes were employed as nucleophilic catalysts for the ring-opening polymerization (ROP) of cyclic ester monomers. The catalysts is used in combination with an initiator, such as an alcohol, which generates an alpha-end group bearing the ester from the initiating alcohol upon ring-opening and a hydroxyl functional omega-chain end that propagates the chain. This class of catalyst proved to be more reactive than tertiary amine and phosphine nucleophiles, producing narrowly dispersed polymers of predictable molecular weights at room temperature in 1-2 h. Catalysis with respect to both initiating alcohol and monomer was observed. Control of the alpha and omega end-groups was demonstrated with a pyrene-labeled initiator, allowing the preparation of well-defined macromolecular architectures. Analogous to the ROP of cyclic esters using biocatalysts, the polymeriztion pathway using the N-heterocyclic carbenes is believed to ensue through a monomer-activated mechanism.

Caproates↗
PubMed2002

Polymeric nanogels produced via inverse microemulsion polymerization as potential gene and antisense delivery agents.

Polymeric nanogel vectors were developed for cellular gene and antisense delivery. Inverse microemulsion polymerization was utilized to synthesize biocompatible nanogels with controlled size, morphology, and composition. The chemical composition, size, polydispersity, stability, and swelling behavior of the nanogels were investigated by NMR, light scattering, transmission electron microscopy, and atomic force microscopy. The cell viability, uptake, and physical stability of nanogel-DNA complexes were evaluated under physiological conditions. Monodisperse nonionic and cationic nanogels were produced with controllable sizes ranging from 40 to 200 nm in diameter. The nanogels demonstrated extended stability in aqueous media and exhibited low toxicity in cell culture. Cationic nanogels formed monodisperse complexes with oligonucleotides and showed enhanced oligonucleotide uptake in cell culture. The nanogels synthesized in this study demonstrate potential utility as carriers of oligonucleotides and DNA for antisense and gene delivery.

Acrylates↗
PubMed2003

Solventless polymerization: spatial migration of a catalyst to form polymeric thin films in microchannels.

This paper reports a simple, additive process to generate patterned polymer films without using any solvent. This process involves a highly efficient catalyst, a Grubbs's catalyst, and a volatile monomer, norbornene. The catalyst and monomers have higher local concentrations inside the microchannels, formed by contacting poly(dimethylsiloxane) stamps to a solid surface, and allow the polymeric thin films to be defined by the microchannels. The patterned thin film serves as an excellent resistant to reactive ion etching, which promises that this process is a complementary, useful alternative to spin-coating and plasma polymerization in microfabrication.

Journal Article↗