Showing papers on "Polymerization published in 2000"

Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials

Abstract: This review aims at reporting on very recent developments in syntheses, properties and (future) applications of polymer-layered silicate nanocomposites. This new type of materials, based on smectite clays usually rendered hydrophobic through ionic exchange of the sodium interlayer cation with an onium cation, may be prepared via various synthetic routes comprising exfoliation adsorption, in situ intercalative polymerization and melt intercalation. The whole range of polymer matrices is covered, i.e. thermoplastics, thermosets and elastomers. Two types of structure may be obtained, namely intercalated nanocomposites where the polymer chains are sandwiched in between silicate layers and exfoliated nanocomposites where the separated, individual silicate layers are more or less uniformly dispersed in the polymer matrix. This new family of materials exhibits enhanced properties at very low filler level, usually inferior to 5 wt.%, such as increased Young’s modulus and storage modulus, increase in thermal stability and gas barrier properties and good flame retardancy.

Neutral, single-component nickel (II) polyolefin catalysts that tolerate heteroatoms

Abstract: More than half of the 170 million metric tons of polymers produced each year are polyolefins. Current technology uses highly active cationic catalysts, which suffer from an inability to tolerate heteroatoms such as oxygen, nitrogen, and sulfur. These systems require scrupulously clean starting materials and activating cocatalysts. A family of catalysts has been developed whose members are tolerant of both heteroatoms and less pure starting materials. These heteroatom-tolerant neutral late transition metal complexes are in fact highly active systems that produce high-molecular-weight polyethylene, polymerize functionalized olefins, and require no cocatalyst.

Degradable Poly(β-amino esters): Synthesis, Characterization, and Self-Assembly with Plasmid DNA

Abstract: Poly(β-aminoesters) 1−3 were synthesized via the addition of N,N‘-dimethylethylenediamine, piperazine, and 4,4‘-trimethylenedipiperidine to 1,4-butanediol diacrylate. Polymerization proceeded exclusively via the conjugate addition of the secondary amines to the bis(acrylate ester). Polymers were isolated in up to 86% yields with molecular weights ranging up to 31 200 relative to polystyrene standards. The polymers degraded hydrolytically in acidic and alkaline media to yield 1,4-butanediol and β-amino acids 4a−6a and the degradation kinetics were investigated at pH 5.1 and 7.4. In general, the polymers degraded more rapidly at pH 7.4 than at pH 5.1. In initial screening assays, both the polymers and their degradation products were determined to be noncytotoxic relative to poly(ethylene imine), a polymer conventionally employed as a synthetic transfection vector. Polymers 1−3 interacted electrostatically with polyanionic plasmid DNA in water and buffer at physiological pH, as determined by agarose gel elec...

Living free radical polymerization with reversible addition : fragmentation chain transfer (the life of RAFT)

Abstract: Free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) is discussed with a view to answering the following questions: (a) How living is RAFT polymerization? (b) What controls the activity of thiocarbonylthio compounds in RAFT polymeriza- tion? (c) How do rates of polymerization differ from those of conventional radical polymerization? (d) Can RAFT agents be used in emulsion polymerization? Retardation, observed when high concentra- tions of certain RAFT agents are used and in the early stages of emulsion polymerization, and how to overcome it by appropriate choice of reaction conditions, are considered in detail. Examples of the use of thiocarbonylthio RAFT agents in emulsion and miniemulsion polymerization are provided. # 2000 Society of Chemical Industry

Synthesis of branched polyethylene using (α-diimine)nickel(II) catalysts : influence of temperature, ethylene pressure, and ligand structure on polymer properties

Abstract: Detailed investigations of the polymerization of ethylene by (α-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were [{(2,6-RR‘C6H3)−NC(Nap)−C(Nap)N−2,6-RR‘C6H3)}NiBr2] (Nap = 1,8-naphthdiyl) (4a, R = CF3, R‘ = H; 4b, R = CF3, R‘ = CH3; 4c, R = C6F5, R‘ = H, 4c, R = C6F5, R‘ = H; 4d, R = C6F5, R‘ = CH3; 4e, R = CH3, R‘ = H, 4f, R = R‘ = CH3; 4g, R = R‘ = CH(CH3)2), [{(2,6-C6H3(i-Pr)2)−NC(CH2CH2CH2CH2)CN−(2,6-C6H3(i-Pr)2)}NiBr2] (5), and [{(2,6-C6H3(i-Pr)2)−NC(Et)C(Me)N−(2,6-C6H3(i-Pr)2)}NiBr2] (6). Active polymerization catalysts were formed in situ by combination of 4−6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polyme...

Latrunculin alters the actin-monomer subunit interface to prevent polymerization.

Abstract: atrunculin-A is a drug that is capable of rapidly, reversibly and specifically disrupting the actin cytoskeleton. The efficacy of its action has made it a compound of choice in many cell-biology laboratories, supplanting the classic actin-depolymerizing drug cytochalasin-D. One reason for this is that the mode of action of latrunculin seems to be less complex than that of cytochalasin. Whereas the latter affects the kinetics of actin-filament polymerization at both the barbed and pointed ends, latrunculin-A seems to associate only with actin monomers, thereby preventing them from repolymerizing into filaments. The association of latrunculin with monomeric, rather than filamentous, actin gave us the opportunity to further our understanding of this interaction by detailed structural analysis of actin monomers using crystallographic techniques. Here we show the first high-resolution structure of an actin-disrupting drug in association with actin and discuss how its interactions with actin, and the conformational changes that its binding causes, may explain its mode of action within the cell. Latrunculin (Fig. 1a) is purified from Latrunculia magnificans, a Red Sea sponge that exudes a noxious, red fluid that kills fish within minutes. Two related compounds, latrunculin-A and latrunculinB, isolated from the fluid were shown to depolymerize actin structures both in vitro and in vivo. The in vitro studies showed that latrunculin binds only to the actin monomer and that the kinetics of this interaction are consistent with the complex being unable to polymerize. Unlike cytochalasin, latrunculin can disrupt the actin cytoskeleton in yeast cells. This has enabled genetic studies to be carried out that have facilitated the identification of point mutations in the actin gene that cause cells to become resistant to the effects of the drug (Fig. 1b). The mutations that give rise to latrunculin resistance were found to be clustered around a distinct site, close to the nucleotide-binding site, which indicated that they might identify a potential binding site for latrunculin. However, as this site is not close to recognized subunit contacts in the filament, or to known binding sites for other proteins that associate with actin, the mechanism by which latrunculin exerts its effects has remained unclear. Actin has never been known to crystallize in the absence of a binding protein that keeps it in a monodispersed state. Of the three known examples of such binding proteins, profilin is inappropriate as it promotes nucleotide exchange, whereas deoxyribonuclease1 binds to domains that have been implicated, in studies of yeast genetics, in latrunculin binding. In contrast, gelsolin domain 1 in complex with actin leaves these domains free and also reduces nucleotide exchange, as does latrunculin. We therefore soaked latrunculin-A L

Synthesis and Characterization of Multiresponsive Core−Shell Microgels

Abstract: We report the synthesis and characterization of temperature and pH responsive hydrogel particles (microgels) with core−shell morphologies. Core particles composed of cross-linked poly(N-isopropylacrylamide) (p-NIPAm) or poly(NIPAm-co-acrylic acid) (p-NIPAm-AAc) were synthesized via precipitation polymerization and then used as nuclei for subsequent polymerization of p-NIPAm-AAc and p-NIPAm, respectively. The presence of a core−shell morphology was confirmed by transmission electron microscopy (TEM). Thermally initiated volume phase transitions were interrogated via temperature-programmed photon correlation spectroscopy (TP-PCS) as a function of solution pH. The p-NIPAm-AAc core hydrogel displays both a strong temperature and pH dependence on swelling. However, both p-NIPAm-AAc (core)/p-NIPAm (shell) and p-NIPAm (core)/p-NIPAm-AAc (shell) particles display a more complex pH dependence than the homogeneous particles. Specifically, a multistep volume phase transition appears when the AAc component becomes hi...

Critically evaluated rate coefficients for free-radical polymerization, 2.. Propagation rate coefficients for methyl methacrylate

Abstract: Propagation rate coefficients, k(P), for free-radical polymerization of butyl acrylate (BA) previously reported by several groups are critically evaluated. All data were determined by the combination of pulsed-laser polymerization (PLP) and subsequent polymer analysis by size exclusion (SEC) chromatography. The PLP-SEC technique has been recommended as the method of choice for the determination of k(P) by the IUPAC Working Party on Modeling of Polymerization Kinetics and Processes. Application of the technique to acrylates has proven to be very difficult and, along with other experimental evidence, has led to the conclusion that acrylate chain-growth kinetics are complicated by intramolecular transfer (backbiting) events to form a mid-chain radical structure of lower reactivity. These mechanisms have a significant effect on acrylate polymerization rate even at low temperatures, and have limited the PLP-SEC determination of k(P) of chain-end radicals to low temperatures (<20 degreesC) using high pulse repetition rates. Nonetheless, the values for BA from six different laboratories, determined at ambient pressure in the temperature range of -65 to 20 degreesC mostly for bulk monomer with few data in solution, fulfill consistency criteria and show excellent agreement, and are therefore combined together into a benchmark data set. The data are fitted well by an Arrhenius relation resulting in a pre-exponential factor of 2.21 x 10(7) L (.) mol(-1) (.) s(-1) and an activation energy of 17.9 kJ (.) mol(-1). It must be emphasized that these PLP-determined k(P) values are for monomer addition to a chain-end radical and that, even at low temperatures, it is necessary to consider the presence of two radical structures that have very different reactivity. Studies for other alkyl acrylates do not provide sufficient results to construct benchmark data sets, but indicate that the family behavior previously documented for alkyl methacrylates also holds true within the alkyl acrylate family of monomers. [GRAPHICS] Arrhenius plot of propagation rate coefficients, k(P), for BA as measured by PLP-SEC.

Non-fluorous polymers with very high solubility in supercritical CO2 down to low pressures

Abstract: Liquid and supercritical carbon dioxide have attracted much interest as environmentally benign solvents, but their practical use has been limited by the need for high CO2 pressures to dissolve even small amounts of polar, amphiphilic, organometallic, or high-molecular-mass compounds. So-called 'CO2-philes' efficiently transport insoluble or poorly soluble materials into CO2 solvent, resulting in the development of a broad range of CO2-based processes, including homogeneous and heterogeneous polymerization, extraction of proteins and metals, and homogeneous catalysis. But as the most effective CO2-philes are expensive fluorocarbons, such as poly(perfluoroether), the commercialization of otherwise promising CO2-based processes has met with only limited success. Here we show that copolymers can act as efficient, non-fluorous CO2-philes if their constituent monomers are chosen to optimize the balance between the enthalpy and entropy of solute-copolymer and copolymer-copolymer interactions. Guided by heuristic rules regarding these interactions, we have used inexpensive propylene and CO2 to synthesize a series of poly(ether-carbonate) copolymers that readily dissolve in CO2 at low pressures. Even though non-fluorous polymers are generally assumed to be CO2-phobic, we expect that our design principles can be used to create a wide range of non-fluorous CO2-philes from low-cost raw materials, thus rendering a variety of CO2-based processes economically favourable, particularly in cases where recycling of CO2-philes is difficult.

Polymerized ABA Triblock Copolymer Vesicles

Abstract: The synthesis and the characterization of a poly(2-methyloxazoline)-block-poly(dimethylsiloxane)-block-poly(2-methyloxazoline) (PMOXA−PDMS−PMOXA) triblock copolymer carrying polymerizable groups at both chain ends are described. This copolymer forms vesicular structures in dilute aqueous solution, the size of which can be controlled in the range from 50 nm up to about 500 nm. The methacrylate end groups of the triblock copolymer can be polymerized in the vesicular aggregates using an UV-induced free radical polymerization. Static and dynamic light scattering, scanning electron microscopy, and transmission electron microscopy on both the resulting nanocapsules and their nonpolymerized precursors clearly show that the cross-linking polymerization does not lead to morphological changes in the underlying vesicles. Moreover, due to their cross-linked structure, the nanocapsules are shape persistent, thus maintaining their integrity even after their isolation from the aqueous solution.

Molecular Design of Single-Site Metal Alkoxide Catalyst Precursors for Ring-Opening Polymerization Reactions Leading to Polyoxygenates. 1. Polylactide Formation by Achiral and Chiral Magnesium and Zinc Alkoxides, (η3-L)MOR, Where L = Trispyrazolyl- and Trisindazolylborate Ligands

Abstract: Single-site achiral and chiral C3-symmetric complexes LMOR, where M = Mg and Zn, L = an η3-trispyrazolyl- or η3-trisindazolyl-borate ligand and R = Et, tBu, Ph, or SiMe3, have been synthesized and employed in the ring-opening polymerization of l-, rac-, and meso-lactide in CH2Cl2 at 25 °C and below. The polymerization occurs by acyl cleavage and gives rise to polylactide, PLA, with PDI of 1.1−1.25 up to 90% conversion. Studies of the kinetics of polymerization reveal first order behavior in both lactide and metal catalyst. For L = tris(3-tert-butylpyrazolyl)borate, (tBupz)3BH, polymerization of ∼500 equiv of l-lactide proceeds to 90% conversion within 1 h and 6 d for the magnesium and zinc catalysts, respectively. The zinc complexes are, however, more tolerant to air and moisture and solid samples where R = SiMe3 are persistent in air for several days. The rate of polymerization is also significantly influenced by the nature of the η3-L spectator ligand. Chiral C3-symmetric catalysts, where L = tris(indaz...

Kinetics and mechanism of cyclic esters polymerization initiated with Tin(II) octoate. 3. Polymerization of L, L-dilactide

Abstract: Following our previous papers on the mechanism of cyclic esters polymerization induced by tin(II) octoate (Sn(Oct) 2 ) and particularly papers on ∈-caprolactone (CL), the present work shows that L,L-dilactide/Sn(Oct)2 does not differ mechanistically from the CL/Sn(Oct) 2 system. Sn atoms bonded through alkoxide groups to macromolecules were also observed by MALDI-TOF mass spectrometry. Formation of the actual initiator from Sn(Oct) 2 and a hydroxy group-containing compound (ROH) was envisaged by kinetic arguments. The appropriate experiments were carried out to show that some mechanisms put forward during the past few decades by several research groups were not sufficiently substantiated. Eventually, we conclude that L,L-dilactide/Sn(Oct)2 polymerization proceeds by simple monomer insertion into the ...-Sn-OR bond, reversibly formed in the reaction ...-SnOct + ROH ⇄ ...-Sn-OR + OctH, where ROH is either the low molar mass co-initiator (an alcohol, hydroxy acid, or H 2 O) or a macromolecule fitted with a hydroxy end group. These interconversions take place throughout the whole polymerization process. Sn(Oct) 2 itself does not play an active role in the polymerization.

Stereoselective polymerization of a racemic monomer with a racemic catalyst: Direct preparation of the polylactic acid stereocomplex from racemic lactide

Abstract: PLA is prepared by the ring opening polymerization (ROP) of lactide, the cyclic dimer of lactic acid. Commercial polylactides usually are synthesized from lactide monomers prepared from a single lactic acid enantiomer, and because the resulting polymers are stereoregular, they have high degrees of crystallinity. 2 The mechanical properties of crystalline polymers are stable to near the polymer melting point, and thus they have higher use temperatures than their amorphous analogues. For example, polymerization ofL-lactide gives a semicrystalline polymer with a melting transition near 180 °C andTg ∼ 67 °C,3 properties that make it useful for applications ranging from degradable packaging to surgical implants and matrices for drug delivery. 4 In contrast, the polymer derived fromrac-lactide, a 1:1 mixture ofDand L-lactide, yields amorphous polymers with glass transitions near room temperature. AlthoughL-lactide can be prepared with relatively high enantiopurity from corn fermentation, the requirement for an enantiopure monomer places restrictions on the polymer synthesis. As shown in Scheme 1, chiral catalysts have been employed to effect kinetic resolution of racemic lactide. Spassky et al. have reported kinetic resolutions of rac-lactide by employing a chiral Schiff’s base complex of aluminum, ( -)1.5 At low conversions high enantiomeric enrichment in the polymer is observed. 6 This finding is significant because the catalyst overrides the tendency for syndiotactic placements that are typically favored by chainend control. 7 At higher conversions the enantiomeric enrichment in the polymer decreases. The drop in selectivity can be attributed to the fact that the relative concentration of the “wrong” isomer increases in the monomer pool as the desired enantiomer is incorporated in the polylactide. In a recent report, Coates et al. effected the syndiotactic polymerization of meso-lactide by using the isopropoxide catalyst ( -)-2.8 Sincemeso-lactide possesses two stereocenters of opposite configuration, the concentration of D andL stereocenters remains constant and the intrinsic selectivity of the catalyst is not diminished by statistical depletion of the preferred stereocenter. An interesting effect of stereoregularity on lactide properties was first reported by Tsuji and co-workers. 9-11 As shown in Scheme 1,L-PLA and D-PLA form a stereocomplex that has a Tm 50 °C higher than theTm for the homochiral polymers. Preparation of this stereocomplex presently requires parallel ROP of Dand L-lactide with subsequent combination of the chiral polylactide chains. Despite its improved mechanical properties, practical applications of the stereocomplex are prohibited by the requirement that separate pools of enantiopure lactide monomers must be polymerized to enantiopure polymers. If the same material could be prepared from the rac-lactide, it is conceivable that applications of the stereocomplex could be realized. Spassky’s and Coates’ results in stereoselective ROP of lactides suggests a strategy for the direct preparation of the polylactide stereocomplex fromrac-lactide. Specifically, the racemic catalyst, rac-2, should lead to parallel syntheses of isotactic D-PLA and L-PLA chains since ( -)-2 preferentially polymerizes D-lactide and (+)-2 preferentially polymerizesL-lactide. In contrast to kinetic resolution ofrac-lactide with (-)-1, theD:L ratio in the monomer pool should remain constant at high conversion since polymerization by the racemic catalyst will remove D and L isomers at equal rates. Thus, the high enantioselectivity that is realized at low conversion in kinetic resolutions using chiral 1 should be maintained at high conversion with rac-1 to give a 1:1 mixture of isotactic chains (Scheme 2). Polymerization ofrac-lactide withrac-2 yields nearly monodisperse chains ( Mw/Mn ) 1.05) consistent with a “living” polymerization and the absence of transesterification. This is supported by the linear relationship between the monomer conversion to polymer andMn. More importantly, the1H NMR spectrum (Figure 1a) is consistent with formation of chains

Polymer Science and Technology

Abstract: FUNDAMENTALS: Introduction Historical Development Basic Concepts and Definitions Classification of Polymers Polymerization Mechanisms Chain-Reaction Polymerization Ionic and Coordination Polymerizations Step-Growth Polymerization Ring-Opening Polymerization Chemical Bonding and Polymer Structure Chemical Bonding Primary Structure Secondary Structure Tertiary Structure Crystallinity and Polymer Properties Thermal Transitions in Polymers The Glass Transition Temperature The Crystalline Melting Point Polymer Modification Copolymerization Postpolymerization Reactions Functional Polymers POLYMER PREPARATION AND PROCESSING METHODS: Condensation (Step-Reaction) Polymerization Mechanism of Condensation Polymerization Kinetics of Condensation Polymerization Stoichiometry in Linear Systems Molecular Weight Control Molecular Weight Distribution in Linear Condensation Systems Molecular Weight Averages Ring Formation vs. Chain Polymerization Three-Dimensional Network Step-Reaction Polymers Prediction of the Gel Point Morphology of Cross-Linked Polymers Chain Reaction (Addition) Polymerization Vinyl Monomers Mechanism of Chain Polymerization Steady-State Kinetics of Free-Radical Polymerization Autoacceleration (Trommsdorff Effect) Kinetic Chain Length Chain-Transfer Reactions Temperature Dependence of Degree of Polymerization Ionic and Coordination Chain Polymerization Copolymerization The Copolymer Equation Types of Copolymerization Polymer Composition Variation with Feed Conversion Chemistry of Copolymerization The Q-e Scheme Polymer Additives and Reinforcements Plasticizers Fillers and Reinforcements (Composites) Alloys and Blends Antioxidants and Thermal and UV Stabilizers Flame Retardants Colorants Antistatic Agents (Antistats) Polymer Reaction Engineering Polymerization Processes Polymerization Reactors Unit Operations in Polymer Processing Extrusion Injection Molding Blow Molding Rotational Molding Thermoforming Compression and Transfer Molding Casting PROPERTIES AND APPLICATIONS: Solution Properties of Polymers Solubility Parameter (Cohesive Energy Density) Conformations of Polymer Chains on Solution Thermodynamics of Polymer Solutions Solution Viscosity Mechanical Properties of Polymers Mechanical Tests Stress-Strain Behavior of Polymers Deformation of Solid Polymers Compression vs. Tensile Tests Effects of Structural and Environmental Factors on Mechanical Properties Polymer Fracture Behavior Polymer Viscoelasticity Simple Rheological Responses Viscoelasticity Mechanical Models for Linear Viscoelastic Response Material Response Time -The Deborah Number Relaxation and Retardation Spectra Superposition Principles Polymer Properties and Applications The Structure of the Polymer Industry Raw Materials for the Polymer Industry Polymer Properties and Applications Other Vinyl Polymers Acrylics Engineering Polymers Elastomers Thermosets Appendices Polymer Nomenclature Answers to Selected Problems Conversion Factors Index **An Introduction is included at the beginning of each chapter and Problems and References are included at the end of each chapter**

Optimization of atom transfer radical polymerization using Cu(I)/ tris(2-(dimethylamino)ethyl)amine as a catalyst

Abstract: The optimization of the synthesis of homopolymers from n-butyl acrylate (BA), styrene (Sty), and methyl methacrylate (MMA), using copper-based atom transfer radical polymerization (ATRP) with tris(2-dimethylaminoethyl)amine (Me6-TREN) as a ligand is reported. Catalyst concentration (copper bromide complexed with 1 equiv of Me6-TREN) as low as 1% relative to initiator (50 ppm in the reaction mixture) was sufficient to successfully prepare well-defined poly(n-butyl acrylate) in bulk. With 50% catalyst to initiator, styrene was polymerized in bulk. The deactivator species had poor solubility in nonpolar solvents. Finally, the polymerization of methyl methacrylate with CuBr/Me6-TREN required the use of a high concentration of catalyst and the addition of Cu(II)Cl2 at the beginning of the reaction; the resulting polymers were of higher molecular weight than predicted by theory and had relatively broad molecular weight distributions.

Facile Atom Transfer Radical Polymerization of Methoxy-Capped Oligo(ethylene glycol) Methacrylate in Aqueous Media at Ambient Temperature

Abstract: We report the facile atom transfer radical polymerization (ATRP) of a commercially available hydrophilic monomer, methoxy-capped oligo(ethylene glycol) methacrylate (OEGMA), under remarkably mild conditions. Various bromide-based initiators, in conjunction with a copper-based catalyst, allow rapid homopolymerization of OEGMA in water at 20 °C. Good living character was achieved with two ligands, namely 2,2‘-bipyridine (bpy) and 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA). Kinetic studies confirm that, for bpy, 90% conversion is typically achieved within 20 min, and the polymerization is first order even up to very high conversions. Molecular weight increases linearly with conversion and is close to the target molecular weight in all cases. Polydispersities remain narrow (Mw/Mn = 1.15−1.30) throughout the polymerization. It is possible to lower the copper catalyst by a factor of 10 (i.e., [Cu(I)]/[initiator] = 0.10) without significant loss of control over the polymerization, which is good eviden...

High-stiffness propylene polymers and a process for the preparation thereof

Abstract: The present invention concerns high-stiffness polymer composition and a process for producing a polymer composition nucleated with a polymeric nucleating agent containing vinyl compound units. The method comprises modifying a catalyst by polymerizing a vinyl compound in the presence of said catalyst and a strongly coordinating donor in a medium, which does not essentially dissolve the polymerized vinyl compound, and by continuing the polymerization of the vinyl compound until the concentration of unreacted vinyl compounds is less than about 0.5 wt-%. The thus obtained modified catalyst composition is used for polymerizing propylene optionally together with comonomers in the presence of said modified catalyst composition. Modification of the catalyst according to the present invention will reduce production costs and provide highly reliable catalyst activity and polymers of high stiffness.