Judit E. Puskas

Living carbocationic polymerization of resonance-stabilized monomers

Abstract Living polymerization has been at the forefront of polymer science and engineering in recent years. The term “living” is used to distinquish polymerizations in which chain breaking processes such as termination and transfer reactions are absent. Living polymerization provides for the control over the molecular weight (MW) and molecular weight distribution (MWD) of the polymer sample. This paper will give a general overview about living carbocationic polymerization, followed by a review of recent developments with emphasis on resonance-stabilized monomers and the use of the living concept for the design and synthesis of new polymeric structures.

Electron pair donors in carbocationic polymerization: I. Introduction into the synthesis of narrow molecular weight distribution polyisobutylenes

SummaryLow molecular weight (¯Mn ∼900–5000) narrow molecular weight distribution (MWD; ¯Mw/Mn = 1.1–1.2)tert.-chlorine telechelic polyisobutylenes (PIBs) have been synthesized by the use of thetrans-2,5-dimethyl-2,5-diacetoxy-3-hexene (DiOAcDMeH6)/BCl3 initiating system in the presence of the electron donor (ED) dimethyl sulfoxide (DMSO) in methyl chloride diluent at −30°C. The living character of the polymerization was demonstrated by linear Mn versus Wp (weight of polymer) plots starting at the origin with the slope of 1/[Io] (where [Io] = initiator concentration). DMSO reduces the overall rate of polymerization, however, it increases the initiator efficiency (Ieff) to ∼100%. The number averagetert.-chlorine end functionality is -Fn=1.97±0.04, by1H NMR spectroscopy. Polymerization mechanistic details are discussed. This is the first time narrow MWDtert.-chlorine telechelic PIB has been prepared close to the reflux temperature of methyl chloride.

The effect of hard and soft segment composition and molecular architecture on the morphology and mechanical properties of polystyrene–polyisobutylene thermoplastic elastomeric block copolymers

This paper reports the effects of hard (polystyrene, PS) and soft (polyisobutylene, PIB) segment composition and the molecular architecture (linear versus star, PS and PIB block length) on the morphology and mechanical properties of polystyrene/polyisobutylene (SIBS) block copolymers synthesized by living carbocationic polymerization. Atomic force microscopy, dynamic mechanical thermal analysis and tensile testing verified the phase-separated nature of the block copolymers, which behaved as thermoplastic elastomers (TPEs). The morphology of these TPEs is similar to polydiene-based TPEs, and is defined by the soft/hard segment composition. Interestingly, topology (linear vs star) did not have a major influence on morphology. Tensile testing showed that for both linear and three-arm star block copolymers, the modulus and tensile strength increased while elongation at break decreased with higher PS content. However, three-arm star block copolymers showed larger moduli than their linear homologues with similar PS content and PIB arm length, indicating the influence of molecular architecture on mechanical properties. These results might serve as a foundation for macromolecular engineering design for optimizing properties.

Effect of sample thickness on the mechanical properties of injection-molded polyamide-6 and polyamide-6 clay nanocomposites

The effect of the thickness on the mechanical properties of injection-molded specimens of pure polyamide-6 (PA6) and polyamide-6 clay nanocomposites (PA6-NC) with 5 wt% of layered silicates was investigated. Plates of 0.5, 0.75, 1 and 2 mm thickness were characterized in the injection direction using Dynamic Mechanical Analysis under torsion and tension respectively, and tensile tests. The fracture surfaces were analyzed by Scanning Electron Microscopy. In contrast with PA6, PA6-NC showed thickness effect and clear differences in the mechanical and thermomechanical properties between skin and core, especially in the 2 mm thick samples. Increasing thickness in PA6-NC led to a reduction of tensile modulus and yield stress. In the fracture surface of the thicker tensile specimens the formation of a sheet-like structure was observed. Multiple voiding in the core causing initial failure in this region and a stiffer skin with a better orientation of the layered silicates in the injection direction are two important elements of a micromechanical model proposed in this paper to explain the fracture mechanism in PA6-NC.

Electron-pair donors in carbocationic polymerization. III: Carbocation stabilization by external electron-pair donors in isobutylene polymerization

Abstract The polymerization of isobutylene (IB) initiated by a variety of tertiary chloride/TiCl4 or BC13 systems can be very beneficially influenced by deliberately added electron-pair donors (EDs). EDs whose Gutmanns donor number is larger than ∼26, e.g., dimethylsulfoxide (DMSO) and dimethylacetamide (DMA), are most suitable to mediate numerous most desirable effects. Thus, in the presence of DMSO or DMA: (1) quasi-living polymerizations initiated by cumyl chloride (CumCl)/TiCl4 or 2-chloro-2,4,4-trirnethylpentane (TMPCl)/TiCl4 give rise to very narrow molecular weight distribution (MWD) polyisobutylene (PIB) ( M w / M n = 1.1–1.2); (2) the CumCl/BCl3/IB inifer system becomes living and yields PIB with very narrow MWD; (3) the polymerization-inactive TMPCl/BCl3 system becomes active and induces living IB polymerization to narrow-MWD product; (4) indanyl end-group formation, which may occur in the CumCl/BCl3 or p-dicumyl chloride/BCl3, inifer systems, is eliminated. The effect of reaction conditions,...

Drug-eluting stent coatings.

This paper reviews the development of coronary stents from a polymer scientists view point, and presents the first results of an interdisciplinary team assembled for the development of new stent systems. Poly(styrene-b-isobutylene-b-styrene) block copolymer (SIBS), a nanostructured thermoplastic elastomer, is used in clinical practice as the drug-eluting polymeric coating on the Taxus coronary stent (trademark of Boston Scientific Co.). Our group has been developing new architectures comprising of arborescent (dendritic) polyisobutylene cores (D_SIBS), which were shown to be as biocompatible as SIBS. ElectroNanospray (Nanocopoeia Inc.) was used to coat test coupons and coronary stents with selected D(S)IBS polymers loaded with dexamethasone, a model drug. The surface topology varied from smooth to nanosized particulate coating. This paper will demonstrate how drug release profiles were influenced by both the molecular weight of the polyisobutylene core and spraying conditions of the polymer-drug mixture.

A nanostructured carbon-reinforced polyisobutylene-based thermoplastic elastomer

This paper presents the synthesis and characterization of a polyisobutylene (PIB)-based nanostructured carbon-reinforced thermoplastic elastomer. This thermoplastic elastomer is based on a self-assembling block copolymer having a branched PIB core carrying -OH functional groups at each branch point, flanked by blocks of poly(isobutylene-co-para-methylstyrene). The block copolymer has thermolabile physical crosslinks and can be processed as a plastic, yet retains its rubbery properties at room temperature. The carbon-reinforced thermoplastic elastomer had more than twice the tensile strength of the neat polymer, exceeding the strength of medical grade silicone rubber, while remaining significantly softer. The carbon-reinforced thermoplastic elastomer displayed a high T(g) of 126 degrees C, rendering the material steam-sterilizable. The carbon also acted as a free radical trap, increasing the onset temperature of thermal decomposition in the neat polymer from 256.6 degrees C to 327.7 degrees C. The carbon-reinforced thermoplastic elastomer had the lowest water contact angle at 82 degrees and surface nano-topography. After 180 days of implantation into rabbit soft tissues, the carbon-reinforced thermoplastic elastomer had the thinnest tissue capsule around the microdumbbell specimens, with no eosinophiles present. The material also showed excellent integration into bones.

Highly Hydrophobic Electrospun Fiber Mats from Polyisobutylene-Based Thermoplastic Elastomers

This paper is the first report of electrospinning neat polyisobutylene-based thermoplastic elastomers. Two generations of these materials are investigated: a linear poly(styrene-b-isobutylene-b-styrene) (L_SIBS) triblock copolymer and a dendritic poly(isobutylene-b-p-methylstyrene) (D_IB-MS), also a candidate for biomedical applications. Cross-polarized optical microscopy shows birefringence, indicating orientation in the electrospun fibers, which undergo large elongation and shear during electrospinning. In contrast to the circular cross section of L_SIBS fibers, D_IB-MS yields dumbbell-shaped fiber cross sections for the combination of processing conditions, molecular weight, and architecture. Hydrophobic surfaces with a water contact angle as high as 146 ± 3° were obtained with D_IB-MS that had the noncircular fiber cross section and a hierarchical arrangement of nano- to micrometer-sized fibers in the mat. These highly water repellent fiber mats were found to serve as an excellent scaffold for bovine chondrocytes to produce cartilage tissue.

Polyisobutylene-based thermoplastic elastomers : A review

Abstract This paper honours the 40th anniversary of the discovery of the living polymerization concept. Polymeric materials exhibiting both thermoplastic and elastomeric characteristics have a variety of unique properties which makes them valuable articles of commerce. Such thermoplastic elastomers or TPEs, schematically represented in Scheme 1, are block copolymers — ABA linear triblock, A(BA)n linear alternating block or (AB)n−X radial block, where A is a thermoplastic glassy block with a high glass transition temperature (Tg) while B is an elastomeric block with a low Tg. These TPEs behave like vulcanized rubbers at room temperature and like thermoplastics at elevated temperatures. Thus the materials can be melt extruded like plastics, while retaining their beneficial rubbery properties upon cooling. This ability is not only of advantage during processing, but allows the materials to be reprocessed, which is of importance from both the material savings and the environmental protection point of view. Th...

Quasiliving Carbocationic Poiymerization. III. Quasiliving Polymerization of lsobutylene

Abstract The polymerization of isobutylene has been investigated by the use of the steady, slow, continuous monomer addition technique in the presence of a variety of initiating systems, i.e., “H2O”/TiCl4, “H2O”/AlCl3, C6H5C(CH3)2Cl/TiCl4, p-ClCH2 C6(CH3)4* CH2Cl/AlCl3 at -50°C. Quasiliving polymerizations have been obtained with the “H2O” and C6H5(CH3)2Cl/TiC14 systems in 60/40 v/v n-hexane/methylene chloride solvent mixtures with very slow monomer input. After a brief “flash” polymerization, the M n of PIB increased linearly with the cumulative amount of monomer added (consumed); however, the number of polymer molecules formed also increased, indicating the presence of chain transfer to monomer. With the “H2O”/TiCl4 initiating system, M n,max was 56,000 and M w /M n < 2.0. By the use of the C6H5C(CH3)2CL/TiCl4 initiating system, quasiliving polymerization has been achieved and chain transfer could virtually be eliminated.