A poly(styrene-b-isobutylene-b-styrene) (SIBS) triblock polymer is employed as the polymer drug carrier for the TAXUS Express2 Paclitaxel-Eluting Coronary Stent system (Boston Scientific Corp.). It has been shown that the release of paclitaxel (PTx) from SIBS can be modulated by modification of either drug-loading ratio or altering the triblock morphology by blending. In the present work, results toward achieving release modulation of PTx by chemical modification of the styrenic portion (using hydroxystyrene or its acetylated version) of the SIBS polymer system are reported. The synthesis of the precursor poly[(p-tert-butyldimethylsilyloxystyrene)]-b-isobutylene-b-[(p-tert-butyldimethylsilyloxystyrene] triblock copolymers was accomplished by living sequential block copolymerization of isobutylene (IB) and p-(tert-butyldimethylsiloxy)styrene (TBDMS) utilizing the capping-tuning technique in a one-pot procedure in methylcyclohexane/CH3Cl at -80 degrees C. This procedure involved the living cationic polymerization of IB with the 5-tert-butyl-1,3-bis(1-chloro-1-methylethyl)benzene/TiCl4 initiating system and capping of living difunctional polyisobutylene (PIB) chain ends with 1,1-ditolylethylene (DTE) followed by addition of titanium(IV) isopropoxide (Ti(OIp)4) to lower the Lewis acidity before the introduction of TBDMS. Deprotection of the product with tetrabutylammonium fluoride yielded poly(hydroxystyrene-b-isobutylene-b-hydroxystyrene), which was quantitatively acetylated to obtain the acetylated derivative. The hydroxystyrene and acetoxystyrene triblock copolymers have acceptable mechanical properties for use as drug delivery coatings for coronary stent applications. It was concluded that the hydrophilic nature of the endblocks and polarity effects on the drug/polymer miscibility lead to enhanced release of PTx from these polymers. The drug-polymer miscibility was confirmed by differential scanning calorimetry and atomic force microscopy evaluations.
Linear soluble polybenzyls, although deceptively simple in structure, have been strangely elusive. We report for the first time the synthesis of perfectly linear soluble polybenzyls by the polycondensation of 1,2,4,5-tetrasubstituted benzenes with formaldehyde using CHCl 3 /trifluoroacetic acid (TFA) as the medium, wherein TFA served both as an acidic catalyst as well as a cosolvent. The number-average molecular weights (M n 's) of the polymers, as determined by gel permeation chromatography, varied from about 1000 to 37,000, depending on the nature of the substituent on the benzene ring; M n was highest when all four substituents were alkoxy groups and was lowest when they were all alkyl groups. This correlated well with susceptibility of the aromatic ring toward electrophilic aromatic substitution, which is the underlying polymerization mechanism. Differential scanning calorimetry of the polymers showed that most of the samples were amorphous with glass-transition temperatures ranging from about Ϫ80°to ϩ80°C, whereas a few that were either symmetrically substituted or possessed a long alkyl substituent were partially crystalline. Preliminary studies suggested that the methylene unit linking the phenyl rings in these polybenzyls could be readily oxidized to generate conjugated polymers that may be perceived as carbon analogues of polyaniline-poly(arylmethine)s.
Recycling of thermoplastic elastomers based on poly (phenylene ether) (PPE) was studied in detail. The quaternary blend comprising of styrene-ethylene-butylene-styrene (SEBS)/ethylene vinyl acetate (EVA)/PPE-PS (polystyrene) showed improvement in mechanical properties upon recycling, which was correlated with the formation of crosslinked network in the system. Presence of crosslinked network was confirmed by the gel content analysis. The blend components involved in the crosslinking were evaluated by gel morphology analysis. Fourier transform infrared spectroscopy revealed the chemical composition of the crosslinked gel. Crosslinking mechanism was established based on the reactivity of allylic EVA radical during recycling. Rheological study supported the notion of crosslinking upon recycling that resulted in higher storage modulus (G 0 ) as a manifestation of restrained flow by network formation. On the basis of the earlier data, a reaction mechanism for crosslinking was proposed. Finally, structure-property correlation was developed through morphological, chemical, and rheological analysis to understand the anomalous enhancement in mechanical properties upon recycling. POLYM. ENG. SCI., 48:496-504, 2008. ª
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