Syntheses and Characterization of Poly(cyclohexyl vinyl ether-<i>s</i><i>tat</i>-vinyl\nalcohol)-<i>b</i>-polyisobutylene-<i>b</i>-poly(cyclohexyl vinyl ether-<i>s</i><i>tat</i>-vinyl\nalcohol) Triblock Copolymers and Their Application as Coatings To\nDeliver Paclitaxel from Coronary Stents

Abstract

Novel ABA-type thermoplastic elastomers, poly(vinyl alcohol) (PVA)-<i>b</i>-polyisobutylene (PIB)-<i>b</i>-PVA triblock copolymers, were prepared by hydrolysis of poly(<i>tert</i>-butyl vinyl ether) (P<i>t</i>BVE)-<i>b</i>-PIB-<i>b</i>-P<i>t</i>BVE triblock copolymers. Attempts to dissolve the triblocks in a variety of solvents and solvent\nmixtures for solution casting remained unsuccessful due to the large difference in the solubility parameters\nof PVA and PIB. To decrease crystallinity and increase the solubility of the end segments, cyclohexyl\nvinyl ether (CHVE) was copolymerized with <i>t</i>BVE. According to the reactivity ratios determined by the\nKelen−Tudos method (<i>r</i>_CHVE = 1.37; <i>r</i><i>_t</i>_BVE = 0.65), the product poly(CHVE-<i>stat</i>-<i>t</i>BVE) was highly random.\nWell-defined P(CHVE-<i>stat</i>-<i>t</i>BVE)-<i>b</i>-PIB-<i>b</i>-P(CHVE-<i>stat</i>-<i>t</i>BVE) triblock copolymers (PDI = ∼1.10) were\nprepared with various CHVE/<i>t</i>BVE ratios. After hydrolysis, the P(CHVE-<i>stat</i>-VA)-<i>b</i>-PIB-<i>b</i>-P(CHVE-<i>stat</i>-VA) triblock copolymers exhibited various amounts of coupled products irrespective of the end block\ncomposition and extent of hydrolysis. Model studies with P<i>t</i>BVE indicated that this side reaction is due\nto the methoxy end groups, which are unstable under acidic conditions of hydrolysis. Well-defined P(CHVE-<i>stat</i>-VA)-<i>b</i>-PIB-<i>b</i>-P(CHVE-<i>stat</i>-VA) triblock copolymers (DP_n ≈ 200−1250−200) with 25, 50, and 75 mol\n% VA in the end block with negligible coupled product were prepared by hydrolysis of P(CHVE-<i>stat</i>-<i>t</i>BVE)-<i>b</i>-PIB-<i>b</i>-P(CHVE-<i>stat</i>-<i>t</i>BVE) capped with methallyltrimethylsilane. Differential scanning calorimetry confirmed that the end blocks were completely amorphous, but compression molding of the samples\nled to extensive cross-linking. Tensile properties were measured on samples cast from solution. All three\ntriblock copolymers with 25, 50, and 75 mol % VA in the end blocks exhibited excellent elastomeric\nproperties. With the increase of VA content, the tensile strength increased from 15.9 to 22.6 MPa while\nthe elongation at break remained similar, ∼500%. The P(CHVE-<i>stat</i>-VA)-<i>b</i>-PIB-<i>b</i>-P(CHVE-<i>stat</i>-VA) block\ncopolymers were evaluated as paclitaxel eluting coatings for coronary stents. The mechanical properties\nof these polymers led to the formation of robust stent coatings. The release of paclitaxel could be modulated\nby varying the ratio of CHVE/VA in the hard block segments.