Syntheses and Characterization of Poly(cyclohexyl vinyl ether-stat-vinyl alcohol)-b-polyisobutylene-b-poly(cyclohexyl vinyl ether-stat-vinyl alcohol) Triblock Copolymers and Their Application as Coatings To Deliver Paclitaxel from Coronary Stents

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