SYNDIOSPECIFIC RING-OPENING POLYMERIZATION OF BETA-BUTYROLACTONE TO FORM PREDOMINANTLY SYNDIOTACTIC POLY(BETA-HYDROXYBUTYRATE) USING TIN(IV) CATALYSTS

In a previous report, we have documented the ability of tri-n-butyltin methoxide (Sn(n-Bu)3OCH3) to catalyze the ring-opening polymerization of racemic beta-butyrolactone ((+/-)-BL) to form poly(beta-hydroxybut yrate) (PHB) with a preference for syndiotactic (syn) placement. In th is report, bis(tri-n-butyltin) oxide ((n-Bu3Sn)2O), bis(triphenyltin) oxide (Ph3Sn)2O), and di-n-butyltin dimethoxide (Sn(n-Bu)2(OCH3)2) wer e all shown to catalyze the syndiospecific polymerization of (+/-)-BL. Of these catalysts, the Sn(n-Bu)2(OCH3)2 system showed dramatically d ecreased polymerization times for correspondingly high monomer convers ion. This catalyst system was used to form syn-PHB with an M(n) and sy n diad fraction of 8.4 X 10(4) and 0.62, respectively. Analysis of the stereochemical sequence distributions at various polymerization tempe ratures for all of the Sn(IV) catalysts investigated showed an (E(s) - E(i)) of ca. -2 kcal/mol for syndiotactic versus isotactic diad forma tion. Therefore, the syndiospecificity exhibited little dependence on the catalyst structure over a limited, but significantly broad range o f Sn(IV) organometallic systems. The triad stereosequence distribution s of syn-PHB samples agrees very well with the Bernoulli model of chai n end stereocontrol. Furthermore, the degree of Sn(IV)-catalyst syndio specificity increased at correspondingly lower polymerization temperat ures. Polymerizations carried out at -15 and +90-degrees-C with the Sn (n-Bu)2(OCH3)2 catalyst system gave syn-PHB with syn diad fractions of 0.72 and 0.54, respectively. The polymers formed from (R)-BL (>98% ee ) all showed significant (approximately 13%) degrees of configurationa l inversion at the stereogenic center, with little dependence on the c atalyst used or the polymerization temperature. This result indicates that while the preferred mode of ring opening is primarily acyl cleava ge (bond breaking between the carbonyl carbon and oxygen of the lacton e), a mechanism for stereocenter inversion is operative.