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 /3-butyrolactone ((±)-BL) to form poly-(d-hydroxybutyrate) (PHB) with a preference for syndiotactic (syn) placement. In this report, bis(tri-nbutyltin) oxide ((rc-Bu3Sn)20), bis (triphenyl tin) oxide (Ph3Sn)20), and di-n-butyltin dimethoxide (Sn(n-Bu)2(OCH3)2) were all shown to catalyze the syndiospecific polymerization of (±)-BL. Of these catalysts, the Sn(n-Bu)2(OCH3)2 system showed dramatically decreased polymerization times for correspondingly high monomer conversion. This catalyst system was used to form syn-PHB with an Af" and syn diad fraction of 8.4 X104 and 0.62, respectively. Analysis of the stereochemical sequence distributions at various polymerization temperatures for all of the Sn(IV) catalysts investigated showed an (E, -E¡) of ca. -2 kcal/mol for syndiotactic versus isotactic diad formation. Therefore, the syndiospecificity exhibited little dependence on the catalyst structure over a limited, but significantly broad range of Sn(IV) organometallic systems. The triad stereosequence distributions of syn-PHB samples agrees very well with the Bernoulli model of chain end stereocontrol. Furthermore, the degree of Sn(IV)-catalyst syndiospecificity increased at correspondingly lower polymerization temperatures. Polymerizations carried out at -15 and +90 °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 (fi)-BL (>98% ee) all showed significant (~13%) degrees of configurational inversion at the stereogenic center, with little dependence on the catalyst used or the polymerization temperature. This result indicates that while the preferred mode of ring opening is primarily acyl cleavage (bond breaking between the carbonyl carbon and oxygen of the lactone), a mechanism for stereocenter inversion is operative.
Use of a nerve conduit filled with collagen-GAG matrix to bridge a motor or mixed nerve defect may result in superior functional motor recovery compared with commercially available empty collagen conduit. However, nerve autograft remains the gold standard for reconstruction of a segmental motor nerve defect.
Tight control of pore architecture in porous scaffolds for bone repair is critical for a fully elucidated tissue response. Solid freeform fabrication (SFF) enables construction of scaffolds with tightly controlled pore architecture. Four types of porous scaffolds were constructed using SFF and evaluated in an 8-mm rabbit trephine defect at 8 and 16 weeks (n = 6): a lactide/glycolide (50:50) copolymer scaffold with 20% w/w tri-calcium phosphate and random porous architecture (Group 1); another identical design made from poly(desaminotyrosyl-tyrosine ethyl ester carbonate) [poly(DTE carbonate)], a tyrosine-derived pseudo-polyamino acid (Group 2); and two poly(DTE carbonate) scaffolds containing 500 microm pores separated by 500-microm thick walls, one type with solid walls (Group 3), and one type with microporous walls (Group 4). A commercially available coralline scaffold (Interpore) with a 486-microm average pore size and empty defects were used as controls. There was no significant difference in the overall amount of bone ingrowth in any of the devices, as found by radiographic analysis, but patterns of bone formation matched the morphology of the scaffold. These results suggest that controlled scaffold architecture can be superimposed on biomaterial composition to design and construct scaffolds with improved fill time.
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