Ferenc Molnar

Variably Isotactic Poly(hydroxybutyrate) from Racemic β-Butyrolactone: Microstructure Control by Achiral Chromium(III) Salophen Complexes†

Produced by bacteria and microorganisms, the highly isotactic poly(hydroxybutyrate) (PHB) is an interesting biodegradable polymeric material. Owing to its comparable mechanical properties, it is an attractive alternative to polyolefins, especially isotactic polypropylene (i-PP). However, as long as PHB fabrication through biotechnological routes remains cost intensive, polyolefins win the contest. Therefore, efficient chemical synthesis pathways for this natural polyester motif are of significant importance. There are two major strategies, which are based on the cheap industrial monomers propylene oxide (PO) and carbon monoxide (CO). In addition to the newly discovered direct alternating copolymerization of epoxides and CO, the twostep procedure of combining PO carbonylation to give bbutyrolactone (BL) with subsequent ring-opening polymerization (ROP) is promising. In the ROP of racemic BL, aluminoxane (AlR3/H2O) catalysts generate mixtures of different tacticities, which can be isolated by fractionation. Medium, in some cases higher molecular weights and broad polydispersity indexes (PDIs) are reported, however, reaction times of several days are required. A sometimes higher stereoregularity can be achieved with chiral aluminum(III) salen or -porphyrin complexes. Zinc alkoxides (e.g. from ZnEt2/H2O) are more active, but only amorphous polymers are produced. To circumvent this disadvantage chiral ligands have been introduced, however, although the isotactic polymer was formed the activity was reduced. Syndiotactic PHB of various molecular weights is generated with tin(II,IV)-based catalysts. Recently, the group of Carpentier showed that a syndiotactic PHB (with interesting properties) can even be obtained by yttrium(III)-catalyzed ROP. Perfectly isotactic PHB (bacterial (R)-PHB) is brittle and stiff with a decomposition temperature slightly above the melting transition. Isotacticities ranging from 60 to 80% are desired to generate properties comprising those of elastomeric to plastomeric materials. A major second requirement to establish a new family of high-performance polymers is a molecular weight exceeding 2 > 10, or even more desirable, 5 > 10 gmol . Herein we report chromium(III) salophen complexes for the stereoselective ROP of racemic b-butyrolactone (Scheme 1). Surprisingly, for the first time, these achiral