Syndiotactic-Enriched Poly(3-hydroxybutyrate)s via Stereoselective Ring-Opening Polymerization of Racemic β-Butyrolactone with Discrete Yttrium Catalysts

Abstract

The mechanism of the ring-opening polymerization of rac-β-butyrolactone using yttrium complexes supported by dianionic aminoalkoxybis(phenolate) ligands as initiators has been investigated by NMR and shown to occur via a coordination-insertion pathway. The microstructure of the resulting syndiotactic-enriched poly(3-hydroxybutyrate)s (PHBs, P r up to 0.94) has been studied by 13C NMR spectroscopy, enabling a detailed assignment of resonances at the diad and triad levels. On this basis, a statistical Bernoullian analysis has been performed which evidenced that syndioselectivity originates from a chain-end control. Some thermal properties of these PHBs have been studied by WAXD and thermoanalytical techniques and shown to be markedly affected by the syndiotacticity degree. This is especially the case for the melting temperature which raises up to 183 °C for P r = 0.94, a temperature higher than that of pure isotactic PHB (ca. 180 °C).

Polymerization of Enantiopure Monomers Using Syndiospecific Catalysts: A New Approach To Sequence Control in Polymer Synthesis

“…The 13 C NMR spectra of both crude and precipitated polymers reveal a fine structure in two diagnostic peaks of PHAs. Based on previous assignments, 27 these peaks correspond to the carbonyl and methylene carbons of PHAs. The overall pattern of signals remains persistent despite the changing content of lactones incorporated in those alternating copolymers and is consistent with the presence of triad (methylene) and dyad (carbonyl) sequences alternatively distributed throughout the polymer chain.…”

“…13 C NMR spectroscopy allowed us to clarify the alternating tendency of the pseudoenantiomeric comonomer units in these copolymers using previously reported assignments. 21c,d, 26,27 Possible sequences available in PHA copolymers derived from enantiomeric -lactones are depicted in Scheme 1. The 13 C NMR spectra of both crude and precipitated polymers reveal a fine structure in two diagnostic peaks of PHAs.…”

New Insights into the Ring‐Opening Polymerization of β‐Butyrolactone Catalyzed by Chromium(III) Salphen Complexes

“…Indeed, more detailed information on the microstructure of PHBs can be obtained from the analysis of the 13 CNMR spectroscopic signals of the methylene group in the polymer backbone. [12] These resonances are usually sufficiently resolved for their assignment on at riad level even if recorded with low-field spectrometers (e.g., 75 MHz or 300 MHz for 1 H). For an atacticP HB polymer,t he signals of the rm-, mm-, rr-,a nd mr-triads ( Figure 1, trace a) are equal in their intensity in the first approximation,w hich is also predicted from statistical considerations.…”

“…The relatively broad lines in the scattering pattern of Cr-2 indicate, however,t he fairly small size of the crystals. No reflections that correspond to isotactic or syndiotactic crystalline PHB are presenti nt he diffractogram of Cr-2, [14,12] which signifies that the opalescence originates from the crystallization of the catalyst under the reaction conditions.…”

Synthesis, structure and catalytic behavior of yttrium complexes bearing a diaminobis(phenolate) ligand

“…Systematic studies have demonstrated that both of the initiating group and the ligand architecture have great influence on polymerization behavior. Of these recent advances in catalyst design, the application of bridged bis(phenolate) groups as ancillary ligands in groups 3, 4 and lanthanide chemistry has received increasing attention [4,5] . As a result, several well-defined single-site yttrium complexes bearing amino-bridged bis(phenolate) ligands have been developed recently, which have been found to be efficient initiators for ROP of rac-lactide [6] and stereoselective ROP of rac-lactide [6−8] and rac--butyrolactone [5,8,9] .…”