Ring-Opening Copolymerizaton of Cyclohexene Oxide and Succinic Anhydride by Zinc and Magnesium Schiff-Base Complexes Containing Alkoxy Side Arms.

The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is a promising method for the synthesis of new polyesters with various polymer properties. Among previously reported metal catalysts for ROCOP, the Schiff-base complexes have gained significant attention because of their ease of synthesis and modification. In this work, zinc and magnesium complexes containing Schiff-base ligands with different alkoxy side arms [-(CH2)2O- and -(CH2)3O-] were synthesized and shown to have a cubane metal core by X-ray crystal structures. All complexes were studied in the ROCOP of cyclohexene oxide (CHO) and succinic anhydride (SA) in toluene at 110 °C. The zinc complex having a shorter side arm is the most active catalyst for copolymerization, giving poly(CHO-alt-SA) with narrow dispersity and negligible ether linkage. On the other hand, magnesium complexes were not active because of the formation of stable carboxylate species. The detailed analysis of polyester obtained from zinc complexes unexpectedly revealed three different types of polymer structures occurring at different polymerization times. Cyclic polymer was generated at the beginning by intramolecular transesterification of the alkoxy side arm, giving a low-molecular-weight polyester. At higher conversion, cyclization diminished, giving just a linear polyester but with minor competitive formation of higher-molecular-weight polyester having cyclohexanediol as an end group. On the basis of a thorough understanding of the polymerization mechanism, the desired cyclic poly(CHO-alt-SA) was successfully synthesized using a low monomer/catalyst ratio.

Comparative bindings of lactones, lactide, and cyclic carbonates: experimental insights into the coordination step of polymerization.

Comparative bindings of several renowned monomers were investigated experimentally using B(C6F5)3 as a Lewis acid model for the coordination step in ring-opening polymerization. A complete series of the X-ray crystal structures of the B(C6F5)3 adducts with the monomers was reported. The X-ray structural studies and spectroscopic data revealed a coordination strength in the order lactones > tetrahydrofuran > cyclic carbonates > lactide.

Highly efficient metal(iii) porphyrin and salen complexes for the polymerization of rac-lactide under ambient conditions.

Ligated metal(iii) complexes, Al(iii), In(iii), Cr(iii), and Co(iii), containing tetraphenylporphyrin (TPP) and cyclohexylsalen (Cy-salen) ligands were investigated for the polymerization of rac-lactide (rac-LA). A combination of metal complexes, co-catalyst, and co-initiator was shown to be highly efficient for the ring-opening polymerization of rac-LA at room temperature. Influences of metal centers, ligands, and co-catalysts were investigated. Higher Lewis acidity of metal centers and ligands enhanced catalytic activity. Ionic co-catalysts showed increased polymerization rates. Polymerization of rac-LA catalyzed by tetraphenylporphyrin aluminum chloride ((TPP)AlCl) and bis(triphenylphosphine)iminium chloride (PPN+Cl-) in cyclohexene oxide (CHO) as a solvent produced isotactic-enriched polylactide. The order of reactivities based on the metals supported by the TPP ligand is Cr(iii) > Al(iii) > In(iii) > Co(iii) while the reactivities of the catalysts supported by Cy-salen ligands are in the order In(iii) > Cr(iii) > Al(iii) > Co(iii).

A new route for the preparation of enriched iso-polylactide from rac-lactide via a Lewis acid catalyzed ring-opening of an epoxide.

Tetraphenylporphyrin aluminum(iii) salts, TPPAlX, where X = Me, OEt, OiPr, OCHMeCH2Cl, and Cl, and bis(triphenylphosphine)imminium chloride, PPN+Cl- (1 : 1) react with rac-lactide, rac-LA, in neat propylene oxide, PO, to yield chains of enriched isotactic polylactide, PLA, with end groups of PO-Cl and with time these yield cyclic polymers (PO)n(PLA) where n = 2 or 3 and even higher. There is no reaction between TPPAlOR (R = Et or iPr), PPN+Cl-, and rac-LA in neat THF at 25 °C even though TPPAlOR (R = Et or iPr) and PPN+Cl- in neat PO yields polypropylene oxide with a terminal OR group, H(PO)nOR. Taken together, Al(iii) acts as a Lewis acid in the ring-opening of PO, in which PPN+Cl- is present and the incipient ClCH2CHMeO- initiates the ROP of LA to yield anion chains of [(PLA)-OCHMeCH2Cl]-, and then the ring-opening of PO yields cycles, (PO)n(PLA), with the liberation of Cl-. The polymer was isolated by the addition of MeOH/HCl and end group analysis by mass spectrometry.

Synthesis of cyclic polylactide catalysed by bis(salicylaldiminato)tin(II) complexes.

Eight bis(salicylaldiminato)tin(II) complexes have been synthesized from the reaction of Sn[N(SiMe(3))(2)](2) and 2 equiv of the corresponding ligands at room temperature. The ligands, synthesized from salicylaldehyde and amines, were designed to have different electronic and steric properties using different amines to synthesize the tin(II) complexes as aniline (2a), 2,6-dimethylaniline (2b), 2,6-diisopropylaniline (2c), 4-methoxyaniline (2d), 4-trifluoromethylaniline (2e), methylamine (2g), and tert-butylamine (2h). Ligand variation at the salicyl group synthesized from 4-bromosalicylaldehyde and 2,6-diisopropylaniline was used to form complex 2f. Complex 2c was characterized crystallographically. All catalysts were active for the neat polymerization of L-lactide at 115 °C. At a lactide : Sn molar ratio of 10 : 1, cyclic polylactide (PLA) was obtained as demonstrated by (1)H NMR and mass spectrometry. Addition of 1 equiv of benzyl alcohol in the polymerization produced linear PLA. At a higher lactide : Sn molar ratio of 200 : 1, high molecular weight PLAs with M(n) up to 132,200 Daltons were obtained. Results from GPC coupled with light scattering detector and viscometer suggested that they are cyclic PLA. The order of reactivity based on conversion was determined to be 2c < 2b < 2a in accordance with lower steric hindrance. For electronic contribution, the order of 2e < 2a < 2d was observed in agreement with the increasing electron donation of the ligands. Complex 2g having the smallest substituents was found to be the most active catalyst.