Air‐stable copper derivatives as efficient catalysts for controlled lactide polymerization: Facile synthesis and characterization of well‐defined benzotriazole phenoxide copper complexes

Air‐stable copper catalysts supported by bis‐ BTP ligands ( BTP = N,O‐bidentate benzotriazole phenoxide) were synthesized and structurally characterized. The reactions of Cu(OAc)₂·H₂O with 2.0 molar equivalents of sterically bulky 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐bis(1‐methyl‐1‐phenylethyl)phenol ( ^CMe2PhBTP ‐H) and 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐di‐tert‐butylphenol ( ^t‐BuBTP ‐H) in refluxing ethanol solution afforded monomeric copper complexes [(^CMe2PhBTP)₂Cu] ( 1 ) and [(^t‐BuBTP)₂Cu] ( 2 ), respectively. The four‐coordinated copper analogue [(^TMClBTP)₂Cu] (3 ) resulted from treatment of 2‐tert‐butyl‐6‐(5‐chloro‐2H‐benzotriazol‐2‐yl)‐4‐methylphenol ( ^TMClBTP ‐H) as the ligand under the same synthetic method with ligand to metal precursor ratio of 2:1, but treatment of complex 3 in acetone gave five‐coordinated monomeric complex [(^TMClBTP)₂Cu(Me₂CO)] (4 ). X‐ray diffraction of single crystals indicates that Cu complex 4 assumes a distorted square pyramidal geometry, penta‐coordinated by two BTP ligands, and one Me₂CO molecule. Catalysis for lactide (LA) polymerization of BTP ‐containing Cu complexes in the presence of various alcohol initiators was investigated. Complex 3 initiated by 9‐anthracenemethanol catalyzes the ring‐opening polymerization effectively not only in a “living” fashion but also in an “immortal” manner, yielding polymers with the predictable molecular weights and narrow molecular weight distributions. Initiations from multifunctional alcohols were able to produce PLLAs with two‐arm linear and three‐arm star‐shaped molecular architectures. The controlled character of Cu complex 3 also enabled us to synthesize the PEG‐b‐PLLA copolymer. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3840–3849     

Efficient catalysts for ring‐opening polymerization of ε‐caprolactone and β‐butyrolactone: Synthesis and characterization of zinc complexes based on benzotriazole phenoxide ligands

A series of efficient zinc catalysts supported by sterically bulky benzotriazole phenoxide ( BTP ) ligands are synthesized and structurally characterized. The reactions of diethyl zinc (ZnEt₂) with ^CMe2PhBTP ‐H, ^t‐BuBTP ‐H, and ^TMClBTP ‐H yield monoadduct [(μ‐ BTP )ZnEt]₂ ( 1 – 3 ), respectively. Bisadduct complex [( ^t‐BuBTP )₂Zn] ( 4 ) results from treatment of ZnEt₂ with ^t‐BuBTP ‐H (2 equiv.) in toluene, but treatment of ^TMClBTP ‐H with ZnEt₂ in the same stoichiometric proportion in Et₂O produces five‐coordinated monomeric complex [( ^TMClBTP )₂Zn(Et₂O)] ( 5 ). The molecular structures of compounds 1 , 4 , and 5 are characterized by X‐ray crystal structure determinations. All complexes 1 – 5 are efficient catalysts for the ring‐opening polymerization of ε‐caprolactone (ε‐CL) in the presence of 9‐anthracenemethanol. Experimental results indicate that complex 3 exhibits the greatest activity with well‐controlled character among these complexes. The polymerizations of ε‐CL and β‐butyrolactone catalyzed by 3 are demonstrated in a “living” character with narrow polydispersity indices (monomer‐to‐initiator ratio in the range of 25–200, PDIs ≤ 1.10). The “immortal” character of 3 provides a way to synthesize as much as 16‐fold polymer chains of poly(ε‐CL) (PCL) with narrow PDI in the presence of a catalyst in a small proportion. The controlled fashion of complex 3 also enabled preparation of the PCL‐b‐poly(3‐hydroxybutyrate) copolymer. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Synthesis,characterization, and properties of poly(ester‐phosphoester)s by lanthanum triphenolate‐catalyzed ring‐opening copolymerization

The ring‐opening copolymerization of methyl ethylene phosphate (MEP, 2‐methoxy‐2‐oxo‐1,3,2‐dioxaphospholane) and ε‐caprolactone (CL) was performed in bulk with lanthanum tris(2,6‐di‐tert‐butyl‐4‐methylphenolate)s as single‐component catalyst, resulting in poly(ester‐phosphoester) random copolymers with high molecular weight and moderate molecular weight distribution. The properties of the copolymers were characterized by differential scanning calorimetry, X‐ray diffractometer, dynamic mechanical analysis, and static water contact angle measurement. The crystallinities of the copolymers were reduced with the increase of MEP molar fraction in the products. Moreover, copolymers with enhanced hydrophilicity and lower glass transition temperature could be obtained with higher MEP content, which may provide potential applications in biomedical field. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

2D‐ and 3D‐microstructured biodegradable polyester resins

Biodegradable polyester resins were prepared via photo crosslinking of functional polyesters obtained by copolymerization of ε‐caprolactone and the functional cyclic esters γ‐acryloyloxy‐ε‐caprolactone (ACL) and γ‐methacryloyloxy‐ε‐caprolactone (MCL). The cyclic esters were prepared via Baeyer‐Villiger oxidation of the corresponding 4‐acyloyloxy‐cyclohexanone derivatives. Copolymers with different content of either acryloyloxy or methacryloyloxy functional groups were prepared via catalyzed ring‐opening polymerization (ROP) of γ‐acyloyloxy‐ε‐caprolactones and ε‐caprolactone using Al(OiPr)₃ as catalyst and initiator. 2D‐ and 3D‐micropatterning of the copolymers was performed via UV‐crosslinking of polymer films on a suitable substrate and by UV replica molding on both rigid and elastic masters, showing the processability of these novel functional polyesters and their potential as substrates for biomedical devices. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6789–6800, 2008     

Polynonanolactone synthesized from vegetable oil: Evaluation of physical properties,biodegradation, and drug release behavior

A systematic investigation of the synthesis, physical properties, biodegradation, and drug release behavior of an aliphatic polynonanolactone from vegetable oil was performed. The chemical structure of the lactone monomers and polylactones were confirmed by NMR spectrometry and molecular weights were determined by gel permeation chromatography (GPC). The thermal behavior of the polymers was assessed by modulated differential scanning calorimetry (MDSC) and thermogravimetric analysis (TGA). The polynonanolactones are crystalline with melting enthalpies (ΔH_m) ranging from 90 to 135 J/g. The crystalline nature of the polylactides was further evaluated by X‐ray diffraction (XRD) and peaks corresponding to planes (110), (200), and (210) were detected. The hydrolytic and enzymatic degradation properties of the polynonanolactones were studied and the degradation rate is comparable to that of widely used polycaprolactone. The enzyme proteinase K was used for the degradation of polynonanolactones. The extent of degradation was evaluated by scanning electron microscopy (SEM). Drug incorporation and release traits due to hydrolytic degradation of the polymer film was carried out with 5‐fluorouracil (5‐FU) as a model drug. This new class of polynonanolactones obtained from vegetable oil was demonstrated to be a potentially competent candidate to replace petroleum‐based polycaprolactone especially for drug delivery applications where slow release of drugs is a requisite. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6373–6387, 2009     

Sorption and swelling of poly(DL‐lactic acid) and poly(lactic‐co‐glycolic acid) in supercritical CO2: An experimental and modeling study

Tissue engineering scaffolds require a controlled pore size and structure to host tissue formation from cell populations. Supercritical carbon dioxide (scCO₂) processing can be used to form porous scaffolds in which the escape of CO₂ from a plasticized polymer melt generates gas bubbles that shape the pores. The process is difficult to control with respect to changes in final pore size, porosity, and interconnectivity, while the solubility of CO₂ in the polymers strongly affects the foaming process. An in‐depth understanding of polymer CO₂ interaction will enable a successful scaffold processing. Amorphous poly(DL ‐lactic acid) (P_DL LA) and poly(lactic acid‐co‐glycolic acid) (PLGA) polymers are attractive candidates for fabricating scaffolds. In this study, CO₂ sorption and swelling isotherms at 35 °C and up to 200 bar on a variety of homo‐ and copolymers of lactic acid and glycolic acids are presented. Sorption is measured through a gravimetric technique using a suspension microbalance and swelling by visualization. The obtained results are modeled using the Sanchez‐Lacombe equation of state. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 483–496, 2008     

Influence of chain microstructure on the hydrolytic degradation of copolymers from 1,3‐trimethylene carbonate and L‐lactide

Ring‐opening copolymerization of L ‐lactide (LLA) and 1,3‐trimethylene carbonate (TMC) blends with LLA/TMC feed ratios from 90/10 to 50/50 was realized at 110 or at 180 °C for various time periods, using low toxic zirconium (IV) acetylacetonate (Zr(Acac)₄) as initiator. The resulting copolymers exhibit different chain microstructures. Copolymers obtained at 110 °C exhibit a gradient chain structure with the presence of lactidyl sequences next to very short ones, and are semicrystalline. In contrast, copolymers obtained at 180 °C are amorphous because of a more random chain microstructure with the presence of larger amounts of medium sequences. Degradation of the copolymers was carried out in pH 7.4 phosphate buffer at 37 °C. Analytical techniques such as ¹H NMR, DSC, GPC, and XRD were used to monitor the degradation. Initially amorphous copolymers can remain amorphous during degradation because of the highly random unit's distribution, and equivalent LLA and TMC contents. However, initially amorphous copolymers containing larger amounts of lactidyl units are able to crystallize during degradation because of the presence of relatively long LLA blocks. Insofar, as initially semicrystalline copolymers are concerned, degradation occurs preferentially in the amorphous zones. Therefore, various degradation behaviors and degradation rates can be obtained by varying the chemical composition, chain microstructure, and morphology of PLLA‐PTMC copolymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3869–3879, 2009     

Michael addition for crosslinking of poly(caprolactone)s

Poly(caprolactone) (PCL) networks have received significant attention in the literature because of many emerging potential applications as biodegradable materials. In this study, the Michael addition reaction was used for the first time to synthesize biodegradable networks using crosslinking of acetoacetate‐functionalized PCL (PCL bisAcAc) oligomers with neopentyl glycol diacrylate. Hydroxyl‐terminated PCL telechelic oligomers with number‐average molecular weights ranging from 1000 to 4000 g/mol were quantitatively functionalized with acetoacetate groups using transacetoacetylation. In addition to difunctional PCL oligomers, hydroxyl‐terminated trifunctional star‐shaped PCL oligomers were functionalized with acetoacetate groups. Derivatization of the terminal hydroxyl groups with acetoacetate groups was confirmed using FTIR spectroscopy, ¹H NMR spectroscopy, mass spectrometry, and base titration of hydroxyl end groups. PCL bisAcAc precursors were reacted with neopentyl glycol diacrylate in the presence of an organic base at room temperature. The crosslinking reactions yielded networks with high gel contents (>85%). The thermomechanical properties of the networks were analyzed to investigate the influence of molecular weight between crosslink points. The glass transition and the extent of crystallinity of the PCL networks were dependent on the molecular weight of the PCL segment. Dynamic mechanical analysis indicated that the plateau modulus of the networks was dependent on the molecular weight of PCL, which was related to the crosslink density of the networks. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5437–5447, 2009     

Annealing behavior of the novel morphology of poly(butylene succinate) lath‐shaped single crystals

Lamellar single crystals of poly(butylene succinate) (PBS) with novel morphologies were prepared from a chloroform/methanol solution by self‐seeding methods. Crystal structures and morphologies were investigated by means of atomic force microscopy (AFM). Lath‐shaped crystal and hexagonal‐shaped crystals coexist in one PBS single crystal and this has a lamellar thickness of around 5–6 nm as determined by AFM. The thickening of lamellae from 5–6 to 7–9 nm occurred during heating from 41 to 84 °C. In situ temperature‐controlled AFM observations demonstrated that the lath‐shaped crystal sections melted first and then the hexagonal sections while the edge of the single crystals remained regular during annealing. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1492–1496, 2009