Employing (half-)titanocene complexes as initiators for the synthesis of end-functionalized polylactides by coordination polymerization

Seven (half-)titanocene alkoxide complexes, [Cp₂TiCl(OEt)], [Cp₂TiCl(O-2-Bu)], [Cp₂TiCl(OCH₂CF₂CF₂H)], [CpTiCl₂(OEt)], [CpTiCl₂(O-(S)-2-Bu)], [CpTiCl₂(OCH₂CF₃)], and [Cp₂TiCl(OCH₂CF₃)], were synthesized and employed in lactide coordination polymerization. These organotitanium (IV) compounds proved to be very efficient initiators for the ring-opening polymerization of enantiomeric l -lactide and racemic mixture of rac-lactide, leading to end-functionalized polymers. Kinetic studies illustrated that these compounds produce well-defined polymers in a controlled manner. This was further demonstrated by synthesizing poly(l -lactide-b-hexyl isocyanate) diblock copolymer. The effect of the alkoxy end groups on the thermal behavior of the polymers was also studied by thermogravimetric analysis and differential scanning calorimetry. The polymers were characterized using size exclusion chromatography and proton nuclear magnetic resonance spectroscopy. The optical properties of the diblock copolymer were also investigated. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2192–2202     

Spirocyclic bisphenoxides of Ge,Zr, and Sn as catalysts for ring-expansion polymerizations of l- and meso-lactide

Spirocyclic phenoxides of germanium, zirconium, and tin were prepared from 2,2′-dihydroxybiphenyl and 2,2′-dihydroxy-1,1′-binaphthyl. Ring-expansion polymerizations of l -lactide are mainly studied at 160 or 180 °C. The reactivity of the catalysts increases in the order: Zr < Ge < Sn. Regardless of catalyst, the weight-average molecular weights (M_w) never exceed 50,000 g mol⁻¹. The resulting poly(l -lactide)s are optically pure and have a cyclic architecture. Decreasing temperature and time favor formation of even-numbered cycles, and at 102° cyclics, almost free of odd-numbered rings are obtained. Analogous polymerizations of meso-lactide give similar results >120 °C, but different results at 100 or 80 °C. Surprisingly, bell-shaped narrow molecular weight distributions are obtained <140 °C, resembling the pattern of living polymerizations found for alcohol-initiated polymerizations. An unusual transesterification mechanism yielding narrow distributions of odd-numbered cycles is discovered too. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2730–2738     

Size dependence of the folding of multiply charged sodium cationized polylactides revealed by ion mobility mass spectrometry and molecular modelling

Ion mobility spectrometry coupled with mass spectrometry was used to experimentally determine the three-dimensional structure of multiply charged sodium cationized polylactides (PLA). In particular, the experiments were conducted to evaluate the influence of the charge state and the size on the gas-phase conformation of cationized PLA. The measured collision cross sections were then compared to calculated values obtained by computational chemistry methods. The most striking feature was the experimental and theoretical observation of a breaking point in the quasilinear relationship between the average collision cross sections and the number of monomer units for the triply charged cations. This breaking point was theoretically demonstrated, for the doubly and triply charged cations, to be associated with a significant folding of the polymer chains around the cationizing agents. The occurrence of such breaking points could be exploited to correlate the charge state of the most intense ion series observed upon electrospray ionization with the number-average molecular mass of a polymer.     

Hydrogen‐Bonding Catalysts Based on Fluorinated Alcohol Derivatives for Living Polymerization

Recognize this! A hydrogen‐bonding motif based on hexafluorinated alcohol derivatives (see picture; O red, F yellow) activates electrophilic substrates. The catalytic activity of the hydrogen‐bonded systems was demonstrated for the ring‐opening polymerization of a variety of strained heterocycles. Narrowly dispersed polymers with predictable molecular weights were obtained with end‐group fidelity.

     

Polylactide/exfoliated graphite nanocomposites with enhanced thermal stability,mechanical modulus,and electrical conductivity

We have prepared a series of polylactide/exfoliated graphite (PLA/EG) nanocomposites by melt‐compounding and investigated their morphology, structures, thermal stability, mechanical, and electrical properties. For PLA/EG nanocomposites, EG was prepared by the acid treatment and following rapid thermal expansion of micron‐sized crystalline natural graphite (NG), and it was characterized to be composed of disordered graphite nanoplatelets. It was revealed that graphite nanoplatelets of PLA/EG nanocomposites were dispersed homogeneously in the PLA matrix without forming the crystalline aggregates, unlike PLA/NG composites. Thermal degradation temperatures of PLA/EG nanocomposites increased substantially with the increment of EG content up to ～3 wt %, whereas those of PLA/NG composites remained constant regardless of the NG content. For instance, thermal degradation temperature of PLA/EG nanocomposite with only 0.5 wt % EG was improved by ～10 K over PLA homopolymer. Young's moduli of PLA/EG nanocomposites increased noticeably with the increment of EG content up to ～3 wt %, compared with PLA/NG composites. The percolation threshold for electrical conduction of PLA/EG nanocomposites was found to be at 3–5 wt % EG, which is far lower graphite content than that (10–15 wt % NG) of PLA/NG composites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 850–858, 2010     

Clinical Application of Resorbable Polymers in Guided Bone Regeneration

Summary : Guided bone regeneration was shown to be successful in vitro and in vivo using resorbable or nonresorbable materials. Resorbable material has the advantage of progressive substitution by bone. Resorbable polymers of ∝-hydroxy acids like polylactide or polyglycolide are commonly used for tissue engineering and in guided bone regeneration. In clinical studies, guided bone regeneration was successful in non-weight bearing bone, e.g. in dental surgery and craniofacial surgery. This paper reports the preliminary result of using resorbable poly(L/DL-lactide) 80/20% scaffolds in weight bearing bone with infected large segmental defects as well as in small bony defects of hand due to benign tumour, bone graft donor sites and as an adjunct for joint fusion. Resorbable polylactide implants were used in the form of membranes, large 3-D sponges, chips or as injectable paste. Implants were impregnated with marrow blood to add an osteoinductive component. Long-term follow up revealed that these implants are promising candidates for bone graft substitutes.     

Homogeneous poly(L‐lactic acid)/chitosan blended films

This work reports an optimized and simple methodology for the preparation of poly(L‐lactic) acid/chitosan (CHT) blends by solvent casting based on the use of a common solvent: hexafluor‐2‐propanol. Films with different component fractions were successfully prepared and did not show visible phase separation. Such biodegradable films have potential to be used in distinct biomedical and environmental applications. The composition effect on film wettability and morphology was investigated by contact angle measurements and scanning electron microscopy. Swelling measurements were also conducted. The composition effect on their thermal properties was analyzed by differential scanning calorimetry. It was found that crystallization is almost suppressed for CHT fractions above 50%. The film miscibility as a function of their composition was evaluated by optical microscopy and Fourier transform infrared spectroscopy imaging. These results evidenced the good miscibility at the microscopic level of the blends. The viscoelastic behavior of the developed films was also studied for the first time by dynamical mechanical analysis (DMA) in an unconventional way: their mechanical properties were measured while they were immersed in gradient compositions of water/ethanol mixtures. This allowed to analyze the glass transition dynamics of the CHT fraction, which would not be possible with conventional DMA tests. DMA temperature scans were also conducted. Copyright © 2014 John Wiley & Sons, Ltd.     

Organically Modified Layered Titanate: A New Nanofiller to Improve the Performance of Biodegradable Polylactide

Summary: The preparation and characterization of a new type of nanocomposite material that is based on biodegradable polylactide (PLA) and organically modified layered titanate, is reported. Layered titanate, modified with a N‐(cocoalkyl)‐N,N‐[bis(2‐hydroxyethyl)]‐N‐methylammonium cation, was used as a new nanofiller (OHTO) for the nanocomposite preparation. Wide‐angle X‐ray diffraction and transmission electron microscopy (TEM) analyses confirmed that titanate layers were intercalated and nicely distributed in the PLA matrix. The materials properties of neat PLA improved remarkably after nanocomposite preparation.

UV/vis transmission spectra of neat PLA and a nanocomposite.