Synthesis,physical properties,and crystallization of optically active poly(L‐phenyllactic acid) and poly(L‐phenyllactic acid‐co‐L‐lactic acid)

Optically active poly(L ‐phenyllactic acid) (Ph‐PLLA), poly(L ‐lactic acid) (PLLA), and poly(L ‐phenyllactic acid‐co‐L ‐lactic acid) with weight‐average molecular weight exceeding 6 × 10³ g mol^?1 were successfully synthesized by acid catalyzed direct polycondensation of L ‐phenyllactic acid and/or L ‐lactic acid in the presence of 2.5–10 wt % of p‐toluenesulfonic acid. Their physical properties and crystallization behavior were investigated by differential scanning calorimetry, thermogravimetry, and polarimetry. The absolute value of specific optical rotation ([α]) for Ph‐PLLA (?38 deg dm^?1 g^?1 cm³) was much lower than that of [α] for PLLA (?150 deg dm^?1 g^?1 cm³), suggesting that the helical nature was reduced by incorporation of bulky phenyl group. PLLA was crystallizable during solvent evaporation, heating from room temperature, and cooling from the melt. Incorporation of a very low content of bulky phenyllactyl units even at 4 mol % suppressed the crystallization of L ‐lactyl unit sequences during heating and cooling, though the copolymers were crystallizable for L ‐phenylactyl units up to 6 mol % during solvent evaporation. The activation energy of thermal degradation (ΔE_td) for Ph‐PLLA (200 kJ mol^?1) was higher than that for PLLA (158 kJ mol^?1). The ΔE_td for the copolymers increased with an increase in L ‐phenyllactyl unit content. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mechanical and Thermal Properties of Poly(L‐lactide) Incorporating Various Inorganic Fillers with Particle and Whisker Shapes

Poly(L ‐lactide) (PLLA) composites incorporating various inorganic fillers (ifR‐PLA) were prepared by the melt blending technique, and their mechanical and thermal properties were evaluated. The filler types influenced the mechanical properties of ifR‐PLA; for those incorporating particle‐ and whisker‐type fillers the tensile moduli were 3.1–3.7 and 3.7–4.5 GPa, respectively, and the flexural moduli were 4.1–4.8 and 4.8–6.1 GPa. It was found that the tensile strength and modulus, as well as the flexural modulus, of ifR‐PLA incorporating whisker‐type fillers increased in proportion to the volume percent of the fillers (V_f). The flexural strength of ifR‐PLA incorporating 9Al₂O₃ · 2B₂O₃ whiskers showed a similar increase, while that of ifR‐PLA incorporating CaCO₃ whiskers showed a decrease with increasing V_f. This difference may be because the 9Al₂O₃ · 2B₂O₃ with its large aspect ratio kept its original fibrous shape, while the CaCO₃ lost its fibrous shape during the blending process. However, the reinforcing effect of these fillers was relatively low compared with that known for the corresponding composites of the conventional polymeric materials, probably because of the poor surface adhesion of PLLA to the fillers.

Comparison of effect on storage moduli of different fillers.     

Photodegradation of Poly(L‐lactic acid): Effects of Photosensitizer

Summary: Amorphous and crystallized poly(L ‐lactic acid) (PLLA‐A and PLLA‐C, respectively) films with different contents of N,N,N′,N′‐tetramethyl‐1,4‐phenylenediamine (TMPD) as a photosensitizer were prepared, and the effects of the addition of TMPD on the photodegradation of PLLA films were investigated. It was found that the addition of TMPD effectively enhanced the photodegradation of PLLA films and thereby decreased their molecular weight of PLLA films regardless of their crystallinity, and that PLLA films with different molecular weights can be prepared by the addition of different amounts of TMPD and subsequent UV irradiation. Too high contents of TMPD however caused the brittleness of PLLA films due to a large decrease in molecular weight. The PLLA chains in crystalline regions as well as those in amorphous regions are photodegradable even at an early stage, in marked contrast to their hydrolytic degradation, where the chains in the amorphous regions are selectively degraded. The basic changes in glass transition, cold crystallization, and melting temperatures (T_g, T_cc, and T_m, respectively) of PLLA films during UV irradiation can be ascribed to low‐temperature annealing effects; i.e., annealing‐induced stabilization in chain packing should have elevated T_g, and annealing‐induced formation of crystallite nuclei should have lowered T_cc and increased T_m. The exceptional large decreases in T_cc and T_m of UV‐irradiated PLLA‐A films and in T_g of UV‐irradiated PLLA‐C films at high TMPD contents are attributable to the large decrease in molecular weight, whereas the exceptional decrease in T_m of PLLA‐C films at high TMPD contents can be due to the folding surface structural change of crystalline regions or to the lattice disorder caused by molecular structural changes.

of PLLA‐A films before UV irradiation and after UV irradiation for 60 h as a function of TMPD content.     

One‐Step Cationic Poly(D,L‐lactic acid) Particle Synthesis by Diafiltration and Interactions with Plasmid DNA

Summary: Diafiltration was used for the one step preparation of cationic poly(D ,L ‐lactic acid) particles in the presence of poly(ethylenimine) (PEI). First, parameters impacting the formation of plain PLA particles were investigated: the chemical nature of the solvent and the polymer concentration within the organic phase. Then, PEI was added to the organic solution and this mixture was subsequently dialyzed against water. The molecular architecture of the polycation was essential and colloidal particles were only obtained with a branched PEI of 25 000 g · mol⁻¹. Increasing the PEI concentration in the mixture led to a particle mean size reduction and a zeta‐potential increase. In any case, a maximum of 8% of the initial PEI input was recovered at the interface. Finally, the study of the interactions of these colloids with DNA showed that the amount of DNA bound at the saturation plateau increased with increasing inputs of PEI in the formulation.

SEM micrograph of cationic poly(D ,L ‐lactic acid) particles.     

Optical Resolution Membranes from Polysulfones Bearing Alanine Derivatives as Chiral Selectors

Polysulfones bearing a derivative of alanyl residue employed as chiral selectors were prepared by polymer modification. The specific rotation ([α]_D) of the polysulfone with a derivative of D ‐alanyl residue (PSf‐Ac‐D ‐Ala) was determined to be 2.87 deg · cm² · g^?1 (c = 1.00 g · dL^?1 in DMF) and that with L ‐alanyl residue (PSf‐Ac‐L ‐Ala) to be ‐2.36 deg · cm² · g^?1 (c = 1.00 g · dL^?1 in DMF). The membrane from PSf‐Ac‐D ‐Ala preferentially adsorbed the D ‐isomer of Glu from racemic mixture of Glu and vice versa. Chiral separation ability was studied by applying a potential difference as a driving force for membrane transport. The permselectivity of PSf‐Ac‐D ‐Ala toward D ‐Glu (α_D/L) was determined to be 1.40, and that of PSf‐Ac‐L ‐Ala toward the L ‐isomer (α_L/D) to be 1.48 at 18.0 V, reflecting their adsorption selectivity.

     

α‐L‐Rhamnosyl‐β‐D‐glucosidase (Rutinosidase) from Aspergillus niger: Characterization and Synthetic Potential of a Novel Diglycosidase

We report the first heterologous production of a fungal rutinosidase (6‐O‐α‐L ‐rhamnopyranosyl‐β‐D ‐glucopyranosidase) in Pichia pastoris. The recombinant rutinosidase was purified from the culture medium to apparent homogeneity and biochemically characterized. The enzyme reacts with rutin and cleaves the glycosidic linkage between the disaccharide rutinose and the aglycone. Furthermore, it exhibits high transglycosylation activity, transferring rutinose from rutin as a glycosyl donor onto various alcohols and phenols. The utility of the recombinant rutinosidase was demonstrated by its use for the synthesis of a broad spectrum of rutinosides of primary (saturated and unsaturated), secondary, acyclic and phenolic alcohols as well as for the preparation of free rutinose. Moreover, the α‐L ‐rhamnosidase‐catalyzed synthesis of a chromogenic substrate for a rutinosidase assay – para‐nitrophenyl β‐rutinoside – is described.

     

Methionine‐Containing Poly(amino acid) Hybrid Fibers via Self‐Assembly at Aqueous Interface

Summary: Using sulfonium groups to create a novel fiber material, methionine‐containing hybrid fibers were prepared from S‐methylated poly(L ‐methionine) and poly(L ‐lysine, L ‐methionine) solutions with gellan solution by polyion complex (PIC) formation via self‐assembly at the aqueous interface. The breaking strain of the PIC fibers were increased by incorporation of methionine residues into the poly(L ‐lysine). These findings may provide a new approach for preparing a wool‐like fiber in aqueous media using the synthetic water‐soluble methionine‐containing poly(amino acid)s.

SEM image of Met‐containing PIC fiber: (a) poly[Met¹⁹Met(SMe)⁸¹]‐gellan fiber (magnification, ×500).     

Copper‐Catalyzed Cycloaddition between Secondary Phosphine Oxides and Alkynes: Synthesis of Benzophosphole Oxides

Benzophosphole derivatives have attracted significant attention as promising organic optoelectronic materials. We have successfully developed a copper (2 mol%)/tert‐butyl hydroperoxide (2 equiv.) catalyst system for reaction of readily available secondary phosphine oxides and alkynes in acetonitrile at 60 °C under air, which provides a rapid access to a structurally diverse array of benzophosphole oxides in moderate to good yields within 30 min. The method can be easily used for a large‐scale preparation. Preliminary mechanistic studies revealed that the addition of a phosphoryl radical onto a triple bond followed by cyclization on the phenyl moiety of the secondary phosphine oxide might occur to form the benzophosphole oxide.

     

Reactive Extrusion of Stereocomplexed Poly‐L,D‐lactides: Processing,Characterization, and Properties

This work reports a facile route to synthesize homochiral and stereocomplexed polylactide by reactive extrusion. The effect of the polymerization catalyst (combination of tin(II)octanoate and triphenylphosphine) before and after its deactivation is discussed. Poly‐L ‐lactide (PLLA) exhibits homochiral crystallinity and diblock poly‐L ,D ‐lactide (PDLLA) exhibits stereocomplex crystallinity. The presence of residual monomer leads to a plasticizing effect, reducing glass transition temperature (T_g). Changes of the tacticity (L ,D ‐tacticity) of the stereocomplex are due to the transesterification reactions between L and D units. Deactivation of the catalyst reduces transesterification reactions and preserves the polylactide stereocomplex upon heating.

     

Hydrolytic Degradation of Amorphous Films of L‐Lactide Copolymers with Glycolide and D‐Lactide

Summary: Films of poly(L ‐lactic acid) (PLLA) and copolymers of L ‐lactide (LLA) with either glycolide [P(LLA‐GA)](81/19) or D ‐lactide [P(LLA‐DLA)](77/23) were prepared and an effect of comonomer type on the hydrolytic degradation of the films was studied in phosphate‐buffered solutions at 37 °C. The degraded films were investigated using gravimetry (weight loss and water absorption), gel permeation chromatography, DSC, X‐ray diffractometry, tensile testing and polarization optical microscopy. To exclude the effects of molecular weight and crystallinity on hydrolytic degradation, the films were prepared from polymers with similar molecular weights and were made amorphous by melt quenching. It was found that the hydrolytic degradation rate decreased in the order P(LLA‐GA) > P(LLA‐DLA) > PLLA. The hydrolytic degradation rate constant of PLLA and LLA copolymer films increased with increasing the water absorption (hydrophilicity), or with decreasing the initial glass transition temperature or the L ‐lactyl unit sequence length, indicating that the hydrolytic degradation rate of the copolymers was closely related to these three parameters. The crystallization of P(LLA‐GA) film occurred within hydrolytic degradation for 20 weeks.

M_n of PLLA and LLA copolymer films as a function of hydrolytic degradation time.     

Non‐Isothermal Crystallization Behavior of Poly(L‐lactic acid) in the Presence of Various Additives

Summary: The effects of various additives: poly(D ‐lactic acid) (PDLA), talc, fullerene C₆₀, montmorillonite, and various polysaccharides, on the non‐isothermal crystallization behavior of poly(L ‐lactic acid) (PLLA), during both the heating of melt‐quenched films from room temperature, and the cooling of as‐cast films from the melt, was investigated. When the melt‐quenched PLLA films were heated from room temperature, the overall PLLA crystallization was accelerated upon addition of PDLA or the stereocomplex crystallites formed between PDLA and PLLA, the mixtures containing PDLA, and the mixture of talc and montmorillonite. No significant effects on the overall PLLA crystallization were observed for talc, C₆₀, montmorillonite, and the mixtures containing C₆₀. Such rapid completion of the overall PLLA crystallization upon addition of the aforementioned additives can be ascribed to the increased density (number per unit volume or area) of PLLA spherulites. When the as‐cast PLLA films were cooled from the melt, the overall PLLA crystallization completed rapidly, upon addition of PDLA, talc, C₆₀, montmorillonite, and their mixtures. Such rapid overall PLLA crystallization is attributable to the increased density of the PLLA spherulites and the higher nucleation temperature for PLLA crystallization. In contrast, the addition of various polysaccharides has no significant effect, or only a very small effect, on the overall PLLA crystallization during heating from room temperature or during cooling from the melt. This finding means that the polysaccharides can be utilized as low‐cost fillers for PLLA‐based materials, without disturbing the crystallization of the PLLA. The effect of additives in accelerating the overall PLLA crystallization during cooling from the melt, decreased in the following order: PDLA > talc > C₆₀ > montmorillonite > polysaccharides.

Polarization optical photomicrographs of pure PLLA, and the PLLA‐F film, with the fullerene additive, during cooling from the melt (Process IIB). Both of the photomicrographs were taken at 120 °C.     

Tryptophan C5‐, C6‐ and C7‐Prenylating Enzymes Displaying a Preference for C‐6 of the Indole Ring in the Presence of Unnatural Dimethylallyl Diphosphate Analogues

The behavior of four dimethylallyltryptophan synthases (DMATSs) (5‐DMATS and 5‐DMATS_Sc as tryptophan C5‐prenyltransferases, and 6‐DMATS_Sa and 6‐DMATS_Sv as C6‐prenyltransferases) and one L ‐tyrosine prenyltransferase with a tryptophan C7‐prenyltransferase activity was investigated in the presence of two unnatural alkyl donors (methylallyl and 2‐pentenyl diphosphate) and one benzyl donor (benzyl diphosphate). Detailed biochemical investigations revealed the acceptance of these dimethylallyl diphosphate (DMAPP) analogues by all tested enzymes with different relative activities. Enzyme products with the allyl or benzyl moiety attached to different positions were identified in the reaction mixtures, whereby C‐6 alkylated or benzylated L ‐tryptophan was found as one of the main products. This observation demonstrates a preference of the five prenyltransferases toward C‐6 of the indole ring in the presence of unnatural DMAPP derivatives. Molecular dynamics simulation experiments with a homologous model of 5‐DMATS explained well its reactions with methylallyl and 2‐pentenyl diphosphate. Furthermore this study expands significantly the potential usage of tryptophan prenylating enzymes as biocatalysts for Friedel–Crafts alkylation.

     

Electrospun Composite Mats of Poly[(D,L‐lactide)‐co‐glycolide] and Collagen with High Porosity as Potential Scaffolds for Skin Tissue Engineering

Electrospun composite mats of poly[(D,L ‐lactide)‐co‐glycolide] and collagen with high porosities of 85–90% and extended pore sizes of 90–130 µm were prepared to mimic the ECM morphologically and chemically. The existence of collagen molecules on the fiber surface was confirmed, enabling the cells to find enhanced binding sites for their integrin receptors. The mechanical data for the blended fibrous mats indicated that they were sufficiently durable for dermal tissue engineering. Fibroblasts derived from GFP transgenic C57BL/6 mice were used to directly observe cell proliferation, and the inoculation of collagen enhanced cell attachment, proliferation and extracellular matrix secretion, which were found to be dependent on the amount of collagen in the composite scaffold.

     

Dramatic Improvements in Mechanical Properties of Poly(L‐lactide)/Silica Nanocomposites by Addition of Hyperbranched Poly(ester amide)

Biodegradable hyperbranched poly(ester amide) (HBP) was used as a compatibilizer to modify PLA/SiO₂ nanocomposites for the first time. The ternary composites displayed dramatically improved mechanical properties including excellent toughness and fairly high stiffness. TEM images revealed that an encapsulation structure was formed by HBP surrounding SiO₂ nanoparticles, and their surfaces became flocculent due to the migration process of silica. The linear viscoelastic behavior of the nanocomposites measured by parallel plate rheometer indicated that strong interface adhesion existed between PLA matrix and silica nanofiller after incorporating of HBP. The compatibilization effect of HBP and the enhanced mobility of nanoparticles contributed to the improved mechanical properties.

     

Effects of Temperature and Surfactants on the Equilibrium Swelling Behavior of Poly[acrylamide‐co‐(itaconic acid)] Hydrogels

Summary: The swelling equilibrium of poly(acrylamide) [PAAm] and poly[acrylamide‐co‐(itaconic acid)] [P(AAm/IA)] hydrogels was studied as a function of temperature and IA content in aqueous solutions of surfactants: sodium dodecyl sulfate (SDS, anionic) and hexadecyltrimethylammonium bromide (HTAB, cationic). P(AAm/IA) hydrogels in water exhibited reentrant conformational transitions depending on temperature, whereas PAAm hydrogels were not affected with the change of temperature. The equilibrium‐volume‐swelling ratio of P(AAm/IA) hydrogels increased sharply in SDS solutions, with an increase of the mole percent of IA. However, in HTAB solution, the equilibrium‐volume‐swelling ratio of these hydrogels decreased with an increase of IA content.

The equilibrium volume‐swelling ratios of the hydrogels in water shown as a function of temperature.     

Influence of Low‐Molecular‐Weight Diamines in the Direct Imidation of Poly(propylene)‐Grafted Maleic Anhydride by Melt Reaction

Aminated poly(propylene) was prepared by reacting aliphatic primary diamines with maleic‐anhydride‐functionalized poly(propylene) by in situ melt reaction. Around 60–70% of the initial acid groups had reacted to form amide and imide groups as confirmed by the almost complete disappearance of the maleic anhydride peak in FT‐IR spectra. The molecular weight of the diamines had an influence on changes in molecular structure of the PP‐g‐NH₂ as a result of secondary reactions such as chain extension and cross‐linking. PP‐g‐NH₂ and polycarbonate were pressed into two‐layer films and their adhesion strength was measured. The results showed that PP‐g‐NH₂ was a very effective adhesion promoter.

     

Site‐specific Protein Cross‐Linking with Genetically Incorporated 3,4‐Dihydroxy‐L‐Phenylalanine

Come together right now with L ‐DOPA : Chemical cross‐linking is widely used to study protein–protein interactions. However, many cross‐linking agents suffer from low reactivity or selectivity. An efficient and selective reaction of site‐specific protein cross‐linking was achieved using genetically incorporated 3,4‐dihydroxy‐L ‐phenylalanine.

     

Modeling of Ethylene‐Norbornene Copolymer Microstructure in Solution Polymerization with Homogeneous Metallocene Catalysts

Summary: The microstructure of ethylene‐norbornene copolymer produced in solution polymerization is analyzed through kinetic modeling and experiments using homogeneous rac‐Et(1‐indenyl)₂ZrCl₂/methylaluminoxane catalyst in toluene at 70 °C. The sequence distribution function and average chain length equations are derived for terminal model and penultimate model. The model simulations show that both models provide similar predictions of average copolymer composition, especially at low norbornene concentration in the copolymer. However, at higher norbornene concentrations the penultimate model yields much better predictions of norbornene sequence length distribution than the terminal model. The terminal model has been inadequate in describing the copolymerization rate, whereas the penultimate model yields excellent predictions of rate behavior. The model calculations also indicate that at norbornene concentration in the copolymer larger than about 10 mol‐%, the maximum ethylene block length is smaller than 70, prohibiting the formation of crystalline copolymer.

Ethylene sequence distribution curves at different mol‐% of norbornene in copolymer.     

Efficient and Selective Synthesis of E‐Vinylamines via Tungsten(0)‐Catalyzed Hydroamination of Terminal Alkynes

The hydroamination of terminal alkynes (RCCH=phenylacetylene, 4‐methylphenylacetylene, 4‐fluorophenylacetylene, 1‐hexyne, methyl 2‐propynyl ether, prop‐2‐yn‐1‐ol) with secondary amines (piperidine, pyrrolidine, morpholine, piperazine, methylpiperazine, 4‐methylpiperidine and 3‐methylpiperidine) was achieved in high yield (up to 99%), regioselectivity (only anti‐Markovnikov product) and stereoselectivity (only E‐isomers) within a maximum of 5 h in reactions catalyzed by the tungsten tetracarbonyl complex cis‐[W(CO)₄(piperidine)₂] at 90 °C without any additional solvent.

     

Co(acac)2‐Mediated Radical Polymerization of Acrylonitrile: Control Over Molecular Weights and Copolymerization With Methyl Methacrylate

The cobalt‐mediated radical polymerization of acrylonitrile in DMSO using cobalt (II) acetylacetonate [Co(acac)₂] as mediator is studied. Both the evolution of molecular weight and conversion over time under various conditions are monitored. Molecular weights increase sharply at the beginning of the reaction and subsequently grow linearly with conversion. No branching of the polymer is observed by ¹³C NMR. By a careful design of the reaction parameters, number‐average molecular weights >1.2 · 10⁵ g · mol^?1 with a PDI around 2.4 together with conversions of up to 90% within 24 h are achieved. The copolymerization parameters of acrylonitrile with methyl methacrylate in DMSO at 30 °C are determined using the Kelen‐Tüdõs approach giving r_AN = 0.33, r_MMA = 0.71.