Enantiomeric Poly Research Articles

Stereocomplex formation between enantiomeric poly(α‐methyl‐α‐ethyl‐β‐propiolactones): Effect of molecular weight

AbstractOptically pure S(−) and R(+)‐poly(α‐methyl‐α‐ethyl‐β‐propiolactones) (PMEPLs) of controlled low molecular weights were synthesized by anionic polymerization of the corresponding optically active monomers, and characterized using gel permeation chromatography, Maldi‐TOF mass spectrometry, and NMR spectroscopy. Blends of PMEPLs of opposite configurations and different molecular weights were investigated. All blends lead to the formation of a stereocomplex and its crystallization prevails over a wide range of mixing ratios. The stereocomplex melts 30–40 °C above that of the corresponding pure polymers, depending on the molecular weight; pairs of polymers having similar molecular weights exhibit the highest melting temperatures and enthalpies of fusion. Finally, when the stereocomplex is dispersed in a PMEPL matrix, it acts as a very effective nucleation agent for the crystallization of the polymer in excess. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2380–2389, 2007

Stable Dispersions of Highly Anisotropic Nanoparticles Formed by Cocrystallization of Enantiomeric Diblock Copolymers

Stable dispersions of anisotropic particles with nanometer dimensions can be prepared in a selective solvent through cocrystallization (stereocomplexation) between two enantiomeric poly(lactide) block copolymers: poly(l-lactide)−poly(e-caprolactone) and poly(d-lactide)−poly(e-caprolactone). In the conditions of the experiment, no unspecific aggregation of the homochiral polylactide blocks occurs. The only driving force for self-assembly is between PLLA and PDLA. The good colloidal stability of the dispersion makes it possible to study the evolution of the particle shape and dimensions with time. The formation of these highly anisotropic particles was studied by a combination of techniques (infrared spectroscopy, light scattering, small angle neutron scattering, and atomic force microscopy).

Study of surface properties of blend films using stereocomplexation between enantiomeric polylactide chains

This study investigated the stereocomplex-induced surface structure of enantiomeric poly(lactide) (PLA) blend films by differential scanning calorimeter, electron spectrometer for chemical analysis, and contact angle measurements. The design of the blend systems is based on principles of surface segregation of a modified component with a low surface energy, fluorocarbons (F), as an end group. The two blend systems, uncomplexed (F-l-PLA/l-PLA) and complexed (F-l-PLA/d-PLA), showed quite different surface structures: the surface segregation of fluorocarbon groups in the (F-l-PLA/l-PLA) blend film was observed while the surface structure of the (F-l-PLA/d-PLA) blend film was similar to its bulk one. This indicates that the interchain interaction to form stereocomplexes between l- and d-PLA is strong enough to overcome the driving force of fluorocarbon groups toward the surface.

Thermoplastic elastomers based on poly(lactide)–poly(trimethylene carbonate-co-caprolactone)–poly(lactide) triblock copolymers and their stereocomplexes

Triblock copolymers of poly(l-lactide)–poly(trimethylene carbonate-co-caprolactone)–poly(l-lactide) and poly(d-lactide)–poly(trimethylene carbonate-co-caprolactone)–poly(d-lactide) were prepared by sequential ring-opening polymerizations. These polymers are thermoplastic elastomers (TPEs) with good tensile properties and excellent resistance to creep. In equimolar blends of these enantiomeric copolymers, stereocomplexation between the enantiomeric poly(lactide) segments occurs. Due to the enhanced phase separation, the specimens of stereocomplex have even better resistance to creep than the enantiomeric block copolymers.

Self-Assembly of Stereocomplex-Type Poly(lactic acid)

Enantiomeric poly(lactide)s were assembled by casting solutions of poly(L-lactic acid) and poly(D-lactic acid) with different drying process and annealing procedures. Crystallization in these films was studied by differential scanning calorimetric study and wide-angle X-ray diffraction. Stereocomplex crystallization occurred more predominantly than homo crystal during casting and annealing. Atomic force microscopic observation showed solution-grown single crystal like structure on the substrate surface. We found that stereocomplex crystal formation was enhanced via homo crystal formation. On the basis of these findings, a mechanism is proposed for the crystallization of enantiomeric poly(lactic acid)s when they are cast from dilute solution.

Enantiomeric PLA–PEG block copolymers and their stereocomplex micelles used as rifampin delivery

A novelty approach to self-assembling stereocomplex micelles by enantiomeric PLA–PEG block copolymers as a drug delivery carrier was described. The particles were encapsulated by enantiomeric PLA–PEG stereocomplex to form nanoscale micelles different from the microspheres or the single micelles by PLLA or PDLA in the reported literatures. First, the block copolymers of enantiomeric poly(l-lactide)–poly(ethylene–glycol) (PLLA–PEG) and poly(D-lactide)–poly(ethylene–glycol) (PDLA–PEG) were synthesized by the ring-opening polymerization of l-lactide and d-lactide in the presence of monomethoxy PEG, respectively. Second, the stereocomplex block copolymer micelles were obtained by the self-assembly of the equimolar mixtures of enantiomeric PLA–PEG copolymers in water. These micelles possessed partially the crystallized hydrophobic cores with the critical micelle concentrations (cmc) in the range of 0.8–4.8 mg/l and the mean hydrodynamic diameters ranging from 40 to 120 nm. The micelle sizes and cmc values obviously depended on the hydrophobic block PLA content in the copolymer. Compared with the single PLLA–PEG or PDLA–PEG micelles, the cmc values of the stereocomplex micelles became lower and the sizes of the stereocomplex micelles formed smaller. And lastly, the stereocomplex micelles encapsulated with rifampin were tested for the controlled release application. The rifampin loading capacity and encapsulation efficiency by the stereocomplex micelles were higher than those by the single polymer micelles, respectively. The drug release time in vitro was depending on the composites of the block copolymers and also could be controlled by the polymer molecular weight and the morphology of the polymer micelles.

Surface Structure and Stereocomplex Formation of Enantiomeric Polylactide Blends Using Poly(dimethyl siloxane) as a Probe Polymer

AbstractIn the stereocomplex between enantiomeric poly(l‐lactide) (l‐PLA) and poly(d‐lactide), crystallites formed as a result of stereocomplexation, equimolar l‐ and d‐lactide unit sequences are packed side by side. The stereocomplex exhibits a melting temperature higher by about 50 °C than that of each homopolymer. In this study, we attempt to obtain further insight into the stereocomplex‐induced surface structure of enantiomeric PLA blend films. The design of the blend systems is based on principles of surface segregation of multicomponent polymeric systems with a low surface energy, triblock copolymer (l‐PLA‐b‐PDMS‐b‐l‐PLA) of l‐PLA and poly‐(dimethyl siloxane). (l‐PLA‐b‐PDMS‐b‐l‐PLA/l‐PLA) blend films showed the surface segregation of PDMS, regardless of blend composition while the surface composition of PDMS in the (l‐PLA‐b‐PDMS‐b‐l‐PLA/d‐PLA) blend films was strongly depended on blend composition or a degree of complexation. These results are likely due to strong interaction between d‐ and l‐lactide unit sequences, which prevents the surface segregation of PDMS.

Hydrolytic Behavior of Enantiomeric Poly(lactide) Mixed Monolayer Films at the Air/Water Interface: Stereocomplexation Effects

The Langmuir film balance technique was used to determine the hydrolytic kinetics of monolayers of the stereocomplex formed from mixtures of enantiomeric polylactides, poly(L-lactide) (L-PLA) and poly(D-lactide) (D-PLA), spread at the air-water interface. The present study investigated parameters such as degradation medium, mixture composition, and time on the relative degradation rate. The pi-A isotherms of monolayers of the mixtures provide clear evidence for the presence of a stereocomplex; the isotherms of monolayers of individual polyenantiomer show a transition at about 8.5 mN/m, whereas the transition of monolayers containing a stereocomplex formed from the equimolar mixture shifted to higher surface pressure, about 11 mN/ m. The rate of hydrolysis was recorded by a change in occupied area when the monolayer is maintained at a constant surface pressure. The hydrolysis of the mixture monolayers under basic conditions was slower than that of individual polyenantiomer monolayers, depending on the composition or the degree of complexation. In the presence of proteinase K, the enzymatic hydrolysis rate of mixture monolayers with >50 mol % l-PLA was much slower than that of the single-component L-PLA monolayer. The monolayers formed from mixtures with < or =50 mol % L-PLA did not show any change of occupied areas. This result is explained by the inactivity of D-PLA and stereocomplexed chains to the enzyme. From both results, it can be concluded that the retardation of the hydrolysis of mixture monolayers is mainly due to a strong interaction between D- and L-lactide unit sequences, which prevents the penetration of water or enzyme into the bulk.

Hydrolytic kinetics of langmuir monolayers of enantiomeric poly(lactide)s

The hydrolytic behavior of monolayers of enantiomeric poly(lactide)s (PLAs), d-PLA and l-PLA, and their mixtures spread at the air/water interface has been studied in order to elucidate the hydrolytic stability on the stereocomplex formation of enantiomeric components. The rate of hydrolysis was recorded by a change of occupied area when the monolayer is maintained at a constant surface pressure. Under a basic subphase, the hydrolysis of the (d-PLA/l-PLA 50/50 by mol.%) mixture monolayer was much slower than those of each homopolymer. This result suggests that the stereocomplexed monolayer is more hydrolysis-resistant than each homopolymer one.

Stereocomplex formation between enantiomeric poly(lactic acid)s. 12. spherulite growth of low-molecular-weight poly(lactic acid)s from the melt.

The spherulite growth of stereocomplex crystallites in the blend from low-molecular-weight poly(L-lactide) [i.e., poly(L-lactic acid) (PLLA)] and poly(D-lactide) [i.e., poly(D-lactic acid) (PDLA)] from the melt, together with that of the homocrystallites in pure PLLA and PDLA films, was investigated using polarization optical miscroscopy. The spherulite growth of stereocomplex crystallites occurred at a wider temperature range (</=190 degrees C) compared with that of homocrystallites (</=140 degrees C). At 140 degrees C, the spherulite radius growth rate (G) for the stereocomplex crystallites (136.4 microm min(-1)) was an order of magnitude higher than those for the homocrystallites of PLLA (11.8 microm min(-1)) and PDLA (15.7 microm min(-1)), whereas the induction period was shorter for the spherulties of stereocomplex crystallites (0.0 min) than for the spherulties of homocrystallites of PLLA (2.6 min) and PDLA (0.7 min). In addition to these two factors, the higher spherulite density of stereocomplex crystallites compared with those of the homocrystallites of PLLA and PDLA resulted in rapid completion of overall crystallization of stereocomplex. The front factor (G(0)) and nucleation constant (K(g)) for the stereocomplex crystallites in the temperature range of 140-190 degrees C were estimated to be 3.56 x 10(12) microm min(-1) and 8.42 x 10(5) K(2), respectively. The G(0) value for stereocomplex crystallites was 1 and 2 orders of magnitude higher than those for the homocrystallites of PLLA (9.69 x 10(11) microm min(-1)) and PDLA (8.79 x 10(10) microm min(-1)), whereas the K(g) value for stereocomplex crystallites was twice those for the homocrystallites of PLLA (4.95 x 10(5) K(2)) and PDLA (4.20 x 10(5) K(2)).

Triblock Copolymers Based on 1,3‐Trimethylene Carbonate and Lactide as Biodegradable Thermoplastic Elastomers

AbstractSummary: Biodegradable triblock copolymers based on 1,3‐trimethylene carbonate (TMC) and different lactides (i.e. D,L‐lactide(DLLA), L‐lactide (LLA), D‐lactide (DLA)) designated as poly(DLLA‐TMC‐DLLA), poly(LLA‐TMC‐LLA) and poly(DLA‐TMC‐DLA) were prepared and their mechanical and thermal properties were compared with those of high molecular weight poly(TMC) and poly(TMC‐co‐DLLA) statistical copolymers. Triblock copolymers containing crystallizable LLA or DLA segments perform as thermoplastic elastomers (TPEs) when the poly(lactide) blocks are long enough to crystallize. In blends of poly(LLA‐TMC‐LLA) and poly(DLA‐TMC‐DLA) triblock copolymers, stereo‐complex formation between the enantiomeric poly(lactide) segments occurs as demonstrated by differential scanning calorimetry and light microscopy. These blends have good tensile properties and excellent resistance to creep under static and dynamic loading conditions.Permanent deformation (after 2 h recovery) of compression‐molded poly(TMC) and solvent‐cast poly(LLA‐TMC‐LLA) and poly(ST‐TMC‐ST) films.imagePermanent deformation (after 2 h recovery) of compression‐molded poly(TMC) and solvent‐cast poly(LLA‐TMC‐LLA) and poly(ST‐TMC‐ST) films.

Alkaline Hydrolysis of Enantiomeric Poly(lactide)s Stereocomplex Deposited on Solid Substrates

Alkaline Hydrolysis of Enantiomeric Poly(lactide)s Stereocomplex Deposited on Solid Substrates

Higher Water Intrusion Property on Novel Porous Matrix Composed of Bioinspired Polymer Stereocomplex for Tissue Engineering

Abstract A porous matrix composed of bioinspired phospholipid polymers with enantiomeric poly(lactic acid) segments in side chain, were prepared by formation of stereocomplex between them and characterized for use as new cell culture matrix.

In vitro hydrolysis of blends from enantiomeric poly(lactide)s. 3. Homocrystallized and amorphous blend films.

Homocrystallized and amorphous enantiomeric blend films were prepared from the melt of high molecular weight poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) (1:1) by crystallization and quenching, respectively. A phosphate-buffered solution was used to investigate effects of homocrystallinity via in vitro hydrolysis as well as crystallization process during the hydrolysis, which was performed for a period of 24 months at 37 degrees C and pH 7.4. Results derived from gravimetry, gel permeation chromatography, and tensile testing showed that hydrolyzability was higher for the homocrystallized film than for the amorphous film. Thus, probable mechanisms are proposed for the enhanced hydrolysis of the homocrystallized blend film compared with that of the amorphous blend film. The hydrolysis rate constant (k) values of the homocrystallized and amorphous films estimated from the changes in number-average molecular weight (M(n)) were 5.00 x 10(-3) and 3.32 x 10(-3) day(-1), respectively. Moreover, hydrolyzability of equimolar enantiomeric poly(lactic acid) blends can be altered in the k range of 0.73 x 10(-3) and 5.00 x 10(-3) day(-1) by varying their crystalline species, crystallinity, or molecular weights.

Cell adhesion and morphology in porous scaffold based on enantiomeric poly(lactic acid) graft-type phospholipid polymers.

Poly(D-lactic acid) (PDLA) and poly(L-lactic acid) (PLLA) macromonomers were synthesized for preparation of a novel cytocompatible polymer. The cytocompatible polymer was composed of 2-methacryloyloxyethyl phosphorylcholine (MPC), n-butyl methacrylate (BMA), and the enantiomeric PLLA (or PDLA) macromonomer. The degree of polymerization of the lactic acid in the PLLA and PDLA segments was designed to be ca. 20. The copolymer-coated surface was analyzed with static contact angle by water. From the result, the PLLA (or PDLA) segment and MPC unit were located on the coated surface, and the monomer unit in the copolymer was reconstructed by contacting water. Fibroblast cell culture was performed to evaluate cell adhesion on the coated surface, and the cell morphology was observed. The number of cell adhesion is correlated with the PL(D)LA content, and the cell morphology is correlated with the MPC unit content. The porous scaffold was prepared by the formation of a stereocomplex between the PLLA and PDLA, and the cell adhesion and following cell intrusion was then evaluated. The fibroblast cells adhered on the surface and intruded into the scaffold through the connecting pores after 24 h. The cell morphology became round shape from spreading with the decreasing PLLA (or PDLA) content in the copolymer. It is considered that the change in the cell morphology would be induced by the MPC unit as cytocompatible unit. These findings suggest that the porous scaffold makes it possible to have cytocompatibility and to produce three-dimensional tissue regeneration.

In vitro hydrolysis of blends from enantiomeric poly(lactide)s. Part 4: well-homo-crystallized blend and nonblended films

Well-homocrystallized enantiomeric blend and nonblended films were prepared from poly( l-lactide), i.e., poly( l-lactic acid) (PLLA) and poly( d-lactide), i.e., poly( d-lactic acid) (PDLA) by crystallization from the melt. The effects of enantiomeric blending on the in vitro autocatalytic hydrolysis of the homo-crystallized polylactide, i.e., poly(lactic acid) (PLA) films were investigated in phosphate-buffered solution (pH 7.4) at 37°C for up to 24 months. In the period of 0–12 months, the effects of enantiomeric polymer blending on the autocatalytic hydrolysis were very small. This finding reflects that in the PLLA/PDLA blend film separate homo-crystallization of PLLA and PDLA into the respective crystallites reduced the peculiar strong interaction between PLLA and PDLA chains in the amorphous region between the homo-crystalline regions. In the period of 12–24 months, enantiomeric polymer blending significantly retarded the autocatalytic hydrolysis of the PLLA/PDLA blend film compared with that of the nonblended PLLA and PDLA films. This is attributable to the increased chain mobility and the reduced entanglement effects due to the chain cleavage to a great extent, resulting in the enhanced interaction between PLLA and PDLA chains. It was also revealed that the hydrolyzabilities of the PLA films can be widely varied by enantiomeric polymer blending, crystalline species and their amounts, and molecular weight.

Stereocomplex formation by enantiomeric poly(lactic acid) graft-type phospholipid polymers for tissue engineering.

A porous scaffold as a cell-compatible material was designed and prepared using a phospholipid copolymer composed of 2-methacryloyloxyethyl phosphorylcholine (MPC), n-butyl methacrylate, and enantiomeric macromonomers, the poly(L-lactic acid) (PLLA) macromonomer, and poly(D-lactic acid) (PDLA) macromonomer. On the basis of the wide-angle X-ray diffraction and differential scanning calorimetry measurements, the formation of a stereocomplex between the PLLA and PDLA segments of the copolymer was observed on the porous scaffold. The porous structure was prepared by a sodium chloride leaching technique, and the pore was linked to the scaffold. The pore size was confirmed by scanning electron microscopy and found to be ca. 200 microm. These observations suggest that the porous scaffold makes it possible to produce cell-compatible materials, which may involve the following advantages for tissue engineering: (i) cell compatibility using phospholipid copolymer, (ii) adequate cell adhesion by poly(lactic acid), and (iii) complete disappearance of scaffold by dissociation of stereocomplex. The cell experiment using the porous scaffold will be the next subject and reported in a forthcoming paper.

Stepwise Assembly of Enantiomeric Poly(lactide)s on Surfaces

Enantiomeric poly(lactide)s were assembled on a quartz crystal microbalance (QCM) substrate, which detects the mass of the polymers from the frequency shift, following immersion of QCM into alternating acetonitrile solutions. A quantitative QCM analysis at each step and a differential scanning calorimetric study of the assembly showed racemic crystal (stereocomplex) formation on the substrate surface. Atomic force microscopic observation showed a dotted nanostructure of the assembly. The assembly amount was increased with increasing the PLA concentration and the immersion time, while that was decreased with increasing the assembly temperature. The heterogeneous assembly was also prepared by altering the immersion time. We found that racemic crystal formation was applied to the alternate deposition of certain structurally regulated polymers.

In Vitro Hydrolysis of Blends from Enantiomeric Poly(lactide)s. 2. Well-Stereocomplexed Fiber and Film.

Well-stereocomplexed 1:1 fiber and film were prepared from poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) by the melt-spinning and hot-drawing and the solution-casting methods, respectively, and their hydrolysis in phosphate-buffered solution (pH 7.4) at 37°C was investigated. The decrease rates of weight remaining, tensile strength, elongation-at-break, and melting temperature (Tm) during hydrolysis were higher for the fiber than for the film. The accelerated hydrolysis of the fiber was ascribed to the high concentrations of the catalytic oligomers and monomers during hydrolysis, resulting from their high initial concentration caused by thermal degradation during melt-spinning and hot-drawing and from their retarded elution from the fiber due to the bulky microscopic shape. The changes in Tm, crystallinity, and weight remaining suggest that the hydrolysis of the fiber proceeds mainly via bulk erosion mechanism, that the chain cleavage of the fiber occurred in the crystalline region and at the folding surface as well as in the other amorphous region, and that lamella thickening of the film took place without significant chain cleavage in the crystalline region and at folding surface.

A new class of biodegradable hydrogels stereocomplexed by enantiomeric oligo(lactide) side chains of poly(HEMA‐ g ‐OLA)s

Two enantiomeric amphiphilic graft copolymers consisting of water soluble poly(2-hydroxyethyl methacrylate) (HEMA) and biodegradable oligo(L-lactide) (OLLA) or oligo(D-lactide) (ODLA) were synthesized by free radical copolymerization. HEMA-OL(D)LA macromonomers were synthesized by ring opening polymerization of L- or D-lactide. Both HEMA-OLA macromonomers and graft copolymers were characterized by NMR spectroscopy and gel permeation chromatography. Graft copolymers and their stereocomplexes were analyzed by wide angle X-ray diffraction and differential scanning calorimetry (DSC). Due to the formation of stereocomplex crosslinks between poly(HEMA) main chains, amphiphilic, biodegradable hydrogels prepared by blending of two enantiomeric poly(HEMA-g-OLLA) and poly(HEMA-g-ODLA) degraded more slowly in phosphate buffered saline than individual optically pure poly-(HEMA-g-OL(D)LA).