Stereocomplex Formation Research Articles

A Regio- and Stereoselective Ring-Opening Polymerization Approach to Isotactic Alternating Poly(lactic-co-glycolic acid) with Stereocomplexation.

Poly(lactic-co-glycolic acid) (PLGA) has been widely employed for various biomedical applications owing to its biodegradability and biocompatibility. The discovery of the stereocomplex formation between enantiomeric alternating PLGA pairs underscored its potential as high-performance biodegradable materials with diverse material properties and biodegradability. Herein, we have established a regio- and stereoselective ring-opening polymerization approach for the synthesis of stereocomplexed isoenriched alternating PLGA from racemic methyl-glycolide (rac-MG). The high sequence and tacticity control was accomplished by an optimized enantiopure scandium catalyst bearing a spiro-salen scaffold. Varying polymer stereoregularity Pm from 0.4 to 0.91 led to a transformation of the resulting alternating PLGA from amorphous to semicrystalline materials. Notably, the stereocomplexed alternating PLGA demonstrated enhanced melting transition temperature (Tm up to 191 °C) and crystallization rate. This regio- and stereocontrolled polymerization represented a versatile approach for the preparation of high-performance biodegradable PLGA materials.

Stereoregular Poly(Phenyl Glycidyl Ethers): In Situ Formation of a Polyether Stereocomplex from a Racemic Monomer Mixture.

Polymer stereocomplex formation represents a promising research area as it can improve thermal and mechanical properties of co-crystallized polymer strands of opposite chirality. Polymers that form stereocomplexes commonly feature high stereoregularity and usually require sourcing from enantiopure monomer building blocks. Herein, we report the in situ polyether stereo-complex formation from racemic epoxide monomers, i.e., substituted methyl phenyl glycidyl ethers. The bio-renewable glycidyl ethers were explored in both enantio- and isoselective ring-opening polymerizations (ROPs), resulting in isotactic poly(phenyl glycidyl ether). While the enantio-selective ROP selectively resolves a single enantiomeric, isotactic polyether stereoisomer ([mm]P ≥ 78%), the isoselective ROP leads to the concurrent formation of both isotactic (R)- and (S)-poly(phenyl glycidyl ether) stereoisomers ([mm]P ≥ 92%) and thus results directly in a stereoisomer blend, which forms a stereocomplex. This is one of only a few polymer stereocomplexes generated directly during polymerization from a racemic monomer mixture. Stereo-complexes of the different poly(phenyl glycidyl ether)s show an increase in melting temperature of up to 76 °C, relative to the enantiopure parent polymers. The position of the methyl group at the phenyl ring determines both stereocomplex formation and the thermal properties of the resulting materials.

Stereoregular Poly(Phenyl Glycidyl Ethers): In Situ Formation of a Polyether Stereocomplex from a Racemic Monomer Mixture

AbstractPolymer stereocomplex formation represents a promising research area as it can improve thermal and mechanical properties of co‐crystallized polymer strands of opposite chirality. Polymers that form stereocomplexes commonly feature high stereoregularity and usually require sourcing from enantiopure monomer building blocks. Herein, we report the in situ polyether stereocomplex formation from racemic epoxide monomers, i.e., substituted methyl phenyl glycidyl ethers. The bio‐renewable glycidyl ethers were explored in both enantio‐ and isoselective ring‐opening polymerizations (ROPs), resulting in isotactic poly(phenyl glycidyl ether). While the enantioselective ROP selectively resolves a single enantiomeric, isotactic polyether stereoisomer ([mm]P≥78 %), the isoselective ROP leads to the concurrent formation of both isotactic (R)‐ and (S)‐poly(phenyl glycidyl ether) stereoisomers ([mm]P≥92 %) and thus results directly in a stereoisomer blend, which forms a stereocomplex. This is one of only a few polymer stereocomplexes generated directly during polymerization from a racemic monomer mixture. Stereocomplexes of the different poly(phenyl glycidyl ether)s show an increase in melting temperature of up to 76 °C, relative to the enantiopure parent polymers. The position of the methyl group at the phenyl ring determines both stereocomplex formation and the thermal properties of the resulting materials.

POxylated stereocomplexes from PEtOx-b-PLA diblock copolymers

An ω-hydroxyl terminated poly(2-ethyl-2-oxazoline) (PEtOx) was obtained by termination of the cationic ring-opening polymerization of 2-ethyl-2-oxazoline with acetate and subsequent transesterification with methanol. The resulting PEtOx-OH featuring a degree of polymerization (DP) of 18 was used as a macroinitiator for the ring-opening polymerization of l-lactide as well as d-lactide. The resulting PEtOx-b-PLA block copolymers featured DP values of 25, 50, 100, and 150, respectively, and were characterized by means of size exclusion chromatography, 1H NMR spectroscopy as well as matrix-assisted laser desorption ionization mass spectrometry. Differential scanning calorimetry, wide-angle x-ray scattering and polarized light microscopy indicated the formation of stereocomplexes in racemic blends of PEtOx-b-PLA with opposing chirality. Thereby, the amorphous PEtOx block did not affect the modification of the stereocomplex crystallites, as shown by comparison with data obtained from PLA homopolymer stereocomplexes. Similar to those, the degree of crystallinity and melting temperature decreased with increasing molar mass of the PLA in the stereocomplexes formed from PEtOx-b-PLA block copolymers.

Antioxidation and rheological modification of polylactide by gallic acid at chain end and stereocomplex formation

Abstract This study offers a novel approach that enables both the functionalization of degradable polymers and the modification of polymer properties during processing and use. Degradable polylactide was copolymerized with polyethylene glycol, with the aim to facilitate processing and blending by heating. Moreover, L,L- and D,D-lactide were employed for possible stereocomplexation, with the aim to enhance physical properties during use after processing. Additionally, gallic acid was introduced at both chain ends, which exhibited antioxidant properties as functionalization.

Effect of epoxidized soybean oil on melting behavior of poly(l-lactic acid) and poly(d-lactic acid) blends after isothermal crystallization

Abstract The effect of epoxidized soybean oil (ESO) on homocrystallization (HC) and stereocomplex (SC) formation behavior of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) bends was investigated utilizing differential scanning calorimetry (DSC). Isothermal crystallization was performed on ESO/PLLA/PDLA blends with varying ESO contents (0, 5, 8, and 10 wt%) and temperatures (90 °C, 120 °C, and 150 °C) for a different duration (12.5, 25, and 125 min). It was found that the ESO could effectively inhibit HC crystallization and promote SC crystallization. For the sample without ESO (ESO-0), the isothermal crystallization temperature and duration had little effect on the melting behavior, whereas sample with 5 wt% ESO (ESO-5), HC crystallization decreased while SC crystallization continued to increase with increasing duration. Additionally, at higher crystallization temperatures with constant ESO content, the melting temperature of SC crystals did not significantly change, suggesting that ESO did not degrade PLLA/PDLA blends. These findings imply that ESO modifies crystallization kinetics, suppressing HC formation and enhancing SC formation, which could benefit for specific material properties and applications.

Structural evolution of ultrasonic microinjection molded sterecomplexes of poly(lactide) during heating process

Two stereocomplexes (SCs) with different molecular weights in poly(L-lactide) (PLLA-49k and PLLA-163k) were prepared by mixing equal amounts of poly(D-lactide) (PDLA) and PLLA via the solution method. The exclusive formation of SCs was achieved in SC-PLLA-49k, whereas both homocrystallites (HCs) and SCs were observed in SC-PLLA-163k. Two blends were subjected to further processing via the ultrasonic microinjection molding technique at varying mold temperatures, yielding amorphous samples. As the samples were heated, the heating induced structural reorganization occurs with the concurrent SCs and HCs for all samples at ca. 80 °C. The degree of crystallinity for HCs XHC exhibits a positive correlation with the mold temperature, whereas the degree of crystallinity for SCs XSC shows a negative dependence on mold temperature for both systems. This behavior can be tentatively explained as follows. Firstly, the demixing of enantiomeric PLA chains occurs in the melt state as evidenced by twice heating measurements of blends. The degree of chain segregation is expected to increase with the increase in mold temperature due to lower supercooling, resulting in a decrease of PLLA/PDLA clusters (SC nuclei). Secondly, a high mold temperature is beneficial for the formation of HC nuclei as the chain mobility is improved. The reformulation of SC nuclei is difficult because of a longer diffusion distance required for PLA molecules. The coexistence of SC and HC nuclei causes the simultaneous appearance of SCs and HCs during heating of samples. As HCs start to melt, a transition from HCs to SCs occurs accompanied by a continuous increase in XSC. For both systems, a maximum of XSC is reached when SCs begin to melt followed by a mold temperature independent evolution of XSC upon further heating. Such a maximum is larger in the low molar mass system because it is less entangled. The reservation of SC nuclei is demonstrated by a comparison of the structural evolution and isothermal cold crystallization behavior between the two blends and their ultrasonic microinjection molded samples.

Alternating Graft Copolymer Carrying PLA Graft Chains at Every Other Unit: Sequence Impacts on Crystallization Behaviors.

Alternating graft copolymers were precisely synthesized via selective cyclopolymerization of pendant-transformable divinyl monomer (1), post-polymerization modification via aminolysis with alkylamine, and ring-opening polymerization of l-lactide (LLA) from the hydroxy pendant group in alternating sequence. The poly(LLA) (PLLA) graft chain on the alternating copolymer gave a higher crystallization degree on the isothermal treatment than that on the random counterpart likely because of the periodic sequence. The comonomer pendant group from alkylamine in the aminolysis reaction in the alternating sequence affected the crystallization behaviors, and the oligoethylene glycol (OEG) group promoted the crystallization thanks to the larger free volume effect. As for the stereocomplex formation of the racemic mixture of enantiomeric PLLA and poly(d-lactide) (PDLA) chains, the alternating graft copolymer gave a higher degree of stereocomplex crystallization in the mixture with the enantiomer homopolymer than the random analogue.

The Effect of Polyethylene Glycol on the Non-Isothermal Crystallization of Poly(L-lactide) and Poly(D-lactide) Blends.

The formation of polylactide stereocomplex (sc-PLA), involving the blending of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA), enhances PLA materials by making them stronger and more heat-resistant. This study investigated the competitive crystallization behavior of homocrystals (HCs) and stereocomplex crystals (SCs) in a 50/50 PLLA/PDLA blend with added polyethylene glycol (PEG). PEG, with molecular weights of 400 g/mol and 35,000 g/mol, was incorporated at concentrations ranging from 5% to 20% by weight. Differential scanning calorimetry (DSC) analysis revealed that PEG increased the crystallization temperature, promoted SC formation, and inhibited HC formation. PEG also acted as a plasticizer, lowering both melting and crystallization temperatures. The second heating DSC curve showed that the pure PLLA/PDLA blend had a 57.1% fraction of SC while adding 5% PEG with a molecular weight of 400 g/mol resulted in complete SC formation. In contrast, PEG with a molecular weight of 35,000 g/mol was less effective, allowing some HC formation. Additionally, PEG consistently promoted SC formation across various cooling rates (2, 5, 10, or 20 °C/min), demonstrating a robust influence under different conditions.

Influence of Molecular Weight and Composition of Homopolymers on the Stereocomplex Formation of Polylactides in Gels and Aerogels

Influence of Molecular Weight and Composition of Homopolymers on the Stereocomplex Formation of Polylactides in Gels and Aerogels

Bio-based poly(lactic acid) foams with enhanced mechanical and heat-resistant properties obtained by facilitating stereocomplex crystallization with addition of D-sorbitol

The preparation of bio-based poly(lactic acid) (PLA) foams with high mechanical properties and heat resistance is of great significance for environmental protection and green sustainable development. In this paper, D-sorbitol (DS) containing six hydroxyl groups was introduced into poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) blends for first time to promote the formation of stereocomplex (SC) crystals, which could improve the foaming behavior and enhance mechanical properties and heat resistance of PLA foams. The results showed that DS could improve the formation efficiency and crystallinity of SC crystals by enhancing the hydrogen bonding between the enantiomeric molecular chains. Furthermore, the compression modulus and interactions Vicat softening temperature of the PLLA/PDLA/DS blend foam increased about 854% and 16% compared to the pure PLLA foam, respectively. Besides, when the annealing process was introduced, the compression and heat resistance of the PLA foams increased further. This study provided a feasible strategy for the preparation of bio-based and biodegradable PLA foams with outstanding compressive and heat resistance properties.

Gelation upon the Mixing of Amphiphilic Graft and Triblock Copolymers Containing Enantiomeric Polylactide Segments through Stereocomplex Formation.

Biodegradable injectable polymer (IP) systems that form hydrogels in situ when injected into the body have considerable potential as medical materials. In this paper, we report a new two-solution mixed biodegradable IP system that utilizes the stereocomplex (SC) formation of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA). We synthesized triblock copolymers of PLLA and poly(ethylene glycol), PLLA-b-PEG-b-PLLA (tri-L), and a graft copolymer of dextran (Dex) attached to a PDLA-b-PEG diblock copolymer, Dex-g-(PDLA-b-PEG) (gb-D). We found that a hydrogel can be obtained by mixing gb-D solution and tri-L solution via SC formation. Although it is already known that graft copolymers attached to enantiomeric PLLA and PDLA chains can form an SC hydrogel upon mixing, we revealed that hydrogels can also be formed by a combination of graft and triblock copolymers. In this system (graft vs. triblock), the gelation time was shorter, within 1 min, and the physical strength of the resulting hydrogel (G' > 100 Pa) was higher than when graft copolymers were mixed. Triblock copolymers form micelles (16 nm in diameter) in aqueous solutions and hydrophobic drugs can be easily encapsulated in micelles. In contrast, graft copolymers have the advantage that their molecular weight can be set high, contributing to improved mechanical strength of the obtained hydrogel. Various biologically active polymers can be used as the main chains of graft copolymers, and chemical modification using the remaining functional side chain groups is also easy. Therefore, the developed mixing system with a graft vs. triblock combination can be applied to medical materials as a highly convenient, physically cross-linked IP system.

Preferential formation of stereocomplex crystals in poly(L-lactic acid)/poly(D-lactic acid) blends by a fullerene nucleator

Selective formation of stereocomplex (sc) crystallization in enantiomeric poly(L-lactic acid)/poly(D-lactic acid) (PLLA/PDLA) blends is considered as one of the most effective and promising way to improve the mechanical and thermal properties of polylactide (PLA) materials. However, homocrystallization (hc) prevails over sc crystallization in high-molecular-weight (HMW) PLLA/PDLA blends. Herein, we propose a simple and straightforward approach for fabricating sc crystallization and suppress hc crystallization for HMW PLLA/PDLA blends through the addition of C70 as a nucleator. Non-isothermal crystallization and wide-angel X-ray diffraction studies demonstrate that, the incorporation of 1 wt% C70 overwhelmingly leads to the formation of sc crystallites, while preventing the formation of hc crystallites. Isothermal crystallization experiments at 140 °C reveal a significant reduction in the half-crystallization period of the PLLA/PDLA blend upon the addition of C70. Fourier-transformed infrared spectroscopy suggests that, the improved intermolecular interactions between PLLA and PDLA chains, as well as the inhibition of molecular chain diffusion and mobility, contribute to the accelerated formation of sc facilitated by C70. The enhanced sc crystallization results in a 15.5 °C higher thermal stability in the as-prepared PLLA/PDLA blend with 1 wt% C70 compared to the neat counterpart.

Enhanced stereocomplex crystalline polylactic acids in melt processed enantiomeric bicomponent fiber configurations

The formation of stereocomplex crystalline domains in the bicomponent fiber melt spinning of enantiomeric polylactic acids (PLAs) is systematically explored and enhanced. Here we report a polycrystalline morphology where distinctly different crystalline regions are formed and aligned along the longitudinal direction of the fiber. This approach employs side-by-side and sheath-core bicomponent melt spinning configurations where the two components are the enantiomeric pairs of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA). We demonstrate the formation of the PLA stereocomplexes at the junction interphase through the melt spinning process which subsequently crystallize into a round fibers with stereocomplex and homogeneous crystal lamella morphologies. The fiber morphologies and crystallinities of the melt processed fiber are substantially different from the solution based bicomponent spinning system reported in the prior literature. Furthermore, the different molecular weight in the PLLA/PDLA pairing are found to be crucial to the structural development and properties of the PLA polycrystalline materials. The solid-state annealing does not change the crystal distribution of the crystalline domains and stereocomplex crystalline state, it just enhances the homo-crystallinity in the peripheral of the bicomponent fibers.

Recent advances in the development of the physically crosslinked hydrogels and their biomedical applications

Hydrogels are three-dimensional networks formulated from natural or synthetic polymers with a high capacity to absorb and transport water in their structure. Hydrogels are prepared from the crosslinking of their polymeric chains, which involves two basic mechanisms: chemical crosslinking and physical crosslinking. In chemical crosslinking, hydrogels are held together by covalent bonds; while physically cross-linked hydrogels are produced by non-covalent interactions, such as hydrogen bonds, electrostatic interactions, and hydrophobic forces, among others. Physically cross-linked hydrogels are more similar to biological systems due to their assembly dynamics, so they have wide biomedical applications. The most used approaches in the preparation of hydrogels by physical crosslinking include freeze-thaw, formation of stereocomplexes, ionic interaction, hydrogen bonding, crystallization, and crosslinking by hydrophobic interactions. These approaches are briefly discussed in this review. Some biomedical applications of these hydrogels will also be discussed.

Stereocomplex formation of a poly(D-lactide)/poly(L-lactide) blend on a technical scale

Abstract Poly(D-lactide) (PDLA) and poly(L-lactide) (PLLA), both available on the market, are blended on a technical scale. Using a special process control, the two materials are blended in a twin-screw extruder at a mass throughput rate of 2 kg/h, resulting in a stereocomplex Poly(-lactide) (PLA) blend. Thermal analysis indicates only one melting point at 235 °C. Both the Raman spectra and X-ray powder diffraction patterns show characteristic features for the stereocomplex PLA. With the available amount of this blend PLA fibers with technical strengths can be developed by melt spinning. As such, the application of the biopolymer PLA can be expanded, leading to substitute the conventional plastics for conserving both the resources and the environment.

Poly(L-lactic acid)/graphene composite films with asymmetric sandwich structure for thermal management and electromagnetic interference shielding

The blowout growth of the power density and transmission efficiency of microelectronic devices makes the problems of “overheating” and electromagnetic pollution of electronic devices increasingly prominent, and it is raising an urgent demand to improve the thermal conductivity and electromagnetic interference (EMI) shielding performance of packaging materials. Additionally, the environmental damage caused by fossil-based electronic waste should not be ignored. Hence, a scalable production strategy was proposed to prepare sandwich structure poly(L-lactic acid) (PLLA) composite films with high thermal conductivity and excellent EMI shielding efficiency (SE), which was achieved via solution pouring method and stacking hot-pressing process, in which PLLA/graphene nanoplatelets (PLLA/GNPs) with different GNPs contents was the top and substrate layers, and PLLA/poly(D-lactic acid)/Fe3O4 (PLLA/PDLA/Fe3O4) with a small amount of PDLA and Fe3O4 was the middle layer. Owing to the asymmetric distribution of filler and the successful introduction of magnetic particles, when the GNPs and Fe3O4 contents of the whole film are respectively only 5.61 wt% and 4.39 wt%, and the asymmetric ratio is 1:9, the in-plane thermal conductivity (K‖) and EMI SE of the composite film reaches 7.49 W m−1 K−1 and 41.7 dB, respectively, higher than those of PLLA film with symmetric structure (3.31 W m−1 K−1 and 33.0 dB) at the same contents of fillers. Furthermore, a small amount of PDLA (4.39 wt%) promotes the formation of stereocomplex (SC) crystallites in the middle layer and further induces the interlayer SC microcrystals during the stacking hot-pressing process, enhancing the interlayer interaction to maintain the mechanical robustness of the PLLA film. This work provides a new direction for the manufacture of polymer based composite films with high thermal conductivity and directional EMI shielding, and has broad application potential in the field of large-scale production of green electronic products.

Effect of chitin nanocrystals on stereocomplexation of poly(l-lactide)/poly(d-lactide) blends

Using polysaccharide nanocrystals such as chitin nanocrystals (ChNCs) as nanofiller for biodegradable aliphatic polymers is an attractive way of developing all-degradable nanocomposites. Crystallization study is vital for well regulating final performance of these type polymeric nanocomposites. In this work, ChNCs were incorporated with the poly(l-lactide)/poly(d-lactide) blends and as-obtained nanocomposites were used as target samples for the study. The results showed that ChNCs acted as nucleating agent, promoting the formation of stereocomplex (SC) crystallites and accelerating overall crystallization kinetics as a result. Therefore, the nanocomposites possessed higher SC crystallization temperatures and lower apparent activation energy as compared to the blend. However, the formation of homocrystallites (HC) was dominated by nucleation effect of SC crystallites and accordingly, the fraction of SC crystallites reduced more or less in the presence of ChNCs, despite the nanocomposites possessed higher rate of HC crystallization. This study also provided valuable information on accessing more applications of ChNCs to be used as SC nucleator for polylactide.

Scalable Continuous Manufacturing Process of Stereocomplex PLA by Twin-Screw Extrusion.

A scalable continuous manufacturing method to produce stereocomplex PLA was developed and optimized by melt-blending a 1:1 blend of high molecular weight poly(L-lactide) (PLLA) and high molecular weight poly(D-lactide) (PDLA) in a co-rotating twin-screw extruder. Thermal characteristics of stereocomplex formation were characterized via DSC to identify the optimal temperature profile and time for processing stereocomplex PLA. At the proper temperature window, high stereocomplex formation is achieved as the twin-screw extruder allows for alignment of the chains; this is due to stretching of the polymer chains in the extruder. The extruder processing conditions were optimized and used to produce >95% of stereocomplex PLA conversion (melting peak temperature Tpm = 240 °C). ATR-FTIR depicts the formation of stereocomplex crystallites based on the absorption band at 908 cm-1 (β helix). The only peaks observed for stereocomplex PLA's WAXD profile were at 2θ values of 12, 21, and 24°, verifying >99% of stereocomplex formation. The total crystallinity of stereocomplex PLA ranges from 56 to 64%. A significant improvement in the tensile behavior was observed in comparison to the homopolymers, resulting in a polymer of high strength and toughness. These results lead us to propose stereocomplex PLA as a potential additive/fiber that can reinforce the material properties of neat PLA.

Effects of PDLA molecular weight on the crystallization behaviors and rheological properties of asymmetric PDLA/PLLA blends

The molecular weight of poly(d-lactide) (PDLA) and poly(l-lactide) (PLLA) affects the performance of their blends by controlling the formation and characteristics of stereocomplex (SC) crystals. In this work, the crystallization behaviors and rheological properties of asymmetric PDLA/PLLA (5/95, w/w) blends were carefully investigated considering the molecular weight of PDLA (Mn(PDLA) = 1.5 × 103–18.7 × 103 g/mol) when the molecular weight of PLLA was selected as 5.0 × 104 g/mol. The results revealed that PDLA holding too low or too high Mn(PDLA) was unfavorable to the formation of SC crystallites owing to their weak interaction or difficult diffusion in the PLLA matrix, and the optimal Mn(PDLA) was ca.5.4 × 103 g/mol. Given this optimal Mn(PDLA), the composite viscosity and tensile viscosity of the obtained PDLA/PLLA blend were increased by 2–3 orders of magnitude and 10 times compared with neat PLLA, respectively, due to the formation of SC crystals with a high content (ΔHm-SC of 26.0 J/g). Besides, this PDLA/PLLA blend showed a fast crystallization rate and strong solid-like viscoelastic behavior. This work revealed the regular correspondence between Mn(PDLA) and the performance of asymmetric PDLA/PLLA blends.