Stereoblock Copolymers Research Articles

Access to Stereoblock Polyesters via Irreversible Chain-Transfer Ring-Opening Polymerization (ICT-ROP).

Precise control over polymer microstructure can enable the molecular tunability of material properties and represents a significant challenge in polymer chemistry. Stereoblock copolymers are some of the simplest stereosequenced polymers, yet the synthesis of stereoblock polyesters from prochiral or racemic monomers outside of "simple" isotactic stereoblocks remains limited. Herein, we report the development of irreversible chain-transfer ring-opening polymerization (ICT-ROP), which overcomes the fundamental limitations of single catalyst approaches by using transmetalation (e.g., alkoxide-chloride exchange) between two catalysts with distinct stereoselectivities as a means to embed temporally controlled multicatalysis in ROP. Our combined small-molecule model and catalytic polymerization studies lay out a clear molecular basis for ICT-ROP and are exploited to access the first examples of atactic-syndiotactic stereoblock (at-sb-st) polyesters, at-sb-st polyhydroxyalkanoates (PHAs). We achieve high levels of control over molecular weight, tacticity, monomer composition, and block structures in a temporally controlled manner and demonstrate that stereosequence control leads to polymer tensile properties that are independent of thermal properties.

Helical polyamines.

Polymer microstructures rely on tacticity, yet exploration in polyamines has focused predominantly on atactic polymers. We introduce a method to synthesize a diverse library of ortho and para-cyanobenzenesulfonyl-activated-methyl aziridines using R, S, and racemic alaninol. Living anionic ring-opening polymerization of racemic sulfonyl aziridines yields soluble polymers, while enantiomerically-pure sulfonyl aziridines follow a dispersion polymerization with complete monomer conversion giving access to stereoblock copolymers. Removal of activation groups is achieved using dodecanethiol and tert-butylimino-tri(pyrrolidino)phosphorane to obtain isotactic or atactic linear polypropylene imines (LPPIs). High-purity L-PPIs are obtained in salt and neutral forms with high yields. Stereoblock copolymers of poly-R-block-S-polysulfonamides and respective polypropylene imine stereoblocks are synthesized, revealing helical structures in water influenced by the monomer type and sequence in CD spectroscopy. Molecular dynamics simulations confirm the helical nature of isotactic LPPIs in water. Bulk characterization demonstrates the first crystalline isotactic polyamines via spherulite growth in polarized light, atomic force microscopy and XRD analyses. In cell-transfection studies, the synthesized isotactic LPPIs exhibit lower toxicity and transfection efficiency than commercial hyperbranched polyethylene imine, with longer chains showing increased transfection efficiency. These isotactic polymers open avenues for complex macromolecular architectures with optically active polyamines akin to poly(amino acid)s but lacking hydrolytically cleavable amide links.

Synthesis and Characterization of Well-Defined High-Molecular-Weight PDLA-b-PLLA and PDLA-b-PLLA-b-PDLA Stereo-Block Copolymers

A facile method is developed to synthesize poly(lactide) (PLA) stereo-diblock and stereo-triblock copolymers using a bismuth catalyst. The judicious choice of the catalytic system allows us to synthesize the PLA stereocomplex having a molar mass exceeding 450 kg/mol and a polydispersity of below 2. The synthesis of such a high molar mass became feasible because in the chosen catalytic system, unlike in the reported synthesis routes, the presence of water in traces does not promote chain transfer. The observations are that during polymerization, single crystals of poly(l-lactide)-b-poly(d-lactide) (PLLA-b-PDLA) and poly(d-lactide)-b-poly(l-lactide)-b-poly(d-lactide) (PDLA-b-PLLA-b-PDLA) stereo-block copolymers are formed. These platelet-like single crystals tend to aggregate, forming a globular spherical morphology equivalent to that perceived on the crystallization of the stereocomplex from solution. The adopted synthesis route provides molecular mixing of the stereo-regular PLA chains in the absence of any detectable traces of homo-crystals, which are unavoidable when the mixing is performed using a homopolymer dissolution route. The molecularly mixed stereo-regular PLAs in a polymer melt retain their molecular interaction, even after isothermal crystallization or rheological studies that follow the anticipated viscoelastic response of a polymer melt. The increasing molar masses of the PLLA/PDLA diblock and triblock copolymers show an increase in the tensile modulus and elongation to break, retrospectively indicating the brittle to ductile transformation of the polymer with increasing molar mass. Thermal and imaging experimental methods such as differential scanning calorimetry, nuclear magnetic resonance, thermogravimetric analysis, scanning electron microscopy, electron diffraction, wide-angle X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy are employed to follow the enthalpic relaxation, conformational, and structural changes.

Self-Assembly of Stereoblock Copolymers Driven by the Chain Folding of Discrete Poly(d-lactic acid-b-l-lactic acid) via Intramolecular Stereocomplexation

The encoding of information by defining the sequence of monomers is a highly anticipated strategy for controlling the three-dimensional structures of polymers via information-driven chain folding and self-assembly. In this paper, we report the controlled chain folding of stereoblock poly(lactic acid)s (PLAs) composed of two oligo(lactic acid) domains, [DLAn] and [LLAn], constructed by using precisely defined numbers of d- and l-lactic acids, respectively. Under the crystallization-driven self-assembly (CDSA) condition, block copolymers (BCPs) of stereoblock PLA and poly(ethylene glycol) formed planar nanostructures having unilamellar crystalline cores of stereoblock PLA. [DLAn]-[LLAn] stereoblocks were folded predominantly by intramolecular stereocomplexation (SCN) in dilute solutions in which the intermolecular interaction between BCPs was suppressed. The thickness of the planar nanostructures was precisely defined by the number of repeating units constituted of [DLAn] and [LLAn] domains. The convergent synthesis of PLA permitted the addition of a single monomer unit between the [DLAn] and [LLAn] domains, resulting in the introduction of a desired functional group at the apex of the folded chain. Our results demonstrate that information encoded in the form of a monomer sequence may shape the polymer chain and guide its self-assembly toward specific nanostructures having the desired dimensions and functions.

Substantially Enhanced Stereocomplex Crystallization of Poly(L-lactide)/Poly(D-lactide) Blends by the Formation of Multi-Arm Stereo-Block Copolymers

Stereocomplex-type polylactide (SC-PLA) created by alternate packing of poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA) chains in a crystalline state has emerged as a growingly popular engineering bioplastic that possesses excellent hydrolytic stability and thermomechanical properties. However, it is extremely difficult to acquire high-performance SC-PLA products via melt-processing of high-molecular-weight PLLA/PDLA blends because both SC crystallites and homocrystallites (HCs) are competitively formed in the melt-crystallization. Herein, a facile yet powerful way was employed to boost SC formation by introducing trace amounts of some epoxy-functionalized small-molecule modifiers into the enantiomeric blends during reactive melt-blending. The results show that the SC formation is considerably enhanced with the in situ generation of multi-arm stereo-block PLA copolymers, based on the reaction between epoxy groups of the modifiers and hydroxyl end groups of PLAs. More impressively, it is intriguing to find that the introduction of only 0.5 wt% modifiers can induce exclusive SC formation in the blends upon isothermal and non-isothermal melt-crystallizations. The outstanding SC crystallizability might be attributed to the suppressing effect of such unique copolymers on the separation of the alternately arranged PLLA/PDLA chain segments in molten state as a compatibilizer. Furthermore, the generation of these copolymers does not result in a significant increase in melt viscosity of the blends. These findings suggest new opportunities for the high-throughput processing of SC-PLA materials into useful products.

Living Polymerization of Chiral O-Carboxyanhydride of Mandelic Acid and Precise Stereoblock Copolymer Syntheses Using Highly Active OOO-Tridentate Bis(phenolate) Zinc Complexes

The synthesis of highly stereoregular poly(mandelic acid) (PMA) via controllable polymerization is a considerable challenge because of the easy racemization of monomers during polymerization; consequently, precise stereoblock copolymers of PMA have not been reported so far. In this work, highly stereoregular PMA was synthesized through a living ring-opening polymerization of O-carboxyanhydrides (OCAs) of mandelic acid using highly active OOO-tridentate bis(phenolate)/zinc catalysts. In this polymerization system, the side reaction of racemization was suppressed very well via decreasing the basicity of ligands in zinc complexes. The molecular weight increases linearly as the ratio of monomer to initiator increases and can be high up to 32.5 kg/mol with narrow molecular weight distributions. Meanwhile, for the living polymerization feature, stereodiblock and stereotriblock copolymers with controllable molecular weights were obtained upon sequential one-pot monomer addition within 45 min. Thermal and X-ray diffraction characterizations indicated that these stereoblock copolymers crystallized in a stereocomplex phase with a distinct melting temperature (Tm) up to 178 °C, and the solubility behaviors of stereoblock PMAs and enantiomeric homopolymers of PMAs are remarkably different.

Functionalized Polyesters via Stereoselective Electrochemical Ring-Opening Polymerization of O-Carboxyanhydrides

Ring-opening polymerization is used to prepare polyesters with precisely controlled molecular weights, molecular weight distributions, and tacticities. Herein, we report a Co/Zn catalytic system that can be activated by an electrical current to mediate efficient ring-opening polymerization of enantiopure O-carboxyanhydrides, allowing for the synthesis of isotactic functionalized polyesters with high molecular weights (>140 kDa) and narrow molecular weight distributions (Mw/Mn < 1.1). We also demonstrate that these catalysts can be used for stereoselective ring-opening polymerization of racemic O-carboxyanhydrides to synthesize syndiotactic or stereoblock copolymers with different glass transition temperatures compared with their atactic counterparts.

Self-Assembly of Linear and Cyclic Polylactide Stereoblock Copolymers with a Parallel and Antiparallel Chain Arrangement Distinguishing Their Directions on a Water Surface.

The self-assembly of molecules into a well-ordered structure is one of the most important processes in fabricating sophisticated materials. Here, we show that polymer chains can be self-assembled, distinguishing their direction (parallel or antiparallel), and could be a new useful scaffold for self-assembly in a controlled direction. The system that was used was a stereocomplex (SC) formation of linear and cyclic polylactide (PLA) stereoblock copolymers with a parallel and antiparallel chain arrangement in a Langmuir monolayer. The linear and cyclic stereoblock copolymers with a parallel arrangement formed a well-ordered lamellar SC in the first and second layers upon compression, but the linear and cyclic stereoblock copolymers with an antiparallel arrangement did not form a first-layer lamella and instead formed only the second-layer lamella. These results were only rationally explained by assuming that the enantiomeric PLA chains selectively assembled in a parallel direction, not in an antiparallel direction, in the SC. A simple polymer chain could be self-assembled, distinguishing the direction without a specific interaction group in it.

Synthesis and Properties of Stereoblock Copolymers Composed of Lactide and ε-Caprolactone

Synthesis and Properties of Stereoblock Copolymers Composed of Lactide and ε-Caprolactone

ATRP of N‐Hydroxyethyl Acrylamide in the Presence of Lewis Acids: Control of Tacticity, Molecular Weight, and Architecture

Good control of tacticity, molecular weight, and architecture is attained via atom transfer radical polymerization (ATRP) of N-hydroxyethyl acrylamide (HEAA), in a one-pot process in the presence of Y(OTf)3 . The effect of temperature, ratio of [Y(OTf)3 ]/[HEAA], and ATRP procedure on the tacticity and degree of control over the polymerization is investigated in detail. Under optimal conditions, using photo ATRP and 15% Y(OTf)3, the content of meso dyads (m) can be increased from 42% to 80% in a homopolymer with a dispersity D=1.22. Well-defined stereoblock copolymers, atactic- b -isotactic poly(HEAA), with D=1.27, are obtained by adding Y(OTf)3 at a specific conversion, initially started without Y(OTf)3 .

Tailor-made block copolymers of l-, d- and rac-lactides and ε-caprolactone via one-pot sequential ring opening polymerization by pyridylamidozinc(ii) catalysts.

Three-coordinated Zn(ii) complexes bearing sterically encumbered bidentate monoanionic [N,N−] pyridylamido ligands efficiently catalyze the ring opening polymerization of lactide (LA) and ε-caprolactone (CL). Owing to the polymerization controlled nature and high rate, precise stereodiblock poly(LLA-b-DLA) with different block lengths can be easily produced by one-pot sequential monomer addition at room temperature in short reaction times. NMR, SEC and DSC analyses confirm the production of highly isotactic diblock copolymers which crystallize in the high melting stereocomplex phase. Stereo-triblock and tetrablock copolymers of l-LA, d-LA and rac-LA have been synthesized similarly. Finally, a diblock poly(CL-b-LA) has been easily obtained by sequential addition of ε-caprolactone and lactide under mild conditions.

Enhancement of the crystallization and biocompatibility of poly(TMC-b-(LLA-ran-GA)) by poly(lactide) stereocomplex

A series of poly(1,3-trimethylene carbonate-b-(l-lactide-ran-glycolide)) (PTLG) with the three-armed PDLA and PLLA stereoblock copolymer (sc-PLA) acting as the nucleating agent was preparedviathe solution-casting method.

Screening of crystalline species and enhanced nucleation of enantiomeric poly(lactide) systems by melt-quenching

The effects of quenching from the melt on the isothermal crystallization of star-shaped four-armed stereo diblock poly(lactide) (4-LD) and four-armed poly(l-lactide) (4-L) polymers with different l-lactyl unit contents as low crystallizability models for the stereoblock copolymers or blends of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) were investigated. Quenching induced both stereocomplex (SC)- and homo-crystallization of non-equimolar 4-LD polymers with l-lactyl unit contents around 30 and 70%, wherein both SC- and homo-crystallization were highly prohibited in other procedures, and enhanced nucleation of 4-LD and 4-L polymers compared to the samples without quenching, resulting in formation of a large number of small-sized crystalline assemblies. The results obtained in the present study strongly suggest that the procedure of isothermal crystallization after quenching from the melt is expected to be utilized for screening the crystallizability of SC- and homo-crystallites in various types of block copolymers and blends of PLLA and PDLA and for enhancing nucleation or formation of dense crystalline structure.

Stereoselective photoredox ring-opening polymerization of O-carboxyanhydrides

Biodegradable polyesters with various tacticities have been synthesized by means of stereoselective ring-opening polymerization of racemic lactide and β-lactones but with limited side-chain groups. However, stereoselective synthesis of functional polyesters remains challenging from O-carboxyanhydrides that have abundant pendant side-chain functional groups. Herein we report a powerful strategy to synthesize stereoblock polyesters by stereoselective ring-opening polymerization of racemic O-carboxyanhydrides with the use of photoredox Ni/Ir catalysts and a selected Zn complex with an achiral ligand. The obtained stereoblock copolymers are highly isotactic with high molecular weights ( > 70 kDa) and narrow molecular weight distributions (Mw/Mn < 1.1), and they display distinct melting temperatures that are similar to their stereocomplex counterparts. Furthermore, in one-pot photoredox copolymerization of two different O-carboxyanhydrides, the use of such Zn complex mediates kinetic resolution of the comonomers during enchainment and shows a chirality preference that allows for the synthesis of gradient copolymers.

CO2-Tuned Sequential Synthesis of Stereoblock Copolymers Comprising a Stereoregularity-Adjustable Polyester Block and an Atactic CO2-Based Polycarbonate Block

Stereoblock copolymers composed of atactic CO2-based polycarbonate and various stereoregular polyester blocks with cis-2,3-(exo, exo), cis-2,3-(endo, endo), or partly trans-2,3-(exo, endo) repeating units were successfully synthesized via sequential ring-opening copolymerization (ROCOP) of cyclohexene oxide (CHO) with norbornene anhydride (NA) and then ROCOP of CHO with CO2 using a salcyCrCl/PPNCl binary catalyst. The structure of the block copolymers was confirmed by NMR and GPC. Incorporation of carboxyl groups by the thiol–ene reaction of the pendant norbornenyl groups further confirmed the polymer structures. The geometric structure of polyester units in the block copolymers is tailored simply by varying the NA isomer and the monomer feed ratio of CHO to NA. Notably, CO2 suppressing the configuration transformation from cis-(exo, exo) to trans-(exo, endo) of polyester block has been revealed in the sequential ROCOP. Using this strategy, the stereospecific polyester with an dominant cis-2,3-(exo, exo)-...

Influence of graphene nanosheets on stereocomplex crystallization behaviors of star‐shaped poly (D(L)‐lactide) stereoblock copolymer

The nanocompsites of star‐shaped poly(D‐lactide)‐co‐poly(L‐lactide) stereoblock copolymers (s‐PDLA‐PLLA) with two‐dimensional graphene nanosheets (GNSs) were prepared by solution mixing method. Crystallization behaviors were investigated using differential scanning calorimetry, polarized optical microscopy, and wide angle X‐ray diffraction. The results of isothermal crystallization behaviors of the nanocompsites clearly indicated that the GNS could remarkably accelerate the overall crystallization rate of s‐PDLA‐PLLA copolymer. Unique stereocomplex crystallites with melting temperature about 207.0°C formed in isothermal crystallization for all samples. The crystallization temperatures of s‐PDLA‐PLLAs shifted to higher temperatures, and the crystallization peak shapes became sharper with increasing GNS contents. The maximum crystallization temperature of the sample with 3 wt% GNS was about 128.2°C, ie, 15°C higher than pure s‐PDLA‐PLLA. At isothermal crystallization processes, the halftime of crystallization (t0.5) of the sample with 3 wt% GNS decreased to 6.4 minutes from 12.9 minutes of pure s‐PDLA‐PLLA at 160°C.The Avrami exponent n values for the nanocomposites samples were 2.6 to 3.0 indicating the crystallization mechanism with three‐dimensional heterogeneous nucleation and spherulites growth. The morphology and average diameter of spherulites of s‐PDLA‐PLLA with various GNS contents were observed in isothermal crystallization processes by polarized optical microscopy. Spherulite growth rates of samples were evaluated by using combined isothermal and nonisothermal procedures and analyzed by the secondary nucleation theory. The results evidenced that the GNS has acceleration effects on the crystallization of s‐PDLA‐PLLA with good nucleation ability in the s‐PDLA‐PLLA material.

Group 4 Metal Complexes of Phenylene-Salalen Ligands in rac-Lactide Polymerization Giving High Molecular Weight Stereoblock Poly(lactic acid).

Phenylene-salalens-sequential tetradentate dianionic {ONNO}-type ligands that include an ortho-phenylene group bridging between an imine and amine internal donors bound to phenol arms with a broad variety of substitution patterns are described. Zirconium and hafnium complexes of the type [{ONNO}M(O-tBu)2 ] were formed as single diastereomers, which, according to crystallographic structures, featured the fac(around the amine)-mer(around the imine) wrapping mode. The reactivity and stereoselectivity in rac-lactide (rac-LA) polymerizations were found to depend on the substitution pattern: complexes featuring small groups on the imine-side phenol and bulky groups on the amine-side phenol exhibited the favorable combination of high activity and high isoselectivity (Pm ≤0.91). Isotactic stereoblock copolymers of high molecular weights were prepared. The polymers crystallized in the stereocomplex phase according to thermal (differential scanning calorimetry) and crystallographic analysis.

α-keto-β-diimine nickel-catalyzed olefin polymerization: Effect of ortho-aryl substituents and preparation of stereoblock copolymers

A series of α-keto-β-diimine nickel complexes (Ar-N = C(CH3)-C(O)-C(CH3)=N-Ar)NiBr2; Ar = 2,6-R-C6H3-, R = Me, Et, iPr, and Ar = 2,4,6-Me3-C6H3-) was prepared. All corresponding ligands are unstable even under an inert atmosphere and in a freezer. Stable copper complex intermediates of ligand synthesis and ethyl substituted nickel complex were isolated and characterized by X-ray. All nickel complexes were used for the polymerization of ethene, propylene, and hex-1-ene to investigate their livingness and the extent of chain-walking. Low-temperature propene polymerization with less bulky ortho-substituents was less isospecific than the one with isopropyl derivative. Propene stereoblock copolymers were prepared by iPr derivative combining the polymerization at low temperature to prepare isotactic polypropylene (PP) block and at a higher temperature, supporting chain-walking, to obtain amorphous regioirregular PP block. Alternatively, a copolymerization of propene with ethene was used for the preparation of amorphous block. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017

The origin of memory effect in stereocomplex poly (lactic acid) crystallization from melt state

Three arm poly(propylene oxide) block poly(d-lactide) block poly(l-lactide) (PPO-PDLA-PLLA) stereoblock copolymers melted at various melting temperature and recrystallized. The role of crystallization memory effect of stereocomplex structures was demonstrated utilizing in situ FTIR and WAXD. There is a critical temperature (Tcr) for detachment of stereocomplex structures of melt samples. When the melting temperature (Tmelt) is lower than Tcr, crystallization temperature (Tc) of samples decreases with Tmelt increasing. If the Tmelt is higher than Tcr, the samples’ Tc is as a constant temperature. We attribute the decrease in Tc to crystallization memory effect and correlate it with the detachment of stereocomplex structures in melt. When Tm<Tmelt<Tcr, the stereocomplex crystallites melt, but stereocomplex segments could survive. The remaining stereocomplex segments play as the self-seeds and reduce the crystallization barrier energy. The recrystallization is the same process as a general self-seeds crystallization. Influence of melt time and molecular weight on the stereocomplex behavior in melt were also investigated.

Temperature programed photo-induced RAFT polymerization of stereo-block copolymers of poly(vinyl acetate)

Stereo-triblock copolymers of poly(vinyl acetate) are synthesized with controlled molecular weights based on a temperature-programed photo-induced RAFT in HFIP.