Modulation of polymerization rate of N-carboxyanhydrides in a biphasic system

The recent advances in accelerated polymerization of N-carboxyanhydrides (NCAs) offer an effective strategy to simplify the preparation of polypeptide materials. However, the fine-tuning of polymerization kinetics, which is critical to differentiate the main polymerization and the side reactions, remains largely unexplored. Herein we report the modulation of polymerization rate of NCA in a water/oil biphasic system. By altering the aqueous pH, the initial location of the initiators, and the pK_(a) of initiating amines, we observed the change in polymerization time from several minutes to a few hours. Due to the high interfacial activity and low pKa value, controlled polymerization was observed from multi-amine initiators even if they were initially located in the aqueous phase. This work not only improves our understanding on the biphasic polymerization mechanism, but also facilitates preparation of versatile polypeptide materials.

Accelerated Ring-Opening Polymerization of α-Amino Acid N-Carboxyanhydride via Inorganic Nano-initiators

While the accelerated polymerization of N-carboxyanhydrides (NCAs) has been utilized to synthesize versatile polypeptide materials in an efficient manner with minimized side reactions, the preparation of polypeptide-based inorganic/organic hybrid materials with the acceleration strategy remained largely unexplored. Herein, we report the accelerated ring-opening polymerization (ROP) of NCAs mediated by amine-modified inorganic nano-initiators, such as mesoporous silica nanoparticles (MSN-NH2), which is driven by the cooperative effect of the neighboring α-helical polypeptide chains in a dichloromethane (DCM)/water biphasic system. Well-defined nano-hybrids were prepared within 15 min from non-purified NCA monomers, through in situ purification and subsequent ultrafast polymerization process. NCAs can be rapidly initiated by amino groups of MSN uniformly dispersed at the interface of DCM and water, and subsequently formed the well-defined polypeptides within 15 min. The prepared inorganic/organic nano-hybrid with MSN as the core and polypeptide as the shell adopted spherical morphology and uniform size distribution due to the excellent controllability of ROP. Besides, this system is also suitable for a variety of NCAs and inorganic nano-initiators. This research allows efficient and rapid preparation of inorganic/organic nano-hybrids, and further promotes the extensive application of this material in the biomedical fields.

Alkali-metal hexamethyldisilazide initiated polymerization on alpha-amino acid N-substituted N-carboxyanhydrides for facile polypeptoid synthesis

Polypeptoids have been explored as mimics of polypeptides,owing to polypeptoids'superior stability upon proteolysis.Polypeptoids can be synthesized from one-pot ring-opening polymerization of amino acid N-substituted N-carboxyanhydrides(NNCAs).However,the speed of polymerization of NNCAs can be very slow,especially for NNCAs bearing a bulky N-substitution group.This hindered the exploration on polypeptoids with more diverse structures and functions.Therefore,it is in great need to develop advanced strategies that can accelerate the polymerization on inactive NNCAs.Hereby,we report that lithium/sodium/potassium hexamethyldisilazide(Li/Na/KHMDS)initiates a substantially faster polymerization on NNCAs than do commonly used amine initiators,especially for NNCAs with bulky N-substitution group.This fast NNCA polymerization will increase the structure diversity and application of polypeptoids as synthetic mimics of polypeptides.

Synthesis of α-Hydroxy-ω-Aminotelechelic Polypeptide from α-Amino Acid N-Carboxyanhydrides Catalyzed by Alkali-metal Borohydrides

It is reported that alkali-metal borohydrides （MBH4, M = Li, Na and K） are efficient catalysts for ring opening polymerization （ROP） of α-amino acid N-carboxyanhydrides （NCAs）. Polypeptides are prepared in quantitative yields with relatively narrow molecular weight distributions （MWDs = 1.1-1.5） which depend on the reaction temperature. End groups of the produced polypeptide are studied in detail by MALDI-ToF MS, IH-NMR, 13C-NMR, IH-1H COSY and IH-13C HMQC analyses. The results indicate that α-hydroxy-ω-aminotelechelic polypeptides are formed which are suitable for post- polymerization functionalization.

6-Membered ring intermediates in polymerization of N-carboxyanhydride-L-α-arginine in H_2O

In polymerization of N-carboxyanhydride-L-α-arginine(L-Arg-NCA) in H2O,nucleophilic reaction of guanidine group with the carbonyl group of L-Arg-NCA leads to quick intramolecular rearrangement,yielding a 6-membered ring intermediate 1-amidino-3-amino-2-piperidone,which is either elongated by another L-Arg-NCA yielding arginyl-1-amidino-3-amino-2-piperidone or hydrolyzed to L-α-arginine.The oligoarginines are formed mainly through hydrolysis of arginyl-1-amidino-3-amino-2-piperidones.This is a unique pathway in polymerization of L-Arg-NCA with regard to the usual pathway of elongations by reaction of N-carboxyanhydride-L-α-amino acid with L-α-amino acid or oligopeptides.

Polypeptide Brushes Grown via Surface-initiated Ring-opening Polymerization of α-Amino Acid N-Carboxyanhydrides

Polypeptide brushes are attractive platforms to generate functional and responsive interfaces that are of potential interest due to their possible biodegradability, biocompatibility and tunable secondary structures. Surface-initiated ringopening polymerization（SI-ROP） of α-amino acid N-carboxyanhydrides represents a powerful and versatile strategy to generate polypeptide brushes. This review is an attempt to capture the state-of-the-art in this field and highlights the latest developments in several selected areas. In addition to presenting an overview of the synthetic methods that have been used to generate polypeptide brushes via SI-ROP, this article will discuss the preparation of patterned polypeptide brushes, the conformational properties of surface-tethered polypeptides, ways to control chain orientation at surfaces as well as properties and applications of these thin polymer films.

SYNTHESIS AND CHARACTERIZATION OF POLY(L-GLUTAMIC ACID-co-L-ASPARTIC ACID)

Poly(amino acid)has been widely utilized in drug delivery,tissue engineering and biomedical materials.The biomaterials based on poly(glutamic acid)are usually modified via copolymerization with other monomers such as L-aspartic acid to improve the uncontrolled degradation rate.The ring-opening homo- and co-polymerization ofγ-benzyl-L-glutamate N-carboxyanhydride(BLG-NCA)andβ-benzyl-L-aspartate N-carboxyanhydride(BLA-NCA)were carried out in solution by using triethylamine(TEA)as initiator.The BLG-NCA homopol...

Non-Isothermal Degradation Kinetics of Hybrid Copolymers Containing Thermosensitive and Polypeptide Blocks

Novel, self-associating hybrid copolymers were synthesized via controlled ring-opening polymerization of N-carboxyanhydride of Z-L-lysine (Z-L-Lys-NCA), initiated by amino-functional macroinitiators. A poly(N-isopropylacry-lamide) (PNIPAm)-based macroinitiator containing 10 mol% of polyoxyethylene grafts and a terminal primary amine group in the form of ammonium hydrochloride (PNIPAm-g-PEО) was synthesized and used to initiate the ammonium- mediated ring-opening polymerization of NCA described by Dimitrov and Schlaad [1]. Thus, hybrid copolymers ((PNIPAm-g-PEO)-b-PLys) with controlled molar-mass characteristics and functionality were obtained. The potential applications of PNIPAm-based copolymers in the systems for controlled drug release, immobilization of enzymes and protein purification have aroused great interest in the studies of their properties and behaviour. The thermal stability and thermodynamic properties of the copolymers obtained were studied. The differential thermal analysis of polyfunctional hybrid copolymers (PNIPAm-g-PEO)-b-PLys) showed that thermooxidative destruction occurs in two stages: primary, of the unstable fragments (grafted chains of PEO);and secondary, of the main polymer chains of poly(N-isopropylacry-lamide) and poly(L-lysine). The kinetics of thermal degradation was evaluated and the values of the activation energy of the degradation process, changes of Gibbs free energy, enthalpy and entropy for the formation of the activated complex were also calculated.

Controlled synthesis of polypeptides

Polypeptides are one kind of promising biodegradable and biocompatible biomedical polymers with the structural units of various a-amino acids.Polypeptides were first polymerized by the ring-opening polymerization(ROP)of α-amino acid N-carboxyanhydrides(NCAs)by Leuchs and Hermann in 1906.In the past decades,several effective strategies,including the selection of initiators,the adjustment of reaction conditions,and the introduction of catalysts,have been reported to improve the controllability of the ROP of various a-amino acid NCAs to synthesize different polypeptides with precise chemical structures and low polydispersity indexes.In this Review,the strategies,mechanisms,challenges,and opportunities for controlled synthesis of polypeptides by the ROP of differentα-amino acid NCAs have been declared.

EFFECTS OF MOLECULAR WEIGHT ON THERMAL RESPONSIVE PROPERTY OF PEGYLATED POLY-L-GLUTAMATES

We investigated the ring opening polymerization （ROP） of di- and tri-ethylene glycol monomethyl ether functionalized L-glutamate N-carboxyanhydrides （NCAs） using hexamethyldisilazane （HMDS） as primary initiator and 1,5,7-triazabicyclo-[4.4.0]dec-5-ene （TBD） as co-initiator. The binary initiator system afforded a living ROP for these pegylated NCAs, and a series of homopolypeptides with controlled molecular weight （MW） and low polydispersity were obtained. We then systematically studied the helical content and clouding point （CP） dependence on polypeptide MW using circular dichroism （CD） spectroscopy and turbidity measurements, respectively. We found that the helical content of both homopolypeptides increased with MW, but the triethylene glycol functionalized poly-L-glutamate （poly-L-EG3Glu） intended to form more stable or-helical structure than diethylene glycol functionalized counterpart （poly-L-EG2Glu） at similar MW. Accordingly, the CP of poly-L-EG2Glu with known end group has strong dependence on its helical content, which is essentially determined by MW. Our results suggested that the thermal responsive properties of these unique pegylated poly- L-glutamates not only rely on their chemical structure but also on their secondary structures, wh^ch is different from conventional thermal responsive polymers.

Tunable supramolecular hydrogels from polypeptidePEG-polypeptide triblock copolymers

A series of ABA triblock copolymers of poly(?-(2-methoxy ethoxy)esteryl-glutamate)-block-poly(ethylene glycol)-blockpoly(?-(2-methoxy ethoxy)esteryl-glutamate) with poly(ethylene glycol) as middle hydrophilic B block and oligo(ethylene glycol)-functionalized polyglutamate(poly-L-EG2Glu) as terminal A blocks were prepared via ring-opening polymerization of EG2 Glu N-carboxyanhydride(NCA). The resulting P(EG2Glu)-b-PEG-b-P(EG2Glu) triblocks can spontaneously form hydrogels in water. The intermolecular hydrogen bonding interactions between polypeptides blocks were responsible for the formation of gel network structure. These hydrogels displayed shear-thinning and rapid recovery properties, which endowed them potential application as injectable drug delivery system. The mechanical strength of hydrogels can be modulated by copolymer composition, molecular weight and concentrations. Also, it was found that the hydrogels' strength decreased with temperature due to dehydration of polypeptide segments. Atomic force microscopy and scanning electron microscopy images revealed that these hydrogels were formed through micelle packing mechanism. Circular dichroism and Fourier transform infrared spectroscopy characterizations suggested the poly-L-EG2 Glu block adopted mixed conformation. A preliminary assessment of drug release in vitro demonstrated the hydrogels can offer a sustained release of doxorubicin(DOX) and the release rate could be controlled by varying chemical composition.

Thermal and Redox Dual Responsive Poly(L-glutamate)with Oligo(ethylene glycol)Side-chains

A series of poly（L-glutamate）s grafted with oligo（ethylene glycol） （OEG） side-chains through the thioether linkages （PALGn-g-EGx, x = 2, 3 and 4） were prepared by ring-opening polymerization （ROP） of γ-allyl-L-glutamate N-carboxyanhydride （ALG-NCA） and thiol-ene photoaddition. The chemical structures and physical properties were characterized by 1H-NMR, Fourier transform infrared （FTIR）, circular dichroism （CD）, etc. The PALGn-g-EGx samples with x = 3 and 4 displayed lower critical solution temperature （LCST） in water due to the presence of OEG units. The clouding point （CP） of polypeptides can be finely tuned by changing the side chain structures, molecular weights and sample concentrations. In addition, the thioether linkages in the side chains offer additional redox-responsive properties. The influence of both OEG units and thioether linkages on the LCST behavior was systematically investigated. This work provides an efficient way to prepare multi-stimuli responsive materials with highly tunable properties.

THE AMPHIPHILIC MULTIARM COPOLYMERS BASED ON HYPERBRANCHED POLYESTER AND LYSINE:SYNTHESIS AND SELF-ASSEMBLY

The amphiphilic multiarm copolymers were synthesized through the modification of commercially available hyperbranched polyesters（Boltorn H40） with N-ε-carbobenzoxy-L-Lysine N-carboxyanhydride（ZLys-NCA）.After being condensed with N-Boc-phenylalanine（Boc-^NPhe） and deprotected the Boc-groups in trifluoroacetic acid（TFA）,the original terminal hydroxyl groups were transformed into the amino groups and then initiated the ring-opening polymerization of ZLys-NCA.The hydrophilic poly（L-lysine） was grafted to the surface of Boltorn H40 successfully after the protecting benzyl groups were removed by the HBr solution in glacial acetic acid（33 wt%）.The resulting multiarm copolymers were characterized by the ^1H-NMR,GPC and FTIR.The arm length calculated by NMR and GPC analysis was about 3 and 13 lysine-units for H40-Phe-PLysl and H40-Phe-PLys2 respectively.Due to the amphiphilic molecular structure,they displayed ability to self-assemble into spherical micelles in aqueous solution with the average diameter in the range from 70 nm to 250 nm.The CMC of H40-Phe-PLysl and H40-Phe-PLys2 was 0.013 mg/mL and 0.028 mg/mL,respectively, indicating that H40-Phe-PLysl with shorter arm length is easier to self-assemble than H40-Phe-PLys2 with longer arm length.