Lipase-catalyzed copolymerization of omega-pentadecalactone with p-dioxanone and characterization of copolymer thermal and crystalline properties

Candida antarctica Lipase B (CALB), a metal-free enzyme, was successfully employed as catalyst for ring-opening copolymerization of omega-pentadecalactone (PDL) with p-dioxanone (DO) under mild reaction conditions (<80 degrees C, atmospheric pressure). Poly(PDL-co-DO) with high molecular weight (Mw > 30 000) and a wide range of comonomer contents was synthesized using various PDL/DO feed ratios. During the copolymerization reaction, large ring PDL was found to be more reactive than its smaller counterpart DO, resulting in higher PDL/DO unit ratios in polymer chains than the corresponding PDL/DO monomer ratios in the feed. The copolymers were typically isolated in 50-90 wt % yields as the monomer conversion was limited by the equilibrium between monomers and copolymer. 1H and 13C NMR analysis on poly(PDL-co-DO) formed by CALB showed that the copolymers contain nearly random sequences of PDL and DO units with a slight tendency toward alternating arrangements. Copolymerization with PDL was found to remarkably enhance PDO thermal stability. Differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) results demonstrate high crystallinity in all copolymers over the whole range of compositions. Depending on copolymer composition, the crystal lattice of either PDO or PPDL hosts units of the other comonomer, a behavior typical of an isodimorphic system. In poly(PDL-co-DO), both melting temperature and melting enthalpy display a minimum at 70 mol % DO, that is, at the pseudoeutectic composition. WAXS diffractograms show one crystal phase (that of either PPDL or PDO) on either side of the pseudoeutectic and coexistence of PPDL and PDO crystals at the pseudoeutectic.     

Synthesis, characterization, and bioavailability of mannosylated shell cross-linked nanoparticles

Saccharide-functionalized shell cross-linked (SCK) polymer micelles designed as polyvalent nanoscaffolds for selective interactions with receptors on Gram negative bacteria were constructed from mixed micelles composed of poly(acrylic acid-b-methyl acrylate) and mannosylated poly(acrylic acid-b-methyl acrylate). The mannose unit was conjugated to the hydrophilic chain terminus of the amphiphilic diblock copolymer precursor, from which the SCK nanoparticles were derived, by the growth of the diblock copolymer from a mannoside functionalized atom transfer radical polymerization (ATRP) initiator. Mixed micelle formation between the amphiphilic diblock copolymer and mannosylated amphiphilic diblock copolymer was followed by condensation-based cross-linking between the acrylic acid residues present in the periphery of the polymer micelles to afford SCK nanoparticles. SCKs presenting variable numbers of mannose functionalities were prepared from mixed micelles of controlled stoichiometric ratios of mannosylated and nonmannosylated diblock copolymers. The polymer micelles and SCKs were characterized by dynamic light scattering (DLS), electrophoretic light scattering, atomic force microscopy (AFM), transmission electron microscopy (TEM), and analytical ultracentrifugation (AU). Surface availability and bioactivity of the mannose units were evaluated by interactions of the nanostructures with the model lectin Concanavalin A via DLS studies, with red blood cells (rabbit) via agglutination inhibition assays and with bacterial cells (E. coli) via TEM imaging.     

Cocrystallization of random copolymers of omega-pentadecalactone and epsilon-caprolactone synthesized by lipase catalysis

Random copolymers were prepared by Candida antarctica lipase B (Novozyme-435) catalyzed copolymerization of omega-pentadecalactone (PDL) with epsilon-caprolactone (CL). Over the whole composition range PDL-CL copolymers are highly crystalline (melting enthalpy by differential scanning calorimetry, above 100 J/g; crystallinity degree by wide-angle X-ray scattering, WAXS, 60-70%). The copolymers melt at temperatures that linearly decrease with composition from that of poly(omega-pentadecalactone) (PPDL; 97 degrees C) to that of poly(epsilon-caprolactone) (PCL; 59 degrees C). The WAXS profiles of PCL and PPDL homopolymers are very similar, except for the presence in PPDL of the (001) reflection at 2theta = 4.58 degrees that corresponds to a 19.3 angstroms periodicity in the chain direction. In PDL-CL copolymers the intensity of this reflection decreases with increasing content of CL units and vanishes at 50 mol % CL, as a result of randomization of the ester group alignment and loss of chain periodicity. PDL-CL copolymers crystallize in a lattice that gradually changes from that of one homopolymer to that of the other, owing to comonomer isomorphous substitution. Cocrystallization of comonomer units is also shown by a random PDL-CL copolymer obtained in a polymerization/transesterification reaction catalyzed by C. antarctica lipase B (Novozyme-435) starting from preformed PCL and PDL monomer.     

Synthesis and characterization of biodegradable poly(ethylene glycol)-block-poly(5-benzyloxy-trimethylene carbonate) copolymers for drug delivery

Amphiphilic diblock copolymers with various block compositions were synthesized with monomethoxy-terminated poly(ethylene glycol) (MePEG) as the hydrophilic block and poly(5-benzyloxy-trimethylene carbonate) (PBTMC) as the hydrophobic block. When the copolymerization was conducted using MePEG as a macroinitiator and stannous 2-ethylhexanoate (Sn(Oct)2) as a catalyst, the molecular weight of the second block was uncontrollable, and the method only afforded a mixture of homopolymer and copolymer with a broad molecular weight distribution. By contrast, the use of the triethylaluminum-MePEG initiator yielded block copolymers with controllable molecular weight and a more narrow molecular weight distribution than the copolymers obtained using Sn(Oct)2. GPC and 1H NMR studies confirmed that the macroinitiator was consumed and the copolymer composition was as predicted. Two of the newly synthesized MePEG-b-PBTMC copolymers were evaluated in terms of properties primarily relating to their use in micellar drug delivery. MePEG-b-PBTMC micelles with a narrow monomodal size distribution were prepared using a high-pressure extrusion technique. The MePEG-b-PBTMC copolymers were also confirmed to be biodegradable and noncytotoxic.     

Methoxy poly(ethylene glycol)-block-poly(delta-valerolactone) copolymer micelles for formulation of hydrophobic drugs

Six amphiphilic diblock copolymers based on methoxy poly(ethylene glycol) (MePEG) and poly(delta-valerolactone) (PVL) with varying hydrophilic and hydrophobic block lengths were synthesized via a metal-free cationic polymerization method. MePEG-b-PVL copolymers were synthesized using MePEG with Mn = 2000 or Mn = 5000 as the macroinitiator. 1H NMR and GPC analyses confirmed the synthesis of diblock copolymers with relatively narrow molecular weight distributions (Mn/Mw = 1.05-1.14). DSC analysis revealed that the melting temperatures (Tm) of the copolymers (47-58 degrees C) approach the Tm of MePEG as the PVL content is decreased. MePEG-b-PVL copolymer aggregates loaded with the hydrophobic anti-cancer drug paclitaxel were found to have effective mean diameters ranging from 31 to 970 nm depending on the composition of the copolymers. A MePEG-b-PVL copolymer of a specific composition was found to form drug-loaded micelles of 31 nm in diameter with a narrow size distribution and improve the apparent aqueous solubility of paclitaxel by more than 9000-fold. The biological activity of paclitaxel formulated in the MePEG-b-PVL micelles was confirmed in human MCF-7 breast and A2780 ovarian cancer cells. Furthermore, the biocompatibility of the copolymers was established in CHO-K1 fibroblast cells using a cell viability assay. The in vitro hydrolytic and enzymatic degradation of the micelles was also evaluated over a period of one month. The present study indicates that the MePEG-b-PVL copolymers are suitable biomaterials for hydrophobic drug formulation and delivery.     

Synthesis and characterization of random and block copolypeptides derived from gamma-methylglutamate and leucine N-carboxyanhydrides

The synthesis of random and block copolypolyeptides derived from gamma-methylglutamate and leucine N-carboxyanhydrides using Al-Schiff's base complexes and allylamine as initiators is here reported. The copolymer structures were confirmed by (1)H and (13)C NMR. The calculation of the statistical average block lengths reveals the presence of longer methylglutamate units in the copolymer. The determination of the reactivity ratios indicated a slightly higher reactivity of gamma-methylglutamateNCA as compared to leucineNCA. Block copolypeptides containing glutamate and leucine units were obtained by sequential polymerization of the two NCAs using Al-Schiff's base complexes or allylamine in dioxane as solvent. Based on (13)C NMR spectra of copolymers exhibiting two signals corresponding to peptide linkages, we confirmed the block structure and concluded that the copolymerization proceeds by attack of an amino group present on a glutamate chain end onto a LeuNCA. The copolymerization with allylamine was also shown, from calculation of the average block lengths of sequences, to exhibit living behavior. Viscometry analysis further showed that molar masses of the copolypeptides obtained with Al-Schiff's base were quite close to those derived from allylamine, supporting the proposed mechanism of copolymerization.     

Lipase-catalyzed copolymerization of dialkyl carbonate with 1,4-butanediol and ω-pentadecalactone: synthesis of poly(ω-pentadecalactone-co-butylene-co-carbonate)

Candida antarctica lipase B (CALB) was successfully used to promote synthesis of aliphatic poly(carbonate-co-ester) copolymers from dialkyl carbonate, diol, and lactone monomers. The polymerization reactions were carried out in two stages: first-stage oligomerization under low vacuum, followed by second-stage polymerization under high vacuum. Therefore, copolymerization of ω-pentadecalactone (PDL), diethyl carbonate (DEC), and 1,4-butanediol (BD) yielded PDL-DEC-BD copolymers with a M(w) of whole product (nonfractionated) up to 33?000 and M(w)/M(n) between 1.2 and 2.3. Desirable reaction temperature for the copolymerization was found to be ～80 °C. The copolymer compositions, in the range from 10 to 80 mol % PDL unit content versus total (PDL + carbonate) units, were effectively controlled by adjusting the monomer feed ratio. Reprecipitation in chloroform/methanol mixture allowed isolation of the purified copolymers in up to 92% yield. (1)H and (13)C NMR analyses, including statistical analysis on repeat unit sequence distribution, were used to determine the polymer microstructures. The synthesized PDL-DEC-BD copolymers possessed near random structures with all possible combinations of PDL, carbonate, and butylene units via either ester or carbonate linkages in the polymer chains and are more appropriately named as poly(PDL-co-butylene-co-carbonate).     

Diblock copolymers based on dihydroxyacetone and ethylene glycol: synthesis, characterization, and nanoparticle formulation

Polymeric biomaterials have played an integral role in tissue engineering, biomedical devices, and targeted drug delivery. Block copolymers are especially important because their physical and chemical properties can be controlled by adjusting the ratio, size, and type of constituting blocks. Herein, the synthesis and characterization of diblock copolymers composed of poly(ethylene glycol) and a polycarbonate based on the metabolic intermediate, dihydroxyacetone, are reported. The length of the dihydroxyacetone-based block was controlled by adjusting the reactant feed ratios and initiator injection conditions. Intermediates and final products were characterized via (1)H NMR, GPC, DSC, TGA, and diffusion-ordered NMR spectroscopy. The dihydroxyacetone-based hompolymer is insoluble in water and most organic solvents, but is hydrophilic in nature. This, coupled with poly(ethylene glycol)'s solubility characteristics, allows the block copolymer to form nanoparticles in aqueous and organic anti-solvents. Dynamic light scattering and TEM results indicated the formation of spherical nanoparticles.     

Investigation of polymer and nanoparticle properties with nicotinic acid and p-aminobenzoic acid grafted on poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) via click chemistry

In this study, the grafting of nicotinic acid and p-aminobenzoic acid (PABA) onto poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) was performed by Huisgen's 1,3-dipolar cycloaddition, also known as click chemistry. Concentrations used for grafting were 0.10, 0.20, and 0.30 molar ratios with respect to caproyl units. The grafted copolymers were successfully obtained at all ratios as confirmed by NMR, GPC, and FT-IR. According to the DSC results, the polymorphisms of these grafted copolymers were mostly changed from semicrystalline to amorphous depending on the type and the amount of grafting compounds. TGA thermograms showed different thermal stabilities of the grafted copolymers compared to the original copolymers. Cytotoxicity results from HUVEC models suggested that the toxicity of grafted nanoparticles increased with the molar ratios of grafting units. Due to differences in molecular structure between nicotinic acid and PABA, physicochemical properties (particle size and surface charge) of grafted copolymer nanoparticles were substantially different. With increasing molar ratio of the grafting units, the particle size of blank nanoparticles tended to increase, resulting from an increase in the hydrophobic fragments of the grafted copolymer. Ibuprofen was chosen as a model drug to evaluate the interaction between grafted copolymers and loaded drug. After ibuprofen loading, the particle size of the loaded nanoparticles of both grafted copolymers increased compared to that of the blank nanoparticles. Significant differences in loading capacity between nicotinic acid and PABA grafted copolymer nanoparticles were clearly shown. This is most likely a result of different compatibility between each grafting compound and ibuprofen, including hydrogen bond interaction, π-π stacking interaction, and steric hindrance.     

Monohydroxylated poly(3-hydroxyoctanoate) oligomers and its functionalized derivatives used as macroinitiators in the synthesis of degradable diblock copolyesters

The presence of a hydroxyl group at the end of poly(3-hydroxyoctanoate) oligomers, noted PHO oligomers, is required to prepare diblock copolymers with improved properties by ring-opening polymerization of cyclic monomer as epsilon-caprolactone. Several chemical methods such as basic hydrolysis, acid-catalyzed reaction with APTS, and methanolysis were used to prepare well-defined low molar masses PHO oligomers. The methanolysis reaction was allowed to proceed for 10-60 min to produce PHO oligomers with Mn values ranging from 20,000 to 800 g mol-1 with low polydispersity index. Detailed analysis of the MALDI-TOF mass spectra of the obtained oligomers has revealed the presence of linear structures bearing methyl ester on one side and hydroxyl end group on the other side. The same procedure was applied to poly(3-hydroxyoctanoate-co-3-hydroxyundecenoate), PHOU, a poly(3-hydroxyalkanoate) containing unsaturated units in its side chains. These oligomers were further used to initiate the polymerization of epsilon-caprolactone by varying the PHO (or PHOU) and PCL lengths. By copolymerization with epsilon-caprolactone, the properties of PHO or PHOU have been improved. The crystallinity of the obtained copolymers was modified by controlling the length of the two different blocks. The unsaturations in the side chains of the PHOU block were oxidized in acid carboxylic functions to obtain a novel artificial biopolyester. Moreover, degradation was followed to study the influence of carboxylic groups on the hydrolysis of the copolymers.     

Structural characterization of a lipase-catalyzed copolymerization of epsilon-caprolactone and D,L-lactide

The copolymerization of epsilon-caprolactone (epsilon-CL) and d,l-lactide catalyzed by Candida antarctica lipase B was studied. Copolymerizations with different epsilon-CL-to-lactide ratios were carried out, and the product was monitored and characterized by MALDI-TOF MS, GPC, and (1)H NMR. The polymerization of epsilon-CL, which is normally promoted by C. antarctica lipase B, is initially slowed by the presence of lactide. During this stage, lactide is consumed more rapidly than epsilon-CL, and the incorporation occurs dimer-wise with regard to the lactic acid (LA) units. As the reaction proceeds, the relative amount of CL units in the copolymer increases. The nonrandom copolymer structure disappears with time, probably due to a lipase-catalyzed transesterification reaction. In the copolymerizations with a low content of lactide, macrocycles of poly(epsilon-caprolactone) and copolymers having up to two LA units in the ring were detected.     

Photocurable liquid biodegradable copolymers: in vitro hydrolytic degradation behaviors of photocured films of coumarin-endcapped poly(epsilon-caprolactone-co-trimethylene carbonate)

Coumarin-endcapped tetrabranched liquid copolymers composed of epsilon-caprolactone and trimethylene carbonate (TMC), prepared using pentaerythritol or four-branched poly(ethylene glycol) (PEG) as an initiator, were ultraviolet irradiated to produce photocured solid biodegradable copolymers. The hydrolytic degradation behaviors of photocured films were determined from the weight loss of the films. The initial hydrolysis rate (determined for up to 24 h using a quartz crystal microbalance) was enhanced with aqueous solutions of higher pH. The hydrolysis rate in the early period of immersion was increased with an increase in TMC content, whereas that in the later period (week order) decreased with a increase in TMC content. This inverse relation of composition dependence on the hydrolysis rate between the early and late periods was discussed. Topological measurements using scanning electron microscopy and atomic force microscopy as well as depth profiles of the fluorescein-stained hydrolyzed layer showed that for the pentaerythritol-initiated copolymer, irrespective of copolymer composition, hydrolysis occurred at surface regions and surface erosion proceeded with immersion time. For PEG-based copolymers, both surface erosion and bulk degradation occurred simultaneously. The hydrolyzed surfaces became highly wettable with water and exhibited noncell adhesivity.     

Synthesis of copolymers containing an active ester of methacrylic acid by RAFT: controlled molecular weight scaffolds for biofunctionalization

We report the controlled radical copolymerization of N-(2-hydroxypropyl)methacrylamide (HPMA) with a monomer containing an active ester, N-methacryloyloxysuccinimide (NMS), by reversible addition fragmentation chain transfer (RAFT). The large difference in the reactivity ratios of HPMA and NMS resulted in significant variations in copolymer composition with increasing conversion during batch copolymerization. The use of a semi-batch copolymerization method, involving the gradual addition of the more reactive NMS, allowed uniformity of copolymer composition to be maintained during the polymerization. We synthesized polymers in a wide range of molecular weights (M(n) = 3000-50,000 Da) with low polydispersities (1.1-1.3). The effect of the ratio of monomer to chain transfer agent (CTA) on the molecular weight of the polymer was investigated. Given the numerous applications of poly(HPMA)-based conjugates in designing polymeric therapeutics, these controlled molecular weight activated polymers represent attractive scaffolds for biofunctionalization. As a demonstration, we attached a peptide to the activated polymer backbone to synthesize a potent controlled molecular weight polyvalent inhibitor of anthrax toxin.     

Linear polyethyleneimine grafted to a hyperbranched poly(ethylene glycol)-like core: a copolymer for gene delivery

A block copolymer of a hyperbranched poly(ethylene glycol)-like core and linear polyethylenimine (HBP) was synthesized by a facile synthetic route that included (1) a single-step cationic copolymerization of diepoxy and polyhydroxyl monomers, (2) derivatization of hydroxyl groups of the core HBPEG copolymer with either tosyl or chloromethylbenzoyl chlorides resulting in a corresponding macroinitiator, and (3) synthesis of HBPEG-block-poly(alkyl oxazolines). HBPEG-block-linear polyethyleneimine (HBP) was obtained by hydrolysis of HBPEG-block-poly(alkyl oxazolines). Linear PEI-bearing hyperbranched polycations (HBP) had lower inherent toxicity in cell culture than PEG-grafted linear polyethyleneimines (PEGLPEI). PEGLPEI formed a complex with DNA with an average diameter of 250 nm. The complexes were loosely condensed and formed aggregates and precipitates during storage. By contrast, hyperbranched polycations (HBP) formed approximately 50 nm nanocomplexes with DNA that were stable for several weeks and showed resistance to DNAse I-mediated degradation. The 'inverted' block copolymers showed several orders of magnitude higher transfection efficiency than PEGLPEI in vitro. Because of the biocompatibility and higher transfection efficiency, the 'inverted' block copolymer merits further investigation as a gene carrier.     

Topology characterization by MALDI-ToF-MS of enzymatically synthesized poly(lactide-co-glycolide)

Lipase catalyzed copolymerization of the monomers lactide and glycolide by Pseudomonas cepacia employing a molar ratio of 80L/20G has been studied. The copolymers were characterized by MALDI-ToF-MS, DSC, SEC and NMR. MALDI-ToF-MS has successfully been used not only to determine end groups and chemical composition but even the microstructure of the copolymers. We demonstrated that for this lipase catalyzed copolymerization, the main product of the reaction at 100 degrees C was linear homopolymer of lactide while at 130 degrees C the main product was cyclic random copolymer.     

Mannosylated poly(ethylene oxide)-b-poly(epsilon-caprolactone) diblock copolymers: synthesis, characterization, and interaction with a bacterial lectin

A novel bioeliminable amphiphilic poly(ethylene oxide)-b-poly(epsilon-caprolactone) (PEO-b-PCL) diblock copolymer end-capped by a mannose residue was synthesized by sequential controlled polymerization of ethylene oxide and epsilon-caprolactone, followed by the coupling of a reactive mannose derivative to the PEO chain end. The anionic polymerization of ethylene oxide was first initiated by potassium 2-dimethylaminoethanolate. The ring-opening polymerization of epsilon-caprolactone was then initiated by the omega-hydroxy end-group of PEO previously converted into an Al alkoxide. Finally, the saccharidic end-group was attached by quaternization of the tertiary amine alpha-end-group of the PEO-b-PCL with a brominated mannose derivative. The copolymer was fully characterized in terms of chemical composition and purity by high-resolution NMR spectroscopy and size exclusion chromatography. Furthermore, measurements with a pendant drop tensiometer showed that both the mannosylated copolymer and the non-mannosylated counterpart significantly decreased the dichloromethane/water interfacial tension. Moreover, these amphiphilic copolymers formed monodisperse spherical micelles in water with an average diameter of approximately 11 nm as measured by dynamic light scattering and cryo-transmission electron microscopy. The availability of mannose as a specific recognition site at the surface of the micelles was proved by isothermal titration microcalorimetry (ITC), using the BclA lectin (from Burkholderia cenocepacia), which interacts selectively with alpha-D-mannopyranoside derivatives. The thermodynamic parameters of the lectin/mannose interaction were extracted from the ITC data. These colloidal systems have great potential for drug targeting and vaccine delivery systems.     

Interchain heterogeneity in 2-hydroxyethylglutamine-l-valine random copolymers

Solid state circular dichroism (c.d.) and infrared (i.r.) studies of water soluble and insoluble fractions of poly(hydroxyethylglutamine-valine) random copolymers, prepared from parent γ-benzyl l-glutamate valine copolymers, show that interchain conformational heterogeneity with interchain compositional heterogeneity is present when the respective N-carboxyanhydrides are copolymerized in dioxan or benzene/methylene chloride. Use of previously determined reactivity ratios for the aforementioned copolymer systems permits the determination of the variation of the average copolymer composition, $\text{f}$_G, with conversion. The experimentally determined average copolymer composition.$\text{f}$_G for the use of the respective reactivity ratios and the copolymer hydroxyethylglutamine, valine are predicted by the use of the respective reactivity ratios and the copolymer composition equation. As the valine content of the copolymer chains in the fractions increases, the expected increase in β-sheet contribution is seen. Comparison of the experimentally determined solid state c.d. spectra with Greenfield and Fasman's computer generated c.d. spectra for varying amounts of α-helix, β-sheet and random structures, shows that the water insoluble fractions with their increased valine contents have a greater contribution of β-sheet structure than the respective soluble fractions.     

Biocompatible and biodegradable polymersome encapsulated hemoglobin: a potential oxygen carrier

This work describes the development of polymersome-encapsulated hemoglobin (PEH) self-assembled from biodegradable and biocompatible amphiphilic diblock copolymers composed of poly(ethylene oxide) (PEO), poly(caprolactone) (PCL), and poly(lactide) (PLA). In the amphiphilic diblock, PEO functions as the hydrophilic block, while either PCL or PLA can function as the hydrophobic block. PEO, PCL, and PLA are biocompatible polymers, while the last two polymers are biodegradable. PEH dispersions were prepared by extrusion through 100 nm pore radii polycarbonate membranes. In this work, the encapsulation efficiency of human and bovine hemoglobin (hHb and bHb) in polymersomes was adjusted by varying the initial concentration of Hb. This approach yielded Hb loading capacities that were comparable to values in the literature that supported the successful resuscitation of hamsters experiencing hemorrhagic shock. Moreover, the Hb loading capacities of PEHs in this study can also be tailored simply by controlling the diblock copolymer concentration. In this study, typical Hb/diblock copolymer weight ratios ranged 1.2-1.5, with initial Hb concentrations less than 100 mg/mL. The size distribution, Hb encapsulation efficiency, oxygen affinity (P 50), cooperativity coefficient (n), and methemoglobin (metHb) level of these novel PEH dispersions were consistent with values required for efficient oxygen delivery in the systemic circulation. Taken together, our results demonstrate the development of novel PEH dispersions that are both biocompatible and biodegradable. These novel dispersions show very good promise as therapeutic oxygen carriers.     

Correlation between the sequence-length distribution and structure of statistical copolymers of L-valine and γ-benzyl-L-glutamate

Statistical copolymers were prepared from N-carboxyanhydrides of L -valine and γ-benzyl-L -glutamate in dioxan with triethylamine as an initiator. The copolymerization conversion was determined by ir spectroscopy, the copolymer composition by amino acid analysis, and the molecular weights by light scattering. The monomer reactivity ratios were found to be r_Val = 0.14 and r_Glu(OBzl) = 6.4. High-molecular-weight copolymers are formed even at low conversions. The content of β-structure in the copolymers was estimated from the ir spectra in copolymerization mixtures. The sequence-length distribution of L -valine and γ-benzyl-L -glutamate copolymers was calculated and its dependence on copolymerization conversion is discussed. Relations between the sequence-length distribution and the content of β-structure were studied. It was found that the content of β-structure in samples with the same composition is different for low- and high-conversion copolymers. The formation of β-structure in copolymers in the copolymerization mixture requires a certain minimal sequence length, which has been found to be about 6 valine units.     

Design of polypeptide-functionalized polystyrene microspheres

In this contribution, the principle of spontaneous surface segregation has been applied for the preparation of polypeptide-functionalized polystyrene microspheres. For that purpose, an amphiphilic diblock copolymer was introduced in the mixture styrene/divinylbenzene and polymerized using AIBN as initiator. During the polymerization, cross-linked particles were obtained in which the diblock copolymer was encapsulated. The amphiphilic diblock copolymers used throughout this study contain a hydrophilic polypeptide segment, either poly(L-lysine) or poly(L-glutamic acid) and a hydrophobic polystyrene block. After 4 h of polymerization, rather monodisperse particles with sizes of approximately 3-4 microm were obtained. Upon annealing in hot water, the hydrophilic polypeptides migrate to the interface, hence, either positively charged or neutral particles were obtained when poly(L-lysine) is revealed at the surface and exposed to acidic or basic pH, respectively. On the opposite, negatively charged particles were achieved in basic pH water by using poly(L-glutamic acid) as additive. The surface chemical composition was modified by changing the environment of the particles. Thus, exposure in toluene provoked a surface rearrangement, and due to its affinity, the polystyrene block reorients toward the interface.