Synthesis and characterization of macroinitiator‐amino terminated PEG and poly(γ‐benzyl‐L‐glutamate)‐PEO‐poly(γ‐benzyl‐L‐glutamate) triblock copolymer

Macroinitiator‐amino terminated poly(ethylene glycol) (PEG) (NH₂‐PEO‐NH₂) was prepared by converting both terminal hydroxyl groups of PEG to more reactive primary amino groups. The synthetic route involved reactions of chloridize, phthalimide and finally hydrazinolysis. Furthermore, poly(γ‐benzyl‐L ‐glutamate)‐poly(ethylene oxide)‐poly(γ‐benzyl‐L ‐glutamate) (PBLG‐PEO‐PBLG) triblock copolymer was synthesized by polymerization of γ‐benzyl‐L ‐glutamate N‐carboxyanhydride (Bz‐L‐GluNCA) using NH₂‐PEO‐NH₂ as macroinitiator. The resultant NH₂‐PEO‐NH₂ and triblock copolymer were characterized by FT‐IR, ¹H‐NMR and gel permeation chromatography (GPC) techniques. The results demonstrated that the degree of amination of the NH₂‐PEO‐NH₂ could be up to 1.95. The molecular weight of the PBLG‐PEO‐PBLG triblock copolymer could be adjusted easily by controlling the molar ratio of Bz‐L ‐Glu NCA to the macroinitiator NH₂‐PEO‐NH₂. Copyright © 2004 John Wiley & Sons, Ltd.     

Aggregation behavior of poly(γ‐benzyl‐L‐glutamate)‐b‐polyisoprene‐b‐poly(γ‐benzyl‐L‐glutamate) rod‐coil‐rod triblock copolymer in N,N‐dimethylformamide

Solution property of poly(γ‐benzyl‐L ‐glutamate)‐b‐polyisoprene‐b‐poly(γ‐benzyl‐L ‐glutamate) (GIG copolymer) was studied by using dynamic light scattering and static light scattering for N,N‐dimethylformamide (DMF) solution and DMF/toluene mixed solutions. GIG copolymer proved to aggregate in DMF and under DMF‐rich condition, that is, high‐polar region. The aggregate decreased in size, and completely disappeared under toluene‐rich condition, that is, low‐polar region. The correlation between solubility parameter and aggregate size of GIG copolymer in the DMF/toluene solution systems quantitatively demonstrated how strongly polarity caused by hydrogen bond made an impact on the aggregation behavior. Because the main driving force to the aggregation under DMF‐rich condition originates with polyisoprene (PIP) blocks, the aggregate in DMF is considered to be a core‐shell micelle consisting of flexible PIP core surrounded by rigid poly(γ‐benzyl‐L ‐glutamate) (PBLG) shell. The values of dimensionless parameter ρ, defined as the ratio of radius of gyration 〈S²〉^1/2 to hydrodynamic radius R_H, revealed that a single chain of GIG copolymer had the form of rigid rod with flexibility, that is, once‐broken rod, caused by the incorporation of a flexible PIP chain between two rigid PBLG rods in the DMF/toluene solution system. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1740–1748, 2010     

Convenient synthesis of poly(γ‐benzyl‐L‐glutamate) from activated urethane derivatives of γ‐benzyl‐L‐glutamate

A series of activated urethane‐type derivatives of γ‐benzyl‐L ‐glutamate were synthesized, and their potential as monomers for polypeptide synthesis was investigated. The derivatives of the focus of this work were a series of N‐aryloxycarbonyl‐γ‐benzyl‐L ‐glutamate 1 , of which aryl groups were phenyl, 4‐chlorophenyl, and 4‐nitrophenyl. These urethanes 1 were reactive in polar solvents such as dimethylsulfoxide, N,N‐dimethylformamide (DMF), and N,N‐dimethylacetamide (DMAc), and were efficiently converted into poly(γ‐benzyl‐L ‐glutamate) (poly(BLG)) under mild conditions; at 60 °C without addition of any catalyst. Among the three urethanes, that having 4‐nitrophenoxycarbonyl group 1c was the most reactive to give poly(BLG) efficiently, as was expected from the highly electron deficient nature of the nitrophenoxycarbonyl group. On the other hand, the urethane 1a having phenoxycarbonyl group was also efficiently converted into poly(BLG), in spite of the intrinsically less electrophilicity of the phenoxycarbonyl group. In addition, the successful formation of poly(BLG) by the reaction of 1a favored its diluted concentration (0.1 M) much more than 2.0 M, the optimum initial concentration for 1c . ¹H NMR spectroscopic analyses of the reactions in situ revealed that the predominant pathway from 1 to poly(BLG) involved the intramolecular cyclization of 1 into the corresponding N‐carboxyanhydride, with release of phenol and its successive ring‐opening polymerization with release of carbon dioxide. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2649–2657, 2008     

Synthesis of poly(benzyl glutamate‐b‐styrene) rod‐coil block copolymers by dual initiation in one pot

We have developed a metal free synthetic pathway to homopolypeptide rod‐coil block copolymers. The concept was proven for the synthesis of poly(benzyl‐L ‐glutamate‐b‐styrene). A dual initiator containing a primary amine and a nitroxide group was used in a macroinitiation approach with high initiation efficiency. Good control over the molecular weight in the ring opening polymerization of benzyl‐L ‐glutamate N‐carboxyanhydride was obtained in DMF at 0 °C yielding poly(benzyl‐L ‐glutamates) with low polydispersities around 1.1. The almost quantitative incorporation of the dual initiator was confirmed by MALDI‐ToF analysis. Macroinitiation of styrene by nitroxide‐mediated controlled radical polymerization yielded the block copolymer with high structural control. The diblock structure was confirmed by molecular weight increase upon macroinitiation by size exclusion chromatography and retention time comparison with homopolymers using gradient polymer elution chromatography. Both polymerizations were also successfully conducted in one pot without intermediate isolation owing to the high compatibility of both polymerization techniques. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3068–3077, 2008     

Synthesis and thermoreversible gelation of dendron‐like polypeptide/linear poly(ε‐caprolactone)/dendron‐like polypeptide triblock copolymers

Dendron‐like poly(γ‐benzyl‐L ‐glutamate)/linear poly(ε‐caprolactone)/dendron‐like poly(γ‐benzyl‐L ‐glutamate) triblock copolymers having 2^{m + 1} PBLG branches (denoted as PBLG‐Dm‐PCL‐Dm‐PBLG, m = 0, 1, 2, and 3) were for the first time synthesized by utilizing ring‐opening polymerization (ROP) and click chemistry. The bifunctional azide‐terminated PCL (N₃‐PCL‐N₃) was click conjugated with propargyl focal point PAMAM‐typed dendrons Dm to generate Dm‐PCL‐Dm, which was then used as macroinitiator for the ROP of BLG‐NCA monomer to produce the targeted PBLG‐Dm‐PCL‐Dm‐PBLG triblock copolymers. Their molecular structures and physical properties were characterized in detail by FTIR, NMR, gel permeation chromatography, differential scanning calorimetry, and wide angle X‐ray diffraction (WAXD). The crystallinity of the central PCL segment within these copolymers is increasingly suppressed by the flanking PBLG wedges, whereas the PBLG segments gradually changed from a β‐sheet conformation to an α‐helix conformation with the increasing PBLG branches. These triblock copolymers formed thermoreversible organogels in toluene, and the dendritic topology of PBLG wedges controlled their critical gelation concentrations. The self‐assembled structure of organogels was further characterized by means of transmission electron microscopy, WAXD, and small‐angle X‐ray scattering. The fibers with flat ribbon morphology were clearly shown, and the gelation occurred through a self‐assembled nanoribbon mechanism. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 709–718, 2010     

Controlled ring‐opening polymerization of L‐lactide and 1,5‐dioxepan‐2‐one forming a triblock copolymer

Novel elastomeric A‐B‐A triblock copolymers were successfully synthesized in a new two‐step process: controlled ring‐opening polymerization of the cyclic ether–ester 1,5‐dioxepan‐2‐one as the amorphous middle block (B‐block) followed by addition and polymerization of the two semicrystalline L ‐lactide blocks (A‐block). A 1,1,6,6‐tetra‐n‐butyl‐1,6‐distanna‐2,5,7,10‐tetraoxacyclodecane initiator system was utilized and the reaction was performed in chloroform at 60 °C. A good control of the synthesis was obtained, resulting in well defined triblock copolymers. The molecular weight and chemical composition were easily adjusted by the monomer‐to‐initiator ratio. The triblock copolymers formed exhibited semicrystallinity up to a content of 1,5‐dioxepan‐2‐one as high as 89% as determined by differential scanning calorimetry. WAXS investigation of the triblock copolymers showed a crystal structure similar to that of the pure poly(L ‐lactide). © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1774–1784, 2000     

RGD peptide grafted biodegradable amphiphilic triblock copolymer poly(glutamic acid)‐b‐poly(L‐lactide)‐b‐poly(glutamic acid): Synthesis and self‐assembly

A novel amphiphilic biodegradable triblock copolymer (PGL‐PLA‐PGL) with polylactide (PLA) as hydrophobic middle block and poly(glutamic acid) (PGL) as hydrophilic lateral blocks was successfully synthesized by ring‐opening polymerization (ROP) of L ‐lactide (LA) and N‐carboxy anhydride (NCA) consecutively and by subsequent catalytic hydrogenation. The results of cell experiment of PGL‐PLA‐PGL suggested that PGL could improve biocompatibility of polyester obviously. The copolymer could form micelles of spindly shape easily in aqueous solution. The pendant carboxyl groups of the triblock copolymer were further activated with N‐hydroxysuccinimide and combined with a cell‐adhesive peptide GRGDSY. Incorporation of the oligopeptide further enhanced the hydrophilicity and led to formation of spherical micelles. PGL‐PLA‐PGL showed better cell adhesion and spreading ability than pure PLA and the GRGDSY‐containing copolymer exhibited even further improvement in cell adhesion and spreading ability, indicating that the copolymer could find a promising application in drug delivery or tissue engineering. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3218–3230, 2007     

Poly(ϵ‐caprolactone‐b‐glycolide) and poly(D,L‐lactide‐b‐glycolide) diblock copolyesters: Controlled synthesis,characterization, and colloidal dispersions

Living ω‐aluminum alkoxide poly‐ϵ‐caprolactone and poly‐D,L ‐lactide chains were synthesized by the ring‐opening polymerization of ϵ‐caprolactone (ϵ‐CL) and D,L ‐lactide (D,L ‐LA), respectively, and were used as macroinitiators for glycolide (GA) polymerization in tetrahydrofuran at 40 °C. The P(CL‐b‐GA) and P(LA‐b‐GA) diblock copolymers that formed were fractionated by the use of a selective solvent for each block and were characterized by ¹H NMR spectroscopy and differential scanning calorimetry analysis. The livingness of the operative coordination–insertion mechanism is responsible for the control of the copolyester composition, the length of the blocks, and, ultimately, the thermal behavior. Because of the inherent insolubility of the polyglycolide blocks, microphase separation occurs during the course of the sequential polymerization, resulting in a stable, colloidal, nonaqueous copolymer dispersion, as confirmed by photon correlation spectroscopy. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 294–306, 2001     

Synthesis of star‐shaped poly(D,L‐lactic acid‐alt‐glycolic acid)‐b‐poly(L‐lactic acid) with the poly(D,L‐lactic acid‐alt‐glycolic acid) macroinitiator and stannous octoate catalyst

Two types of three‐arm and four‐arm, star‐shaped poly(D,L ‐lactic acid‐alt‐glycolic acid)‐b‐poly(L ‐lactic acid) (D,L ‐PLGA50‐b‐PLLA) were successfully synthesized via the sequential ring‐opening polymerization of D,L ‐3‐methylglycolide (MG) and L ‐lactide (L ‐LA) with a multifunctional initiator, such as trimethylolpropane and pentaerythritol, and stannous octoate (SnOct₂) as a catalyst. Star‐shaped, hydroxy‐terminated poly(D,L ‐lactic acid‐alt‐glycolic acid) (D,L ‐PLGA50) obtained from the polymerization of MG was used as a macroinitiator to initiate the block polymerization of L ‐LA with the SnOct₂ catalyst in bulk at 130 °C. For the polymerization of L ‐LA with the three‐arm, star‐shaped D,L ‐PLGA50 macroinitiator (number‐average molecular weight = 6800) and the SnOct₂ catalyst, the molecular weight of the resulting D,L ‐PLGA50‐b‐PLLA polymer linearly increased from 12,600 to 27,400 with the increasing molar ratio (1:1 to 3:1) of L ‐LA to MG, and the molecular weight distribution was rather narrow (weight‐average molecular weight/number‐average molecular weight = 1.09–1.15). The ¹H NMR spectrum of the D,L ‐PLGA50‐b‐PLLA block copolymer showed that the molecular weight and unit composition of the block copolymer were controlled by the molar ratio of L ‐LA to the macroinitiator. The ¹³C NMR spectrum of the block copolymer clearly showed its diblock structures, that is, D,L ‐PLGA50 as the first block and poly(L ‐lactic acid) as the second block. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 409–415, 2002     

Temperature‐induced spontaneous sol–gel transitions of poly(D,L‐lactic acid‐co‐glycolic acid)‐b‐poly(ethylene glycol)‐b‐poly(D,L‐lactic acid‐co‐glycolic acid) triblock copolymers and their end‐capped derivatives in water

The spontaneous hydrogel formation of a sort of biocompatible and biodegradable amphiphilic block copolymer in water was observed, and the underlying gelling mechanism was assumed. A series of ABA‐type triblock copolymers [poly(D,L ‐lactic acid‐co‐glycolic acid)‐b‐poly(ethylene glycol)‐b‐poly(D,L ‐lactic acid‐co‐glycolic acid)] and different derivatives end‐capped by small alkyl groups were synthesized, and the aqueous phase behaviors of these samples were studied. The virgin triblock copolymers and most of the derivatives exhibited a temperature‐dependent reversible sol–gel transition in water. Both the poly(D,L ‐lactic acid‐co‐glycolic acid) length and end group were found to significantly tune the gel windows in the phase diagrams, but with different behaviors. The critical micelle concentrations were much lower than the associated critical gel concentrations, and an intact micellar structure remained after gelation. A combination of various measurement techniques confirmed that the sol–gel transition with an increase in the temperature was induced not simply via the self‐assembly of amphiphilic polymer chains but also via the further hydrophobic aggregation of micelles resulting in a micelle network due to a large‐scale self‐assembly. The coarsening of the micelle network was further suggested to account for the transition from a transparent gel to an opaque gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1122–1133, 2007     

Synthesis of poly(D,L‐lactic acid‐alt‐glycolic acid) from D,L‐3‐methylglycolide

D ,L ‐3‐Methylglycolide (MG) was synthesized via two step reactions with a good yield (42%). It was successfully polymerized in bulk with stannous octoate as a catalyst at 110 °C. The effects of the polymerization time and catalyst concentration on the molecular weight and monomer conversion were studied. Poly(D ,L ‐lactic acid‐co‐glycolic acid) (D ,L ‐PLGA50; 50/50 mol/mol) copolymers were successfully synthesized from the homopolymerization of MG with high polymerization rates and high monomer conversions under moderate polymerization conditions. ¹H NMR spectroscopy indicated that the bulk ring‐opening polymerization of MG conformed to the coordination–insertion mechanism. ¹³C NMR spectra of D ,L ‐PLGA50 copolymers obtained under different experimental conditions revealed that the copolymers had alternating structures of lactyl and glycolyl. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4179–4184, 2000     

Poly(L‐glutamic acid) grafted with oligo(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl methacrylate): Thermal phase transition,secondary structure,and self‐assembly

Thermoresponsive and pH‐responsive graft copolymers, poly(L ‐glutamate)‐g‐oligo(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl methacrylate) and poly(L ‐glutamic acid‐co‐(L ‐glutamate‐g‐oligo(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl methacrylate))), were synthesized by ring‐opening polymerization (ROP) of N‐carboxyanhydride (NCA) monomers and subsequent atom transfer radical polymerization of 2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl methacrylate. The thermoresponsiveness of graft copolymers could be tuned by the molecular weight of oligo(2‐(2‐(2‐methoxyethoxy)ethoxy)ethyl methacrylate) (OMEO₃MA), composition of poly(L ‐glutamic acid) (PLGA) backbone and pH of the aqueous solution. The α‐helical contents of graft copolymers could be influenced by OMEO₃MA length and pH of the aqueous solution. In addition, the graft copolymers exhibited tunable self‐assembly behavior. The hydrodynamic radius (R_h) and critical micellization concentration values of micelles were relevant to the length of OMEO₃MA and the composition of biodegradable PLGA backbone. The R_h could also be adjusted by the temperature and pH values. Lastly, in vitro methyl thiazolyl tetrazolium (MTT) assay revealed that the graft copolymers were biocompatible to HeLa cells. Therefore, with good biocompatibility, well‐defined secondary structure, and mono‐, dual‐responsiveness, these graft copolymers are promising stimuli‐responsive materials for biomedical applications. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Living polymerization of D,L‐3‐methylglycolide initiated with bimetallic (Al/Zn) μ‐oxo alkoxide and copolymers thereof

D,L ‐3‐Methylglycolide (MG) was successfully polymerized with bimetallic (Al/Zn) μ‐oxo alkoxide as an initiator in toluene at 90 °C. The effect of the initiator concentration and monomer conversion on the molecular weight was studied. It is shown that the polymerization of MG follows a living process. A kinetic study indicated that the polymerization approximates the first order in the monomer, and no induction period was observed. ¹H NMR spectroscopy showed that the ring‐opening polymerization proceeds through a coordination–insertion mechanism with selective cleavage of the acyl–oxygen bond of the monomer. On the basis of ¹H NMR and ¹³C NMR analyses, the selective cleavage of the acyl–oxygen bond of the monomer mainly occurs at the least hindered carbonyl groups (P₁ = 0.84, P₂ = 0.16). Therefore, the main chain of poly(D,L ‐lactic acid‐co‐glycolic acid) (50/50 molar ratio) obtained from the homopolymerization of MG was primarily composed of alternating lactyl and glycolyl units. The diblock copolymers poly(ϵ‐caprolactone)‐b‐poly(D,L ‐lactic acid‐alt‐glycolic acid) and poly(L ‐lactide)‐b‐poly(D,L ‐lactic acid‐alt‐glycolic acid) were successfully synthesized by the sequential living polymerization of related lactones (ϵ‐caprolactone or L ‐lactide). ¹³C NMR spectra of diblock copolymers clearly show their pure diblock structures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 357–367, 2001     

Preparation and characterization of poly(2‐methacryloyloxyethyl phosphorylcholine)‐block‐poly(D,L‐lactide) polymer nanoparticles

A poly(D,L ‐lactide)–bromine macroinitiator was synthesized for use in the preparation of a novel biocompatible polymer. This amphiphilic diblock copolymer consisted of biodegradable poly(D,L ‐lactide) and 2‐methacryloyloxyethyl phosphorylcholine and was formed by atom transfer radical polymerization. Polymeric nanoparticles were prepared by a dialysis process in a select solvent. The shape and structure of the polymeric nanoparticles were determined by ¹H NMR, atomic force microscopy, and ζ‐potential measurements. The results of cytotoxicity tests showed the good cytocompatibility of the lipid‐like diblock copolymer poly(2‐methacryloyloxyethyl phosphorylcholine)‐block‐poly(D,L ‐lactide). © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 688–698, 2007     

Arborescent micelles: Dendritic poly(γ‐benzyl l‐glutamate) cores grafted with hydrophilic chain segments

Amphiphilic copolymers were obtained by grafting arborescent poly(γ‐benzyl l ‐glutamate) (PBG) cores of generations G1–G3 with polyglycidol, poly(ethylene oxide) (PEO), or poly(l ‐glutamic acid) (PGA) chain segments. The PBG substrates were synthesized by two methods: (1) subjecting PBG samples with a dispersity ? = M_w/M_n < 1.1 to partial acidolysis of the benzyl ester groups, to produce randomly distributed carboxylic acid functionalities, and (2) using PBG chains containing a glutamic acid di‐tert‐butyl ester initiator fragment in the last grafting cycle of the PBG core synthesis, and selective acidolysis of the tert‐butyl ester groups to obtain substrates with carboxylic acid termini. Linear polymers with ? < 1.20 and a primary amine terminus were also synthesized to serve as hydrophilic shell materials: Polyglycidol and PEO by anionic polymerization, and PGA by N‐carboxyanhydride ring‐opening polymerization. These polymers, combined with the two different PGB substrate types, allowed the evaluation of the usefulness of random versus chain‐end grafting in producing arborescent copolymers useful as unimolecular micelles in organic and aqueous media. Size exclusion chromatography served to determine the grafting yield, molar mass, dispersity, and branching functionality of the copolymers. Dynamic light scattering measurements provided information on their aggregation behavior in aqueous environments. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1197–1209     

Dielectric study of the phase diagram of the poly(γ‐benzyl‐L‐glutamate)/dimethylformamide system

The phase diagram for solutions of poly(γ‐benzyl‐L ‐glutamate) in dimethylformamide has been investigated over the entire composition and temperature ranges with dielectric spectroscopy. The dielectric response in this system is dominated by ionic mobility, and phase transitions have been detected as changes in the ionic conductivity. The phase boundaries, determined by dielectric spectroscopy, are consistent with earlier published results obtained by a combination of optical microscopy observations and nuclear magnetic resonance and differential scanning calorimetry studies. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3943–3952, 2004     

One‐step synthesis of poly(N‐vinylpyrrolidone)‐b‐poly(l‐lactide) block copolymers using a dual initiator for RAFT polymerization and ROP

Amphiphilic, biocompatible poly(N‐vinylpyrrolidone)‐b‐poly(l ‐lactide) (PVP‐b‐PLLA) block polymers were synthesized at 60 °C using a hydroxyl‐functionalized N,N‐diphenyldithiocarbamate reversible addition–fragmentation chain transfer (RAFT) agent, 2‐hydroxyethyl 2‐(N,N‐diphenylcarbamothioylthio)propanoate (HDPCP), as a dual initiator for RAFT polymerization and ring‐opening polymerization (ROP) in a one‐step procedure. 4‐Dimethylamino pyridine was used as the ROP catalyst for l ‐lactide. The two polymerization reactions proceeded in a controlled manner, but their polymerization rates were affected by the other polymerization process. This one‐step procedure is believed to be the most convenient method for synthesizing PVP‐b‐PLLA block copolymers. HDPCP can also be used for the one‐step synthesis of poly(N‐vinylcarbazole)‐b‐PLLA block copolymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1607–1613     

Poly(ϵ‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(ϵ‐caprolactone) triblock copolymers: Synthesis and self‐assembly in aqueous solutions

Nontoxic and biodegradable poly(?‐caprolactone)‐b‐poly(ethylene glycol)‐b‐poly(?‐caprolactone) triblock copolymers were synthesized by the solution polymerization of ?‐caprolactone in the presence of poly(ethylene glycol). The chemical structure of the resulting triblock copolymer was characterized with ¹H NMR and gel permeation chromatography. In aqueous solutions of the triblock copolymers, the micellization and sol–gel‐transition behaviors were investigated. The experimental results showed that the unimer‐to‐micelle transition did occur. In a sol–gel‐transition phase diagram obtained by the vial‐tilting method, the boundary curve shifted to the left, and the gel regions expanded with the increasing molecular weight of the poly(?‐caprolactone) block. In addition, the hydrodynamic diameters of the micelles were almost independent of the investigated temperature (25–55 °C). The atomic force microscopy results showed that spherical micelles formed at the copolymer concentration of 2.5 × 10^?4 g/mL, whereas necklace‐like and worm‐like shapes were adopted when the concentration was 0.25 g/mL, which was high enough to form a gel. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 605–613, 2007     

Synthesis and characterization of multiblock copolymers composed of poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one) outer blocks and poly(L‐lactide) inner blocks

Ethylene glycol (EG) initiated, hydroxyl‐telechelic poly(L ‐lactide) (PLLA) was employed as a macroinitiator in the presence of a stannous octoate catalyst in the ring‐opening polymerization of 5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one (MBC) with the goal of creating A–B–A‐type block copolymers having polycarbonate outer blocks and a polyester center block. Because of transesterification reactions involving the PLLA block, multiblock copolymers of the A–(B–A)_n–B–A type were actually obtained, where A is poly(5‐methyl‐5‐benzyloxycarbonyl‐1,3‐dioxan‐2‐one), B is PLLA, and n is greater than 0. ¹H and ¹³C NMR spectroscopy of the product copolymers yielded evidence of the multiblock structure and provided the lactide sequence length. For a PLLA macroinitiator with a number‐average molecular weight of 2500 g/mol, the product block copolymer had an n value of 0.8 and an average lactide sequence length (consecutive C₆H₈O₄ units uninterrupted by either an EG or MBC unit) of 6.1. For a PLLA macroinitiator with a number‐average molecular weight of 14,400 g/mol, n was 18, and the average lactide sequence length was 5.0. Additional evidence of the block copolymer architecture was revealed through the retention of PLLA crystallinity as measured by differential scanning calorimetry and wide‐angle X‐ray diffraction. Multiblock copolymers with PLLA crystallinity could be achieved only with isolated PLLA macroinitiators; sequential addition of MBC to high‐conversion L ‐lactide polymerizations resulted in excessive randomization, presumably because of residual L ‐lactide monomer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6817–6835, 2006