Star‐shaped poly[(trimethylene carbonate)‐co‐(ε‐capro‐lactone)] and its block copolymers with lactide/glycolide: synthesis,characterization and properties

Linear and star‐shaped copolymers of trimethylene carbonate/ε‐caprolactone were synthesized using different polyol initiators and catalysts. Unexpectedly, when dipentaerythritol was used as an initiator cross‐linked rubbers were obtained, that swell in chloroform. This network formation can be understood by ‘in situ’ generation of cross‐linker molecules from trimethylene carbonate and initiator. SEC analysis showed that with D‐sorbitol star‐shaped copolymers are synthesized with an average functionality between 4 and 6. These low molecular weight rubbers were used as a macro‐initiator for the subsequent lactide/glycolide polymerization. Star‐shaped lactide/glycolide block copolymers with a poly[(trimethylene carbonate)‐co‐(ε‐caprolactone)] rubber core based on D ‐sorbitol show good mechanical properties. At relatively low rubber content ductile tensile behavior was observed indicating extensive toughening.     

Organobase‐Catalyzed Synthesis of Multiarm Star Polylactide With Hyperbranched Poly(ethylene glycol) as the Core

Multiarm star copolymers consisting of the polyether‐polyol hyperbranched poly(ethylene glycol) (hbPEG) as core and poly(L ‐lactide) (PLLA) arms are synthesized via the organobase‐ catalyzed ring‐opening polymerization of lactide using hbPEG as a multifunctional macroinitiator. Star copolymers with high molecular weights up to 792 000 g mol^?1 are prepared. Detailed 2D NMR analysis provides evidence for the attachment of the PLLA arms to the core and reveals that the adjustment of the monomer/initiator ratio enables control of the arm length. Size exclusion chromatography measurements show narrow molecular weight distributions. Thermal analysis reveals a lower glass transition temperature, melting point, and degree of crystallization for the star‐shaped polylactides compared to linear polylactide.     

Synthesis and Micellization of Star‐Shaped Poly(ethylene glycol)‐block‐Poly(ε‐caprolactone)

Summary: The relationship between the architecture of block copolymers and their micellar properties was investigated. Diblock, 3‐arm star‐shaped and 4‐arm star‐shaped block copolymers based on poly(ethylene glycol) and poly(ε‐caprolactone) were synthesized. Micelles of star‐shaped block copolymer in an aqueous solution were then prepared by a solvent evaporation method. The critical micelle concentration and the size of the micelles were measured by the steady‐state pyrene fluorescence method and dynamic light scattering, respectively. The CMC decreased in the order di‐, 3‐arm star‐shaped and 4‐arm star‐shaped block copolymer. The size of the micelles increased in the same order as the CMC. Theory also predicts that the formation of micelles becomes easier for 4‐arm star‐shaped block copolymers than for di‐ and 3‐arm star‐shaped block copolymers, which qualitatively agrees with the experiments.

     

Thermal and Mechanical Properties of Diblock Copolymers Based on 2,2‐Dimethyltrimethylene Carbonate and ε‐Caprolactone

Tri(sec‐butoxyaluminium) [Al(Osec‐Bu)₃]‐initiated copolymerization of 2,2‐dimethyl trimethylene carbonate (DTC) with ε‐caprolactone (CL) by stepwise addition of the monomers leads to high‐molecular weight block copolymers of the AB diblock type. By changing the amount of initiator and the molar ratio of the monomers, several diblock copolymers were prepared having either the same composition with different overall molecular weight or the same PDTC block length with various compositions. The thermal and mechanical properties of the copolymers were studied with the aim of assessing their dependence on molecular characteristics. Ageing time at room temperature was found to have a remarkable effect on tensile modulus and on melting of the ordered PDTC phases. This behavior has been attributed to mutual hindrance of the blocks on their crystallization and to a solid‐solid polymorphic transformation taking place in the crystalline PDTC microphases.     

Synthesis,Sequential Crystallization and Morphological Evolution of Well‐Defined Star‐Shaped Poly(ε‐caprolactone)‐b‐poly(L‐lactide) Block Copolymer

Summary: Well‐defined star‐shaped poly(ε‐caprolactone)‐b‐poly(L ‐lactide) copolymers (PCL‐b‐PLLA) were synthesized via sequential block copolymerization, and their molecular weights and arm length ratio could be accurately controlled. Both differential scanning calorimetry and wide angle X‐ray diffraction analysis indicated that the crystallization of both the PLLA and PCL blocks within the star‐shaped PCL‐b‐PLLA copolymer could be adjusted from the arm length of each block, and both blocks mutually influenced each other. The sequential isothermal crystallization process of both the PLLA and PCL blocks within the PCL‐b‐PLLA copolymers was directly observed with a polarized optical microscope, and the isothermal crystallization of the PCL segments was mainly templated by the existing spherulites of PLLA. Moreover, the PLLA blocks within the star‐shaped PCL‐b‐PLLA copolymer progressively changed from ordinary spherulites to banded spherulites when the arm length ratio of PCL to PLLA was increased while concentric spherulites were observed for the linear analog. Significantly, these novel spherulites with concentric or banded textures and the morphological evolution of the spherulites have been observed for the first time in the PCL‐b‐PLLA block copolymers.

     

Polylactones, 45. Homo‐ and Copolymerizations of 3‐Methylmorpholine‐2,5‐dione Initiated With a Cyclic Tin Alkoxide

Morpholine‐2,5‐dione and D ,L ‐3‐methylmorpholine‐2,5‐dione were polymerized with 2,2‐dibutyl‐2‐stanna‐1,3‐dioxepane (DSDOP) as initiator with variation of time and monomer/initiator (M/I)‐ratio. For comparison a few polymerizations were initiated with Sn(II) 2‐ethylhexanoate. The DSDOP initiated polymerizations gave slightly higher molecular weights, but the molecular weights were rather low in all cases and did not parallel the M/I‐ratio. Furthermore, D ,L ‐3‐methylmorpholine‐2,5‐dione was copolymerized with ε‐caprolactone (ε‐CL) or with L ‐lactide. ¹³C NMR spectra proved the formation of nearly random sequences. Therefore, the 1 : 1 copolymers were amorphous. Higher feed ratios of ε‐CL or L ‐lactide yielded higher molecular weights (at constant M/I‐ratio) and they yielded crystalline copolyesters as evidenced by DSC‐measurements.     

Facile Synthesis of Helical Rod–Coil Block Polymers by the Combination of ATRP and Pd(II)‐Initiated Isocyanides Polymerizations

A bifunctional initiator containing propargyl bromoisobutyrate and alkyne‐Pd(II) (PBB‐Pd(II)) is designed and synthesized. The propargyl bromoisobutyrate unit of PBB‐Pd(II) can initiate the atom transfer radical polymerization (ATRP) of vinyl monomers, while the Pd(II) complex can initiate the polymerization of phenyl isocyanides. Both the ATRP and Pd(II)‐mediated isocyanide polymerization are proceeded in living/controlled manner. Thus, combining the two living polymerizations, a series of well‐defined block copolymers bearing rod poly(phenyl isocyanide)s and coil poly(acrylate) segments can be facilely prepared in high yield with tunable composition, controlled molar masses (M_ns), and narrow molar mass distributions (M_w/M_ns). What is more, benefiting from this synthetic strategy, well‐defined core cross‐linked star polymers are readily synthesized. Optically active block copolymers and star polymers can be facilely obtained by using chiral isocyanide monomers due to the formation of predominated one‐handed helix. The chiral star polymers show excellent enantioselective recognition ability in enantioselective crystallization of racemic compounds.     

Tannic Acid‐Inspired Star‐Like Macromolecules via Temporally Controlled Multi‐Step Potential Electrolysis

Inspired by tea stains, a plant polyphenolic‐based macroinitiator is prepared for the first time by partial modification of tannic acid (TA) with 2‐bromoisobutyryl bromide. In accordance with the “grafting from” methodology, a naturally occurring star‐like polymer with a polar gallotannin core and a hydrophobic poly(n‐butyl acrylate) side arms is synthesized via a simplified electrochemically mediated ATRP (seATRP), utilizing multiple‐step potential electrolysis. To investigate the kinetics of the electrochemical catalytic process triggered by reduction of Cu(II) or Fe(III) catalytic complex in the presence of the multifunctional initiator, cyclic voltammetry measurements are conducted. The naturally derived tannin macromolecule shows narrow MWDs (? = 1.57). Moreover, solvolysis of the star polymer to cleave the side arms and characterize them indicates that all chains grow to the same length (homopolymers with M_w/M_n <1.17), which confirms the well‐controlled seATRP. The structure of the obtained TA‐based systems is characterized microscopically (AFM) and spectroscopically (¹H NMR, FT‐IR). Atomic force microscopy measurements precisely determine the diameters of the obtained star polymers (19.7 ± 3.3 nm). These new star polymers may find biomedical applications as drug delivery systems and antifouling or antimicrobial coatings.     

Synthesis of AB2 3‐ and AB4 5‐Miktoarm Star Copolymers Initiated from Dendritic Tri‐ and Penta‐Functional Initiators by Combination of Ring‐Opening Polymerization of ε‐Caprolactone and Nitroxide‐Mediated Radical Polymerization of Styrene

Summary: Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators. Initially, two kinds of dendritic initiators having one benzylic OH and two or four TEMPO‐based alkoxyamine moieties were prepared. Using them, ROP of ε‐caprolactone was carried out at room temperature to give poly(ε‐caprolactone)s carrying two or four alkoxyamine moieties. NMRP of styrene from the poly(ε‐caprolactone)s was carried out at 120 °C to give AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers, which were analyzed by ¹H NMR and SEC. The increased linearly with conversion and the were in the range 1.10–1.37, showing that well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were formed.

Well‐defined AB₂ 3‐ and AB₄ 5‐miktoarm star copolymers were prepared by combination of ring‐opening polymerization (ROP) and nitroxide‐mediated radical polymerization (NMRP) using dendritic tri‐ and penta‐functional initiators.     

Controlled Synthesis and Ti—O Bond Stability of Star‐Shaped Biodegradable Polyesters via Titanate‐Initiated ROP of Cyclic Esters at Ambient Temperature

Four‐arm star‐shaped PCL polymers are synthesized using iPT as initiator for the controlled ROP of CL at 25–40 °C. The number‐average molecular weights of the star‐shaped Ti(O‐PCL)₄ with narrow molecular weight distributions are proportional to the molar ratios of monomer to initiator. The four‐branch star‐shaped structures of Ti(OPCL)₄ are confirmed through polymer hydrolysis monitoring by GPC, which indicates that the stability of the Ti—O bond in the core of the star‐shaped polymer chain increases with the increase of polymer molecular weights. The star‐shaped Ti(O‐PCL)₄ can act as a macroinitiator for successive block copolymerization with d,l ‐Lactide in bulk at 60 °C.     

Synthesis and Characterization of Styrene/Butyl Acrylate Linear and Star Block Copolymers via Atom Transfer Radical Polymerization

Summary: Well‐defined styrene (S) and butyl acrylate (BA) linear and star‐like block copolymers are synthesized via atom transfer radical polymerization (ATRP) using di‐ and trifunctional alkyl halide initiators employing the Cu/PMDETA (N,N,N′,N″,N″‐pentamethyldiethylenetriamine) catalyst system. Initial addition of Cu^II deactivator and utilization of halogen exchange techniques suppresses the coupling of radicals and improves cross‐propagation to a large extent, which results in better control over the polymerization. Two types of star‐like PBA/PS block copolymers are prepared by using core‐first techniques: a trifunctional PBA or PS macroinitiator extended with the other monomer. Block copolymers with a well‐defined structure and low polydispersity (PDI = ) are obtained in both cases. A trifunctional PBA₃ macroinitiator with = 136 000 g · mol^?1 and PDI = 1.15 is extended to (PBA‐PS)₃ star‐like block copolymer with = 171 100 g · mol^?1 and PDI = 1.15. A trifunctional PS₃ macroinitiator with = 27 000 g · mol^?1 and PDI = 1.16 g · mol^?1 is extended to (PS‐PBA)₃ with = 91 500 g · mol^?1 and PDI = 1.40. The individual star‐like macromolecules as well as their aggregates are visualized by atomic force microscopy (AFM) where the PS and PBA adopt the globular and extended conformation, respectively. For the PBA core star block copolymers, PS segments tend to aggregate either intramolecularly or intermolecularly. PS core star block copolymers form aggregates with a PS core and emanating PBA chains. Most aggregates have ‘n × 3’ arms but minor amounts of ‘defective’ stars with 4, 5, 8, or 11 arms are also observed. The AFM analysis shows that PS core star block copolymers contain about 92% three‐arm block copolymers, and the efficiency of cross‐propagation is 97.3%.

Schematic representation of the synthesis of BA/S star‐like block copolymers by ATRP, and their resultant AFM images.     

β‐Cyclodextrin‐Based Star Amphiphilic Copolymers: Synthesis,Characterization, and Evaluation as Artificial Channels

14‐arm amphiphilic star copolymers are synthesized according to different strategies. First, the anionic ring polymerization of 1,2‐butylene oxide (BO) initiated by per(2‐O‐methyl‐3,6‐di‐O‐(3‐hydroxypropyl))‐β‐CD (β‐CD’OH₁₄) and catalyzed by t‐BuP₄ in DMF is investigated. Analyses by NMR and SEC show the well‐defined structure of the star β‐CD’‐PBO₁₄. To obtain a 14‐arm poly(butylene oxide‐b‐ethylene oxide) star, a Huisgen cycloaddition between an α‐methoxy‐ω‐azidopoly(ethylene oxide) and the β‐CD’‐PBO₁₄,whose end‐chains are beforehand alkyne‐functionalized, is performed. In parallel, 14‐arm star copolymers composed of butylene oxide‐b‐glycidol arms are successfully synthesized by the anionic polymerization of ethoxyethylglycidyl ether (EEGE) initiated by β‐CD’‐PBO₁₄ with t‐BuP₄. The deprotection of EEGE units is then performed to provide the polyglycidol blocks. These amphiphilic star polymers are evaluated as artificial channels in lipid bilayers. The effect of changing a PEO block by a polyglycidol block on the insertion properties of these artificial channels is discussed.     

Tetraphenylsilane‐Cored Star‐Shaped Amphiphilic Block Copolymers for pH‐Responsive Anticancer Drug Delivery

Four‐arm star‐shaped poly[2‐(diethylamino)ethyl methacrylate]‐b‐poly[2‐hydroxyethyl methacrylate]s block copolymers using tetraphenylsilane (TPS) as a core with adjustable arm lengths are synthesized through two‐step atom transfer radical polymerizations. For comparison, a linear block copolymer with similar molecular weight is also prepared. The assembled star‐shaped copolymer micelles exhibit sizes of 102–139 nm and critical micelle concentrations of 1.49–3.93 mg L^?1. Moreover, the bulky and rigid TPS core is advantageous for propping up the four star‐shaped arms to generate large intersegmental space. As a result, the drug‐loading capacity in the micelles is up to 33.97 wt%, much surpassing the linear counterpart (8.92 wt%) and the previously reported star‐shaped copolymers prepared using pentaerythritol as the core. Furthermore, the micelles show sensitive pH‐responsive drug release when the pH changes from 7.4 to 5.0. The in vitro cytotoxicity to Hela cells indicates that the doxorubicin (DOX)‐loaded micelles have similar anticancer activity to the pristine DOX. The combination of excellent micelle stability, high drug‐loading, sensitive pH response, and good anticancer activity endows the copolymers with promising application in drug control delivery for anticancer therapy.     

Anionic synthesis of well‐defined,poly[(styrene)‐block‐(propylene oxide)] block copolymers

Well‐defined, narrow molecular weight distribution (M_w/M_n ≤ 1.1) poly[(styrene)‐block‐(propylene oxide)] block copolymers with relatively high molecular weight poly(propylene oxide) blocks [e. g. M_n (PPO) = 10 000–12 000 g/mol] have been prepared by anionic polymerization. The polystyrene block (M_n = 5 000; M_w/M_n = 1.1) was prepared by alkyllithium‐initiated polymerization of styrene followed by chain‐end functionalization with ethylene oxide and protonation with acidic methanol. The resulting ω‐hydroxyl‐functionalized polystyrene was converted to the corresponding alkali metal salts with alkali metals (Na/K alloy, Rb, Cs) and then used to initiate block polymerization of propylene oxide in tetrahydrofuran. The effects of crown ethers (18‐crown‐6 and dicyclohexano‐24‐crown‐8) and added dimethylsulfoxide were investigated. Chain transfer to the monomer resulted in significant amounts of poly(propylene oxide) formation (50%); however, the diblock molecular weight distributions were narrow. The highest molecular weight poly(propylene oxide) blocks (12 200 g/mol) were obtained in tetrahydrofuran with cesium as counterion without additives.     

Cross‐Linked Poly(ε‐caprolactone/D,L‐lactide) Copolymers with Elastic Properties

Cross‐linked ε‐caprolactone (CL) and D ,L ‐lactide (DLLA) copolymers with elastic properties were synthesized in three steps. First, the monomers were copolymerized in ring‐opening polymerization to obtain telechelic star‐shaped oligomers with almost completely random monomer distribution. The oligomers were methacrylated with methacrylic anhydride in the second step and cured in a third. Molar CL/DLLA compositions of 30/70, 50/50, 70/30, 90/10, and 100/0 were used to obtain elastic structures with a wide range of properties. The effect of the average length of the copolymer block on the properties of the networks was evaluated with three different co‐initiator contents (0.5, 1.0, and 2.0/100) in the oligomer synthesis. The oligomers were characterized by ¹³C NMR spectroscopy, size‐exclusion chromatography (SEC), and differential‐scanning calorimetry (DSC). The formation of elastic networks was confirmed by the absence of a flow region in dynamic mechanical analysis (DMA), the increase in T_g in DSC, and the full recovery of the sample dimensions after tensile testing. In addition, gel contents were high and the samples swelled in CH₂Cl₂. The networks possessed break stresses from 0.7–9.7 MPa with elongations from 80–350%. Networks with 100 or 90% of ε‐caprolactone retained their form in vitro for 12 weeks, but an increase in lactide content made the networks more vulnerable to hydrolysis.

Water absorption of the polymers during hydrolysis.     

Chromatographic Investigations of Macromolecules in the Critical Range of Liquid Chromatography, 14. Analysis of Miktoarm Star (μ‐Star) Polymers

Miktoarm (μ‐star) polymers are star‐shaped block copolymers where chemically different homopolymers constitute the different arms. The quantitative determination of different segments of the star polymers by liquid chromatography under critical conditions (LC‐CC) is described. In the present study tri‐arm and tetra‐arm model polymers with polystyrene, polybutadiene, polyisoprene and poly‐α‐methyl styrene arms are analyzed. Operating at the respective critical conditions, homopolymer arms are made chromatographically invisible. Under these conditions the residual (chromatographically visible) arms can be analyzed with regard to their molar masses. The accuracy of the quantification depends on the fit of the calibration procedure and the detector response to the actual chemical compositions. The results clearly indicate that LC‐CC yields valuable structural information on block copolymers with complex architectures.     

Multi‐Arm Star Polyglycerol‐block‐poly(tert‐butyl acrylate) and the Respective Multi‐Arm Poly(acrylic acid) Stars

Summary: Well‐defined multi‐arm star block copolymers, polyglycerol‐block‐poly(tert‐butyl acrylate) (PG‐b‐PtBA), with average arm‐numbers of 17, 27, 36, 66 and 90 arms, respectively, have been prepared by atom transfer radical polymerization (ATRP) of tBA in acetone, using a core‐first strategy. After hydrolysis with excess concentrated HCl in refluxing dioxane, full hydrolysis of the tert‐butyl ester groups was achieved, resulting in multi‐arm star polyelectrolytes, polyglycerol‐block‐poly(acrylic acid) (PG‐b‐PAA). The hyperbranched macroinitiators employed were prepared on the basis of hyperbranched polyglycerols via esterification with 2‐bromoisobutyryl bromide. Both CuBr/PMDETA and CuBr/Me₆TREN catalyst systems have been employed for ATRP of tBA. CuBr/PMDETA was found to permit good control. Polydispersity indices for the new multi‐arm stars were mainly in the range of 1.22 to 1.4, and the absolute data were in agreement with the calculated values. Moreover, kinetic curves show a linear dependence of ln([M]₀/[M]_t) on time, confirming that the polymerizations are controlled.

Relationship between arm numbers of the multi‐arm stars and maximum conversion achieved.     

Novel block copolymers of 2‐vinylpyridine and ε‐caprolactone: Synthesis and characterization

Novel block copolymers based on 2‐vinylpyridine (2VP) and ε‐caprolactone (CL) have been synthesized via sequential anionic polymerization in tetrahydrofuran (THF). These block copolymers are expected to be promising pigment dispersing agents for TiO₂ in e. g. polyester powder coatings. Initiation of CL by the living P2VP polymer occurred instantaneously and without side reactions. Intramolecular transesterification reactions were not observed either. However, part of the living chains was deactivated immediately after the addition of CL, which yielded bimodal molecular weight distributions. This could be attributed to strong aggregation of the alkoxide chain ends. Addition of LiCl to the polymerization mixture prevented aggregation and resulted in well‐defined block copolymers. The block copolymers have been characterized with SEC, NMR and IR.     

Amphiphilic Multi‐Arm Star‐Block Copolymers for Encapsulation of Fragrance Molecules

Novel amphiphilic multi‐arm star‐block copolymers with a hyperbranched core, a hydrophobic inner shell, and a hydrophilic outer shell have been prepared from a commercial hyperbranched polyester macroinitiator by ring‐opening polymerization of ε‐caprolactone, followed by atom transfer radical polymerization of tert‐butyl acrylate (tBuA). Hydrolysis of the tert‐butyl groups was then used to convert the poly(tBuA) blocks to poly(acrylic acid), resulting in stable amphiphilic core‐shell structures with significantly higher degrees of functionality than reported so far in the literature. A strong correlation between the maximum concentration of selected hydrophobic guest molecules and the concentration of amphiphilic star‐block copolymer in aqueous solution was observed by ¹H NMR, demonstrating the capacity of these copolymers to encapsulate and disperse significant loadings (up to about 27 wt.‐%) of volatile hydrophobic molecules such as fragrances in water.

     

Polylactones, 53. Formation of Cyclic Polyesters in the Combined Ring‐Expansion Polymerization/Ring‐Opening Polycondensation of Lactones

Ring‐expansion polymerizations of β‐D ,L ‐butyrolactone (β‐BL) or ε‐caprolactone (ε‐CL) were initiated with 2,2‐dibutyl‐2‐stanna‐1,3‐dioxepane (DSDOP) and the monomer‐initiator ratio (M/I) was varied. The resulting tin‐containing polylactones were polycondensed in situ either with succinyl chloride (in the case of β‐BL) or with suberoyl chlorid (for ε‐CL). The reaction conditions were optimized towards high molecular weights by the addition of bipyridine. The isolated tin‐free polylactones were characterized by MALDI‐TOF mass spectrometry. In the best spectra cyclic poly(ε‐caprolactone)s were detected up to masses around 10 600 Da and cyclic poly(β‐butyrolactone)s up to masses around 17 000 Da. In addition to the cyclic polyesters linear chains having alcoholic OH and/or CO₂H group were found. These results suggest that the chain growth is limited by cyclization and by incomplete conversion of the functional groups.