Photocurable Shape‐Memory Copolymers of ε‐Caprolactone and L‐Lactide

Biodegradable and photocurable block copolymers of ε‐caprolactone and L ‐lactide were synthesized by polycondensation of PLLA diol ( = 10 000 g · mol^?1), PCL diol ( = 10 000 g · mol^?1), and a chain extender bearing a coumarin group. The effect of copolymer composition on the thermal and mechanical properties of the photocured copolymers was studied by means of DSC and cyclic tensile tests. An increase in Young's modulus and a decrease in the tensile strain with increasing PLLA content was observed for the block copolymers. Block copolymers with high PCL content showed good to excellent shape‐memory properties. Random copolymers exhibited R_f and R_r values above 90% at 45 °C for an extremely large tensile strain of 1 000%.

     

Effect of Arrangement of L‐Lactide and D‐Lactide in Poly[(L‐lactide)‐co‐(D‐lactide)] on its Resistance to Hydrolysis Studied by Molecular Modeling

Molecular modeling is used to explain how the resistance of poly[(L ‐lactide)‐co‐(D ‐lactide)] to hydrolysis is affected by the percentages of L ‐ and D ‐lactide and their arrangements in blocks or random arrangements in the polymer. Previous studies on improving the hydrolysis resistance of PLA have involved forming either poly(L ‐lactide)/poly(D ‐lactide) (PLLA/PDLA) polyblends or copolymers of L ‐ and D ‐lactide. In this study, molecular modeling was used to study the hydrolysis resistance of PLA containing various arrangements of L ‐ and D ‐lactide in the polymers. PLA copolymers are found to have less resistance to hydrolysis than a PLLA/PDLA polyblend having the same percentages of L ‐ and D ‐lactide because a polyblend can form more stereocomplexes, which is the most stable structure PLA can form.

     

Synthesis and Characterization of Block Copolymers of ε‐Caprolactone and DL‐Lactide Initiated by Ethylene Glycol or Poly(ethylene glycol)

Biodegradable copolymers were prepared by ring‐opening polymerization of sequentially added ε‐caprolactone and DL ‐lactide in the presence of ethylene glycol or poly(ethylene glycol), using zinc metal as catalyst. Polymerization was performed in bulk and yielded block copolymers with predetermined PEG/PCL/PLA segments. The obtained polymers were characterized by ¹H NMR, SEC, IR, DSC, TGA, and X‐ray diffraction. Data showed that the copolymers preserved the excellent thermal behavior inherent to PCL. The crystallinity of PLA‐containing copolymers was reduced with respect to PCL homopolymer. The presence of both hydrophilic PEG and fast degrading PLA blocks should improve the biocompatibility and biodegradability of the materials, which are of interest for applications as substrate in drug delivery or as scaffolding in tissue engineering.

Block copolymerization of ε‐caprolactone and DL ‐lactide initiated by dihydroxyl PEG.     

“Tree‐Shaped” Copolymers Based on Poly(ethylene glycol) and Atactic or Isotactic Polylactides: Synthesis and Characterization

“Tree‐shaped” copolymers constituted by an m‐PEG trunk and poly(L ‐lactide) or poly(D ,L ‐lactide) branches were obtained. The m‐PEG was functionalized at the terminal chain with two (G1) and four (G2) hydroxyl groups, then reacted with Al(CH₃)₃ to produce aluminum alkoxide species, active as initiators in the ROP of L‐ or D ,L‐ lactide. Copolymers were characterized by ¹H and ¹³C NMR, GPC and DSC, and compared with analogous linear copolymers. Characterization of a low‐molecular‐weight G1 copolymer confirmed the architecture. GPC curves showed monomodal and narrow molecular weight distribution for all the samples, while the melting points of the copolymers seemed more correlated to the architecture than to the composition.

     

Synthesis and pH/Temperature‐Responsive Behavior of PLLA‐b‐PDMAEMA Block Polyelectrolytes Prepared via ROP and ATRP

Well‐defined diblock poly(L ‐lactide)‐block‐poly(dimethylamino‐2‐ethyl methacrylate) (PLLA‐b‐PDMAEMA) copolymers were synthesized by combining ROP of LLA and ATRP of DMAEMA, from a dual‐initiator 2‐hydroxylethyl 2‐bromoisobutyrate. The molecular characterization of these diblock copolymers was performed using ¹H NMR, FT‐IR, and GPC‐MALLS analysis. The responsive behavior of these diblock copolymers in aqueous solutions at different pH and temperatures were investigated using DLS. Results show that both higher pH and temperature result in a higher degree of neutralization, weaker hydrogen bonding, and micellar aggregation. As observed by TEM, changes in micellar morphology are in accordance with DLS results.

     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

Controlled Ring‐Opening Polymerization of Lactones and Lactide Initiated by Lanthanum Isopropoxide, 2. Mechanistic Studies

The mechanism of ring‐opening polymerization of some lactones and lactide initiated by lanthanum isopropoxide has been comprehensively investigated. NMR and viscosity analyses demonstrated that three active polymer chains grow per lanthanum atom and that, depending on the coordinating ability of the monomer, propagation proceeds on aggregated or unaggregated active species. It is also demonstrated that inter‐ and intramolecular side reactions are limited and that lanthanum‐based initiator selectivity in ring‐opening polymerization is analogous to that of aluminum ones.

Part of the proposed mechanism for the lanthanum alkoxide initiated polymerization of ε‐caprolactone.     

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.

     

Controlled Synthesis and Characterization of Poly[ethylene‐block‐(L,L‐lactide)]s by Combining Catalytic Ethylene Oligomerization with “Coordination‐Insertion” Ring‐Opening Polymerization

Model poly[ethylene‐block‐(L ,L ‐lactide)] (PE‐block‐PLA) block copolymers were successfully synthesized by combining metallocene catalyzed ethylene oligomerization with ring‐opening polymerization (ROP) of L ,L ‐lactide (LA). Hydroxy‐terminated polyethylene (PE‐OH) macroinitiator was prepared by means of ethylene oligomerization on rac‐dimethyl‐silylen‐bis(2‐methyl‐benz[e]indenyl)‐zirconium(IV)‐dichloride/methylaluminoxane (rac‐MBI/MAO) in presence of diethyl zinc as a chain transfer agent, and subsequent in situ oxidation with synthetic air. Poly[ethylene‐block‐(L ,L ‐lactide)] block copolymers were obtained via ring‐opening polymerization of LA initiated by PE‐OH in toluene at 100 °C mediated by tin octoate. The formation of block copolymers was confirmed by ¹H NMR spectroscopy, fractionation experiments, thermal behavior, and morphological characterization using AFM and light microscopy techniques.

     

Double‐Gyroid Morphology of a Polystyrene‐block‐Poly(ferrocenylethylmethylsilane) Diblock Copolymer: A Route to Ordered Bicontinuous Nanoscale Architectures

A polystyrene‐block‐poly(ferrocenylethylmethylsilane) diblock copolymer, displaying a double‐gyroid morphology when self‐assembled in the solid state, has been prepared with a PFEMS volume fraction ?_PFEMS = 0.39 and a total molecular weight of 64 000 Da by sequential living anionic polymerisation. A block copolymer with a metal‐containing block with iron and silicon in the main chain was selected due to its plasma etch resistance compared to the organic block. Self‐assembly of the diblock copolymer in the bulk showed a stable, double‐gyroid morphology as characterised by TEM. SAXS confirmed that the structure belonged to the Ia d space group.

     

Synthesis and Photovoltaic Properties of Alternating Conjugated Polymers Derived from Indeno[1,2‐b]fluorene and Bithiophene or Thieno[3,2‐b]thiophene‐Cored Benzothiadiazole

Two narrow bandgap copolymers derived from 6,6′,12,12′‐tetraoctyl‐indeno[1,2‐b]fluorene and bithiophene or thieno[3,2‐b]thiophene‐cored benzothiadiazole are synthesized and characterized. The copolymers show broad absorption in the range 350–700 nm. The application of the copolymers as photovoltaic cells with configurations ITO/PEDOT:PSS/blend/Al and ITO/PEDOT:PSS/blend/interlayer/Al is investigated. A power conversion efficiency (PCE) of approximately 3.0% is achieved under an AM 1.5G solar simulator (80 mW cm^?2) for the cells with ITO/PEDOT:PSS/polymer:PC₇₁BM([6,6]‐phenyl‐C₇₁ butyric acid methyl ester) (1:4)/interlayer/Al.

     

Synthesis and Characterization of Amphiphilic Poly(ethylene oxide)‐block‐poly(hexyl methacrylate) Copolymers

We describe the preparation of amphiphilic diblock copolymers made of poly(ethylene oxide) (PEO) and poly(hexyl methacrylate) (PHMA) synthesized by anionic polymerization of ethylene oxide and subsequent atom transfer radical polymerization (ATRP) of hexyl methacrylate (HMA). The first block, PEO, is prepared by anionic polymerization of ethylene oxide in tetrahydrofuran. End capping is achieved by treatment of living PEO chain ends with 2‐bromoisobutyryl bromide to yield a macroinitiator for ATRP. The second block is added by polymerization of HMA, using the PEO macroinitiator in the presence of dibromobis(triphenylphosphine) nickel(II), NiBr₂(PPh₃)₂, as the catalyst. Kinetics studies reveal absence of termination consistent with controlled polymerization of HMA. GPC data show low polydispersities of the corresponding diblock copolymers. The microdomain structure of selected PEO‐block‐PHMA block copolymers is investigated by small angle X‐ray scattering experiments, revealing behavior expected from known diblock copolymer phase diagrams.

SAXS diffractograms of PEO‐block‐PHMA diblock copolymers with 16, 44, 68 wt.‐% PEO showing spherical (A), cylindrical (B), and lamellae (C) morphologies, respectively.     

Formation of Crystallosolvates Comprising Nano‐Crystals of Stereocomplex in a Ternary Mixture of Poly(L‐lactide)/Poly(D‐lactide)/Diphenyl Ether

Equal amounts of poly(L ‐lactide) (PLLA) and poly(D ‐lactide) (PDLA) were mixed with diphenyl ether (DE) at high temperature in DE‐to‐polymer composition from 0 to 80 wt.‐%. Cooling of the ternary mixtures produced white crystallosolvates without deposition of DE. Various analyses revealed that the crystallosolvates consisted of both homo‐chiral (hc) and stereocomplex (sc) crystals of PLLA and PDLA as well as amorphous domains involving DE. The crystallosolvate obtained at a DE content of 80 wt.‐%, comprised only the sc crystals. Atomic force microscopy (AFM) revealed that many granules having a size of 20 nm in diameter were formed from rod‐like hc crystallites and that the DE was slightly phase‐separated to form nano‐sized reservoirs. The stronger sc interaction between the PLLA and PDLA chains excluded DE from the polymer matrix.

     

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.     

Melt Structure and its Transformation by Sequential Crystallization of the Two Blocks within Poly(L‐lactide)‐block‐Poly(ε‐caprolactone) Double Crystalline Diblock Copolymers

Summary: Sequential crystallization of poly(L ‐lactide) (PLLA) followed by poly(ε‐caprolactone) (PCL) in double crystalline PLLA‐b‐PCL diblock copolymers is studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM), wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS). Three samples with different compositions are studied. The sample with the shortest PLLA block (32 wt.‐% PLLA) crystallizes from a homogeneous melt, the other two (with 44 and 60% PLLA) from microphase separated structures. The microphase structure of the melt is changed as PLLA crystallizes at 122 °C (a temperature at which the PCL block is molten) forming spherulites regardless of composition, even with 32% PLLA. SAXS indicates that a lamellar structure with a different periodicity than that obtained in the melt forms (for melt segregated samples). Where PCL is the majority block, PCL crystallization at 42 °C following PLLA crystallization leads to rearrangement of the lamellar structure, as observed by SAXS, possibly due to local melting at the interphases between domains. POM results showed that PCL crystallizes within previously formed PLLA spherulites. WAXS data indicate that the PLLA unit cell is modified by crystallization of PCL, at least for the two majority PCL samples. The PCL minority sample did not crystallize at 42 °C (well below the PCL homopolymer crystallization temperature), pointing to the influence of pre‐crystallization of PLLA on PCL crystallization, although it did crystallize at lower temperature. Crystallization kinetics were examined by DSC and WAXS, with good agreement in general. The crystallization rate of PLLA decreased with increase in PCL content in the copolymers. The crystallization rate of PCL decreased with increasing PLLA content. The Avrami exponents were in general depressed for both components in the block copolymers compared to the parent homopolymers.

Polarized optical micrographs during isothermal crystallization of (a) homo‐PLLA, (b) homo‐PCL, (c) and (d) block copolymer after 30 min at 122 °C and after 15 min at 42 °C.     

Thermosensitive Hollow Capsules Based on Thermoresponsive Polyelectrolytes

Thermosensitive hollow capsules were successfully fabricated by the layer‐by‐layer deposition onto colloid particles of oppositely charged diblock copolymers each containing a poly(N‐isoproprylacrylamide) (PNIPAM) block and by the subsequent decomposition of the core. The multilayer growth was characterized by electrophoresis and single particle light scattering. By combining confocal microscopy observation and FRAP measurements, we showed that the morphology and the permeability of the capsules change upon heating in aqueous solution. The decrease of size accompanied by a decrease of the permeability with increasing temperature was attributed to structural rearrangements in the shell. However, this process is only partially reversible upon cooling, limiting the thermoresponsive behavior of the capsules.

CLSM images of hollow capsules in presence of 6‐carboxyfluorescein (left) and fluorescein‐labeled dextran (right).     

Poly(carbazole)‐Based Copolymers Containing Deep‐Blue Chromophore for Polymer Light‐Emitting Diode Applications

A new series of two poly(carbazole)‐based copolymers (poly(9‐hexyl‐carbazole‐co‐9‐(6‐(3‐(4‐phenylquinolin‐2‐yl)carbazol‐9‐yl)hexyl)carbazole) (PCVz) and poly(9,9‐dioctylfluorene‐co‐9‐(6‐(3‐(4‐phenylquinolin‐2‐yl)carbazol‐9‐yl)hexyl)carbazole) (PFCVz)) containing carbazoylphenylquinoline pendant groups were synthesized via the Suzuki coupling reaction for polymer light‐emitting diode applications. The electro‐optical properties of ITO/PEDOT/Polymer/PBD/LiF/Al devices based on these copolymers were investigated using UV‐visible, photoluminescence, and electroluminescence spectroscopy. The turn‐on voltages of the copolymer devices were found to be 6.0–8.0 V. The maximum brightness and luminescence efficiency of the copolymers device were found to be 230 cd · m^?2 and 0.28 cd · A^?1 at 11 V, respectively.

     

Well‐Defined Miktocycle Eight‐Shaped Copolymers Composed of Polystyrene and Poly(ε‐caprolactone): Synthesis and Characterization

A series of well‐defined miktocycle number‐eight‐shaped copolymers composed of cyclic polystyrene (PS) and cyclic poly(ε‐caprolactone) (PCL) have been successfully synthesized by a combination of atom transfer radical polymerization (ATRP), ring‐opening polymerization (ROP), and “click” reaction. The synthesis involves three steps: 1) preparation of tetrafunctional initiator with two acetylene groups, one hydroxyl group and a bromo group; 2) preparation of two azide‐terminated block copolymers, N₃‐PCL‐(CH?C)₂‐PS‐N₃, with two acetylene groups anchored at the junction; and 3) intramolecular cyclization of the block copolymer through “click” reaction under high dilution. The ¹H NMR, FT‐IR, and gel permeation chromatography (GPC) techniques are applied to characterize the chemical structures of the resulting intermediates and the target polymers. Their thermal behavior is investigated by differential scanning calorimeter (DSC). The decrease in chain mobility of eight‐shaped copolymers restricts the crystallization of PCL.

     

Synthesis and Characterization of Conducting Copolymers of (S)‐2‐Methylbutyl‐2‐(3‐thienyl)acetate with Pyrrole and Thiophene

Polymerization of methylbutyl‐2‐(3‐thienyl)acetate (MBTA) was achieved by constant current electrolysis at low temperature. Subsequently, the syntheses of block copolymers of polyMBTA were accomplished in the presence of either pyrrole or thiophene by constant potential electrolysis. Moreover, the copolymer of MBTA with thiophene was obtained with constant potential electrolysis.

The synthesis of monomer MBTA.