The Use of a Low‐Molar‐Mass Self‐Assembled Template to Direct the Crystallisation of Poly(ε‐caprolactone)

Summary: We show that small quantities of dibenzylidene sorbitol dispersed in poly(ε‐caprolactone) provide a self‐assembling nanoscale framework to yield high levels of crystal orientation. During modest shear flow of the melt, the additive forms highly extended nanoparticles which adopt a preferred alignment with respect to the flow field and, on cooling, polymer crystallisation is directed by these particles. We speculate that atomistic level epitaxy is unlikely to be the only directing influence.

SAXS pattern of PCL/DBS in the melt at 80 °C and subjected to a shear flow of 10 s⁻¹ for 1 000 shear units. The flow direction is vertical.     

Preparation of Porous Poly(ε‐caprolactone) Structures

Porous poly(ε‐caprolactone) structures have been prepared by leaching of compression moulded salt‐containing polymer precipitates. Coagulation takes place when a PCL solution containing dispersed water‐soluble salt particles is precipitated into an excess of non‐solvent. Porous scaffolds are obtained after leaching of the compression moulded polymer‐salt precipitate. This process yields scaffolds with a very homogeneous pore morphology and independent control of pore size and porosity.     

Crystallization of Poly(ε‐caprolactone)/Poly(vinyl chloride) Miscible Blends Under Strain: The Role of Molecular Weight

Summary: The effect of poly(ε‐caprolactone) (PCL) molecular weight on the orientation of crystalline PCL in miscible poly(ε‐caprolactone)/poly(vinyl chloride) (PCL/PVC) blends, melt crystallized under strain, has been studied by a combination of wide angle X‐ray diffraction (WAXD) and small angle X‐ray scattering (SAXS) studies. An unusual crystal orientation with the b‐axis parallel to the stretching direction was observed in miscible PCL/PVC blends with PCL of high molecular weight (>21 000). SAXS showed the presence of nanosize confined PCL in the PCL/PVC blends, which could be preserved at temperatures higher than the T_m of PCL but lower than the T_g of PVC. A mechanism based on the confinement of PCL crystal growth was proposed, which can explain the formation of b‐axis orientation in PCL/PVC blends crystallized under strain.

SAXS pattern of stretched PCL/PVC blend after annealing at 90 °C for 5 min.     

Crystallization of Poly(ε‐caprolactone) under Nanoparticle Confinement

Solid‐state ¹³C‐NMR spectroscopy has been used to characterize the conformation of the hydrophobic poly(ε‐caprolactone) core of a nanoparticle having a cross‐linked hydrophilic poly(acrylic acid)/polyacrylamide shell. The amphiphilic nanoparticles were synthesized from the diblock copolymer, poly(ε‐caprolactone)₁₂₁‐b‐poly(acrylic acid)₁₆₅ by self‐assembly into polymer micelles, followed by cross‐linking via condensation reactions between the carboxylic acid groups of the hydrophilic shell and the amine groups of 2,2′‐(ethylenedioxy)bis(ethylamine). NMR Experiments performed at −30° on nanoparticles rapidly quenched from 60° show that the core is largely noncrystalline and locally disordered. Heating to 25° results in some crystallization, although far less than that observed for bulk poly(ε‐caprolactone) homopolymer. Storage at −30° results in further crystallization and conversion of most rubbery, mobile regions into more rigid, locally ordered amorphous domains. The absence of dipolar coupling between natural‐abundance ¹³C in the poly(ε‐caprolactone) core of the nanoparticle, and ¹⁵N labels dispersed throughout the cross‐linked shell show that the interface between core and shell is sharp. The dipolar coupling measurements were accomplished by ¹³C{¹⁵N} rotational‐echo double resonance.     

Novel Amphiphilic Degradable Poly(ε‐caprolactone)‐graft‐poly(4‐vinyl pyridine), Poly(ε‐caprolactone)‐graft‐poly(dimethylaminoethyl methacrylate) and Water‐Soluble Derivatives

New amphiphilic graft copolymers that have a poly(ε‐caprolactone) (PCL) biodegradable hydrophobic backbone and poly(4‐vinylpyridine) (P4VP) or poly(2‐(N,N‐dimethylamino)ethyl methacrylate) (PDMAEMA) hydrophilic side chains have been prepared by anionic polymerization of the corresponding 4VP and DMAEMA monomers using a PCL‐based macropolycarbanion as initiator. The water solubility of these amphiphilic copolymers is improved by quaternization, which leads to fully water‐soluble cationic copolymers that give micellar aggregates in deionized water with diameters ranging from 65 to 125 nm. In addition, to improve the hydrophilicity of PCL‐g‐P4VP, grafting of poly(ethylene glycol) (PEG) segments has been carried out to give a water‐soluble double grafted PCL‐g‐(P4VP;PEG) terpolymer.

     

Precision Polyolefin Nanoalloy Polypropylene/Poly(ε‐caprolactone)

This communication reports the first example of precision polyolefin nanoalloys where an exotic immiscible polymer is nanometrically dispersed with stability in a polyolefin matrix in a highly controlled mode. Following the preparation of polypropylene/multiwalled carbon nanotubes nanocomposites (PP/MWCNTs) by in situ Ziegler‐Natta polymerization, the hydroxyl groups on the surfaces of individual MWCNTs are used to initiate ring‐opening polymerization of ε‐caprolactone, resulting in PP/poly(ε‐caprolactone) (PCL) alloy with PCL grafted on MWCNTs. Upon phase formation, the PP/MWCNTs‐g‐PCL alloys exhibit a unique PCL dispersion morphology, which is stable and solely governed by PCL molecular weight.

     

Giant Biodegradable Poly(ethylene glycol)‐block‐Poly(ε‐caprolactone) Polymersomes by Electroformation

Here, the formation of giant enzyme‐degradable polymersomes using the electroformation method is reported. Poly(ethylene glycol)‐block‐poly(ε‐caprolactone) polymersomes have been shown previously to be attractive candidates for the detection of bacterial proteases and protease mediated release of encapsulated reporter dyes and antimicrobials. To maximize the efficiency, the maximization of block copolymer (BCP) vesicle size without compromising their properties is of prime importance. Thus, the physical‐chemical properties of the BCP necessary to self‐assemble into polymeric vesicles by electroformation are first identified. Subsequently, the morphology of the self‐assembled structures is extensively characterized by different microscopy techniques. The vesicular structures are visualized for giant polymersomes by confocal laser scanning microscopy upon incorporation of reporter dyes during the self‐assembly process. Using time correlated single photon counting and by analyzing the fluorescence decay curves, the nanoenvironment of the encapsulated fluorophores is unveiled. Using this approach, the hollow core structure of the polymersomes is confirmed. Finally, the encapsulation of different dyes added during the electroformation process is studied. The results underline the potential of this approach for obtaining microcapsules for subsequent triggered release of signaling fluorophores or antimicrobially active cargo molecules that can be used for bacterial infection diagnostics and/or treatment.     

Anionic polymerization of cyclic ester and amide in miniemulsion: Synthesis and characterization of poly(ε‐caprolactone) and poly(ε‐caprolactone‐co‐ε‐caprolactam) nanoparticles

For the first time, poly(ε‐caprolactone) and poly(ε‐caprolactone‐co‐ε‐caprolactam) nanoparticles were successfully obtained by anionic polymerization of ε‐caprolactone and anionic copolymerization of ε‐caprolactone with ε‐caprolactam, respectively, in heterophase by the miniemulsion technique. After polymerization the resulting dispersions are stable for hours in case of the pure polyester and days for the copolymer. The syntheses were carried out with different continuous phases, amounts of surfactant, initiator, and monomers. The influence of the reaction parameters on the molecular weight of the polymers and on colloidal characteristics like size and morphology of the nanoparticles were studied by dynamic light scattering, gel permeation chromatography, differential scanning calorimetry, nuclear magnetic resonance, and Fourier transform infrared spectroscopy. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

γ‐Butyrolactone Copolymerization with the Well‐Documented Polymer Drug Carrier Poly(ethylene oxide)‐block‐poly(ε‐caprolactone) to Fine‐Tune Its Biorelevant Properties

Polymeric drug carriers exhibit excellent properties that advance drug delivery systems. In particular, carriers based on poly(ethylene oxide)‐block‐poly(ε‐caprolactone) are very useful in pharmacokinetics. In addition to their proven biocompatibility, there are several requirements for the efficacy of the polymeric drug carriers after internalization, e.g., nanoparticle behavior, cellular uptake, the rate of degradation, and cellular localization. The introduction of γ‐butyrolactone units into the hydrophobic block enables the tuning of the abovementioned properties over a wide range. In this study, a relatively high content of γ‐butyrolactone units with a reasonable yield of ≈60% is achieved by anionic ring‐opening copolymerization using 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene as a very efficient catalyst in the nonpolar environment of toluene with an incorporated γ‐butyrolactone content of ≈30%. The content of γ‐butyrolactone units can be easily modulated according to the feed ratio of the monomers. This method enables control over the rate of degradation so that when the content of γ‐butyrolactone increases, the rate of degradation increases. These findings broaden the application possibilities of polyester‐polyether‐based nanoparticles for biomedical applications, such as drug delivery systems.     

Poly(ε‐caprolactone)‐Based Organogels and Hydrogels with Poly(ethylene glycol) Cross‐Linkings

Summary: The reaction of triphosgene with poly(ethylene glycol) yielded poly(ethylene glycol) dichloroformate. This difunctional cross‐linker was allowed to react with poly(ε‐caprolactone) bearing carbanionic sites obtained by activation with lithium diisopropylamide. The reaction resulted in the cross‐linking of poly(ε‐caprolactone) chains by poly(ethylene glycol) segments, giving copolymer networks that gel in both organic and aqueous media.

Schematic of the PCL‐g‐PEG copolymers synthesized here.     

Star‐Pseudopolyrotaxane Organized in Nanoplatelets for Poly(ε‐caprolactone)‐Based Nanofibrous Scaffolds with Enhanced Surface Reactivity

Herein, it is demonstrated that star pseudopolyrotaxanes (star‐pPRs) obtained from the inclusion complexation of α‐cyclodextrin (CD) and four‐branched star poly(ε‐caprolactone) (star‐PCL) organize into nanoplatelets in dimethyl sulfoxide at 35 °C. This peculiar property, not observed for linear pseudopolyrotaxanes, allows the processing of star‐pPRs while preserving their supramolecular assembly. Thus, original PCL:star‐pPR core:shell nanofibers are elaborated by coaxial electrospinning. The star‐pPR shell ensures the presence of available CD hydroxyl functions on the fiber surface allowing its postfunctionalization. As proof of concept, fluorescein isothiocyanate is grafted. Moreover, the morphology of the fibers is maintained due to the star‐pPR shell that acts as a shield, preventing the fiber dissolution during chemical modification. The proposed strategy is simple and avoids the synthesis of polyrotaxanes, i.e., pPR end‐capping to prevent the CD dethreading. As PCL is widely used for biomedical applications, this strategy paves the way for simple functionalization with any bioactive molecules.

     

Preparation of Robust Poly(ε‐caprolactone) Hollow Spheres with Controlled Biodegradability

Summary: This work reports a new type of poly(ε‐caprolactone) (PCL) robust hollow sphere with controllable biodegradability, produced by grafting PCL shells from the surface of silica sphere cores and removing the template cores. Bis(ε‐caprolactone‐4‐yl) (BCY) composed of two ε‐caprolactone molecules was used as the crosslinker, which not only greatly strengthened the PCL hollow spheres but also brought hardly any non‐biodegradable component into the system. Solubility experiments and biodegradation tests show that the crosslinked PCL hollow spheres were robust both in water and acetone, and were completely biodegradable with characteristics of controllable biodegradability according to the content of the BCY. The Rhodamine release test indicated that the release rate of encapsulated drugs in the PCL hollow spheres was controlled by diffusion and the biodegradability of the PCL molecules, and the latter mechanism will dominate when more enzymes are involved.

Reaction scheme for synthesis of linear and crosslinked hollow poly(ε‐caprolactone) spheres.     

Microwave‐assisted graft polymerization of ε‐caprolactone onto magnetite

The graft polymerization of ε‐caprolactone (ε‐CL) onto magnetite was carried out under microwave irradiation in the presence of tin(II) 2‐ethylhexanoate. The molar ratio of ε‐CL to tin(II) 2‐ethylhexanoate was 300, whereas the molar ratio of ε‐CL to magnetite was 5. The chemical structures of the obtained poly(ε‐caprolactone) coated magnetic nanoparticles were characterized by FTIR and XPS spectroscopy. These magnetic‐polymer hybrid nanostructures were further investigated by X‐ray diffraction and magnetization measurements. The morphology of the magnetic core‐shell nanostructures were determined by TEM. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5397–5404, 2009     

Equilibrium thermal behavior and morphology of organophilic montmorillonite/poly(ε‐caprolactone) nanocomposites

With differential scanning calorimetry, we have demonstrated a peculiar behavior under equilibrium conditions of neat poly(ε‐caprolactone) and its organophilic montmorillonite nanocomposites. In particular, in the determination of the equilibrium melting temperature by the extrapolation of the data of the melting temperature (T_m) versus the crystallization temperature (T_c), a bimodal trend has been observed. At the lower T_c's, the data of T_m follow a constant trend, whereas at the higher ones, the usual increasing trend has been obtained. Morphological observations by atomic force microscopy (AFM) have provided evidence of two different crystalline morphologies for the lower and higher T_c ranges. Moreover, AFM has shown that the thermal treatments strongly influence the clay dispersion in the polymer matrix. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 22–32, 2006     

Thermoreversibly Crosslinked Poly(ε‐caprolactone) as Recyclable Shape‐Memory Polymer Network

A new concept to build shape memory polymers (SMP) combining outstanding fixity and recovery ratios (both above 99% after only one training cycle) typical of chemically crosslinked SMPs with reprocessability restricted to physically crosslinked SMPs is demonstrated by covalently bonding, through thermoreversible Diels–Alder (DA) adducts, star‐shaped poly(ε‐caprolactones) (PCL) end‐functionalized by furan and maleimide moieties. A PCL network is easily prepared by melt‐blending complementary end‐functional star polymers in retro DA regime, then by curing at lower temperature to favour the DA cycloaddition. Such covalent network can be reprocessed when heated again at the retro DA temperature. The resulting SMP shows still excellent shape memory properties attesting for its good recyclability.

     

Synthesis and Characterization of Star‐Shaped Diblock Poly(ε‐caprolactone)/Poly(ethylene oxide) Copolymers

Summary: Star‐shaped hydroxy‐terminated poly(ε‐caprolactone)s (ssPCL), with arms of different lengths, were obtained by ring‐opening polymerization (ROP) of ε‐caprolactone initiated by pentaerythritol, and were condensed with α‐methyl‐ω‐(3‐carboxypropionyloxy)‐poly(ethylene oxide)s ( = 550–5 000) to afford four‐armed PCL‐PEO star diblock copolymers (ssPCL‐PEO). The polymers were characterized by ¹H and ¹³C NMR spectroscopy and size‐exclusion chromatography (SEC). The melting behavior of ssPCLs was studied by differential scanning calorimetry (DSC). X‐ray diffraction and DSC techniques were used to investigate the crystalline phases of ssPCL‐PEOs.

The part of the synthesis of four‐armed star‐shaped diblock poly(ε‐caprolactone)‐poly(ethylene oxide) copolymers as described.     

Complex phase behavior and state of miscibility in Poly(ethylene glycol)/Poly(l‐lactide‐co‐ε‐caprolactone) Blends

A miscibility and phase behavior study was conducted on poly(ethylene glycol) (PEG)/poly(l ‐lactide‐ε‐caprolactone) (PLA‐co‐CL) blends. A single glass transition evolution was determined by differential scanning calorimetry initially suggesting a miscible system; however, the unusual T_g bias and subsequent morphological study conducted by polarized light optical microscopy (PLOM) and atomic force microscopy (AFM) evidenced a phase separated system for the whole range of blend compositions. PEG spherulites were found in all blends except for the PEG/PLA‐co‐CL 20/80 composition, with no interference of the comonomer in the melting point of PEG (T_m = 64 °C) and only a small one in crystallinity fraction (X_c = 80% vs. 70%). However, a clear continuous decrease in PEG spherulites growth rate (G) with increasing PLA‐co‐CL content was determined in the blends isothermally crystallized at 37 °C, G being 37 µm/min for the neat PEG and 12 µm/min for the 20 wt % PLA‐co‐CL blend. The kinetics interference in crystal growth rate of PEG suggests a diluting effect of the PLA‐co‐CL in the blends; further, PLOM and AFM provided unequivocal evidence of the interfering effect of PLA‐co‐CL on PEG crystal morphology, demonstrating imperfect crystallization in blends with interfibrillar location of the diluting amorphous component. Significantly, AFM images provided also evidence of amorphous phase separation between PEG and PLA‐co‐CL. A true T_g vs. composition diagram is proposed on the basis of the AFM analysis for phase separated PEG/PLA‐co‐CL blends revealing the existence of a second PLA‐co‐CL rich phase. According to the partial miscibility established by AFM analysis, PEG and PLA‐co‐CL rich phases, depending on blend composition, contain respectively an amount of the minority component leading to a system presenting, for every composition, two T_g's that are different of those of pure components. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 111–121     

Microwave syntheses of poly(ε‐caprolactam‐co‐ε‐caprolactone)

Microwave irradiation was applied to synthesize poly(ε‐caprolactam‐co‐ε‐caprolactone) directly from the anionic catalyzed ring opening of two cyclic monomers, ε‐caprolactam and ε‐caprolactone using a variable frequency microwave furnace, programmed to a set temperature and controlled by a pulsed power on–off system. Dielectric properties of ε‐caprolactam, ε‐caprolactone, and their mixture were measured in the microwave range from 0.4 to 3 GHz, showing that both ε‐caprolactam and ε‐caprolactone exhibited effective absorption of microwave energy to induce a fast chemical reaction. The microwave induced anionic copolymerization of ε‐caprolactam and ε‐caprolactone generated copoly(amide‐ester)s in yields as high as 70%. Conventional thermal and microwave copolymerization studies were also conducted for comparison with the microwave results. These studies demonstrated that an effective and efficient microwave method to copolymerize ε‐caprolactam with ε‐caprolactone in higher yield, higher amide content, and higher T_g 's, relative to the thermal process, has been developed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1379–1390, 2000     

Organometallic and enzymatic catalysis for ring opening copolymerization of ε‐caprolactone and 4‐methyl‐ε‐caprolactone

A series of copolymers containing ε‐caprolactone (CL) and 4‐methyl‐ε‐caprolactone (MeCL) were synthesized by ring‐opening polymerization (ROP) using Tin(II) bis(2‐ethylhexanoate)(Sn(Oct)₂) or Novozym 435 as catalyst. The molecular structure and weight of copolymers were determined by nuclear magnetic resonance (NMR) and size exclusion chromatography (SEC), respectively. Our kinetic study showed that the monomer reactivity ratios for CL (r₁) and MeCL (r₂) using Sn(Oct)₂ as catalyst were estimated to be near unity and r₁ × r₂ = 1, indicating the random distribution of the monomers in the final copolymer. The results of DSC and XRD consistently indicated that the copolymers were inclined to be amorphous with the increasing of MeCL fraction. Microspheres were prepared from copolymers and characterized by SEM. The preliminary degradability and biocompatibility studies on these copolymers were also assessed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011