Cyclic PCL Research Articles

Lithium anthraquinoids as catalysts in the ROP of lactide and caprolactone into cyclic polymers

New lithium anthraquinoids 2b–d active in the synthesis of cyclic PLA and cyclic PCL have been synthesized and fully characterized.

The role of entanglement in crystallization and melting of cyclic poly(ε-caprolactone)s

Entanglement affects the long period L, lamellar thickness dc and the thickness of the amorphous region da of cyclic PCL.

Morphology and growth rate of spherulite of cyclic poly(ε-caprolactone) having a triazole group at the closing point

To clarify the effect of triazole (TR) group at the closing point within the cyclic polymer chain on the crystal growth, we studied morphology and the growth rate of spherulite G of cyclic poly(ε-caprolactone) with a TR group at the closing point (C-TR-PCL) as a function of the degree of supercooling ΔT using polarizing optical microscopy. We prepared C-TR-PCLs synthesized by the ring-closing reaction from their linear precursors with Mw = 30000 and 50000, and cyclic PCL polymerized by ring expansion reaction (C-PCL) having no TR group with Mw = 26000, and linear PCL with/without a TR group at a chain end (L-TR-PCL-OH and L-PCL) with Mw = 26000 and 36000, where Mw was determined by GPC using linear polystyrene standards.We found that C-TR-PCL and L-TR-PCL-OH show banded spherulites at small ΔT and band-less normal spherulites with maltese cross at large ΔT. Whereas, C-PCL and L-PCL only showed band-less normal spherulites at any ΔT. This speculates that steric hindrance of TR group near the surface of crystals will become significant at small ΔT. G obeyed the equation, G = G0exp(−B/TΔT), for all the samples. B of C-PCL and L-PCL is constant irrespective of TΔT, however, that of C-TR-PCL and L-TR-PCL-OH showed two values depending on TΔT. At the region of lower B, i.e., small ΔT, the banded spherulites could be observed. The change of B corresponds to the morphological change of the spherulites. Since the size of the crystalline region closely related to lamellar thickness becomes large at small ΔT, TR groups excluded from the crystals become to assemble near the surface of the crystals. This is the reason why B becomes low at small ΔT. In C-TR-PCL with higher Mw and L-TR-PCL-OH, the decrease of B with decreasing ΔT becomes small compared to that of C-TR-PCL with lower Mw due to relatively small effect of TR group.

Precise Synthesis, Properties, and Structures of Cyclic Poly(ε-caprolactone)s.

Cyclic PCL (c-PCL) has drawn great attention from academia and industry because of its unique, unusual structure and property characteristics due to the absence of end groups in addition to the biocompatibility and biodegradability of its linear analogue. As a result of much research effort, several synthetic methods have been developed to produce c-PCLs so far. Their chain, morphology and property characteristics were investigated even though carried out on a very limited basis. This feature article reviews the research progress made in the synthesis, morphology, and properties of c-PCL; all results and their pros and cons are discussed in terms of purity and molecular weight distribution in addition to the cyclic topology effect. In addition, we attempted to synthesize a series of c-PCL products of high purity by using intramolecular azido-alkynyl click cyclization chemistry and subsequent precise and controlled separation and purification; and their thermal degradation and phase transitions were investigated in terms of the cyclic topology effect.

A twin-tailed tadpole-shaped amphiphilic copolymer of poly(ethylene glycol) and cyclic poly(ε-caprolactone): synthesis, self-assembly and biomedical applications

A tadpole-shaped amphiphilic copolymer containing cyclic PCL and two PEG tails, PEG-b-(c-PCL)-b-PEG, was rationally designed and synthesized.

Synthesis of High Molecular Weight Cyclic Poly(ε-caprolactone)s of Variable Ring Size Based on a Light-Induced Ring-Closure Approach.

High molecular weight cyclic poly(ε-caprolactone)s (cPCLs) with variable ring size are synthesized via light-induced ring closure of α,ω-anthracene-terminated PCL (An-PCL-An). The ring size of cPCL is tunable simply by adjusting the polymer concentration from 10 to 100 mg mL(-1) in THF. The cyclo-addition via the bimolecular cyclization of An-PC-An is well characterized by a variety of analyses such as (1) H NMR and UV-vis spectroscopies, gel-permeation chromatography, and differential scanning calorimetry. The reversible dimerization of An induced by heating enables the cyclic PCL to have a switchable "on-off" capability. This novel light-induced ring-closure technique can be one of the most powerful candidates for producing various well-defined cyclic polymers in highly concentrated polymer solution.

Effective Approach to Cyclic Polymer from Linear Polymer: Synthesis and Transformation of Macromolecular [1]Rotaxane.

We report a convenient and scalable synthesis of cyclic poly(ε-caprolactone) (PCL) from its linear counterpart based on the rotaxane protocol. Cyclic PCL was prepared by ring-opening polymerization of ε-caprolactone (ε-CL) initiated by a pseudo[2]rotaxane initiator in the presence of diphenylphosphate (DPP) as a catalyst, followed by capping of the propagation end by using a bulky isocyanate to afford macromolecular [2]rotaxane. The successive intramolecular cyclization to macromolecular [1]rotaxane at the polymer terminus proceeded with good yield. The attractive interaction of the terminal ammonium/crown ether moiety was removed via N-acetylation. This enabled movement of the crown ether wheel along the axle PCL chain to the urethane region of the other terminus in solution state. Size-exclusion chromatography and 2D diffusion-ordered spectroscopy (DOSY) results demonstrated the formation of cyclic PCL from linear PCL, which is further supported by thermal property or crystallinity change before and after transformation.

Comparing crystallization rates between linear and cyclic poly(epsilon-caprolactones) via fast-scan chip-calorimeter measurements

The vital role of chain ends in the crystallization of linear polymers can be understood by unraveling the mechanisms of crystallization of cyclic polymers. A commercial chip-calorimeter Flash DSC1 was employed to compare the nucleation and overall crystallization rates between linear and cyclic poly(epsilon-caprolactones) (PCL) with similar molar masses of around 2 kg/mol. In the high temperature region, faster overall crystallization of cyclic PCL relative to linear PCL is consistent with previously reported results employing conventional DSC. In the low temperature region, the cyclic PCL exhibits a lower onset temperature of homogenous nucleation when compared to linear PCL analogs. This result was attributed to a higher mobility of free chain ends in the linear PCL as compared to cyclic PCL. A simplified data-treatment method on the nucleation half-time has been proposed.

Nucleation and Antinucleation Effects of Functionalized Carbon Nanotubes on Cyclic and Linear Poly(ε-caprolactones)

Novel nanocomposites were prepared by blending linear or cyclic poly(e-caprolactones) with two types of chemically modified carbon nanotubes (CNTs). The low-polydispersity cyclic PCL samples (C-PCLs) were synthesized by click chemistry with a number-average molecular weight (Mn) of 22 kg/mol. Linear analogues (L-PCLs) with the same Mn value were also prepared. Two types of CNTs were employed (with 1% w/w content): single wall CNTs functionalized with octadecylamine (SWNT-ODA) and multiwall carbon nanotubes grafted with linear PCL chains (i.e., MWNT-g-PCL prepared by ring-opening polymerization on previously functionalized MWNTs with a composition of 10% MWNT and 90% L-PCL). The nanocomposites were characterized by transmission electron microscopy (TEM), polarized light optical microscopy (PLOM), and differential scanning calorimetry (DSC). A nucleating effect was detected in both PCLs when SWNT-ODAs were employed. However, in the case of MWNT-g-PCL, the nanofiller nucleated L-PCL but caused an unexpected ...

Nucleation, crystallization, self-nucleation and thermal fractionation of cyclic and linear poly(ε-caprolactone)s

A series of low polydispersity cyclic PCL samples (C-PCLs), as well as their linear analogs (L-PCLs), were synthesized by click chemistry in a number average molecular weight (Mn) range of 2–22kg/mol. They were investigated by Polarized Light Optical Microscopy (PLOM) and Differential Scanning Calorimetry (DSC). The nucleation and overall crystallization kinetics were studied, as well as their self-nucleation behavior and SSA (Successive Self-nucleation and Annealing) thermal fractionation. Cyclic PCLs were found to nucleate and crystallize faster than linear PCLs due to: (a) faster diffusion of C-PCL chains and (b) larger supercoolings of C-PCLs at any given crystallization temperature, as compared to L-PCLs. A bell shape curve was obtained when the overall crystallization rate was examined as a function of Mn, this effect is probably due to a competition between nucleation and diffusion. It was found for the first time, that since cyclic molecules have lower entanglement densities, they can quickly recover their pseudo-equilibrium compact coil conformations upon melting and therefore exhibit much smaller crystalline memory effects than their linear counterparts of identical chain lengths. SSA revealed that C-PCLs are more sensitive to annealing than L-PCLs because their ring topology and limited lamellar chain folding facilitates crystal thickening.

New insights on the crystallization and melting of cyclic PCL chains on the basis of a modified Thomson–Gibbs equation

Low polydispersity linear and cyclic poly(ε-caprolactone) with molecular weights of 2–7.5 kg/mol were synthesized. Differences in the morphology of cyclic and linear single crystals (obtained from solution at identical temperatures) indicate that cyclic PCL crystals were obtained at higher supercoolings than linear ones. Cyclic PCL crystals (both crystallized from solution and from the melt) exhibited higher melting point values (Tm) than analogous linear PCL lamellae. In addition, cyclic PCLs crystallize from the melt at higher rates than linear PCLs at identical crystallization temperatures. Results showed that cyclic PCLs possess a higher equilibrium melting point (Tm0) than linear PCL crystals. A modified Thomson–Gibbs equation and a modified Hoffman–Weeks equation have been derived in this work to demonstrate that the higher Tm0 of the cyclic polymer stemmed from the negative cyclization entropy associated with the absence of chain ends and the more collapsed conformation of the cyclic chain in the melt state.

Crystallization of Cyclic Polymers: Synthesis and Crystallization Behavior of High Molecular Weight Cyclic Poly(ε-caprolactone)s

High molecular weight poly(ε-caprolactone)s (PCL) were synthesized via zwitterionic polymerization of ε-caprolactone initiated with N-heterocyclic carbenes. Ring-opening polymerization of ε-caprolactone (1 M) with carbenes 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (1) or 1,3,4,5-tetramethylimidazol-2-ylidene (3) affords cyclic PCL with molecular weights up to Mn = 114 000 g/mol. Crystallization of the cyclic PCL was investigated by synchrotron small-angle X-ray scattering experiments and differential scanning calorimetry. High molecular weight cyclic poly(ε-caprolactone) crystallizes with a similar lamellar thickness and long period spacing as linear poly(ε-caprolactone), but the crystallization of cyclic poly(ε-caprolactone) is faster than that of linear poly(ε-caprolactone) for molecular weights greater than 75 000 g/mol. These results imply that the polymer topology does not have a significant influence on the crystal structure or morphology but can have a significant influence on the rate of crystallization from the melt.

Cyclic poly(ε‐caprolactone) synthesized by combination of ring‐opening polymerization with ring‐closing metathesis, ring closing enyne metathesis, or “click” reaction

AbstractThis article described the synthesis of cyclic poly(ε‐caprolactone) (PCL) via ring‐closing metathesis (RCM), ring closing enyne metathesis (RCEM), and “click” reaction of different difunctional linear PCL. Linear PCL precursors were prepared by ring‐opening polymerization (ROP) of ε‐caprolactone in bulk using 10‐undecen‐1‐ol or propargyl alcohol as the initiator, followed by reacting with corresponding acyl chloride containing vinyl or azido end group. The subsequent end‐to‐end intramolecular coupling reactions were performed under high dilution conditions. The successful transformation of linear PCL precursor to cyclic PCL was confirmed by Gel permeation chromatography, 1H NMR, and Fourier transform infrared measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3022–3033, 2009

Amphiphilic Sun-Shaped Polymers by Grafting Macrocyclic Copolyesters with PEO

An amphiphilic sun-shaped copolymer was successfully prepared by esterification of carboxylic acid terminated PEO with the pendent hydroxyl groups of high molecular weight (Mn = 28 000) cyclic PCL. The cyclic structure of the copolyester originally resulted from the polymerization initiation by a cyclic tin dialkoxide and was ultimately stabilized by the intramolecular cross-linking of a few unsaturated groups.

Synthesis of tadpole-shaped copolyesters based on living macrocyclic poly(ɛ-caprolactone)

Synthesis of an asymmetric tadpole-shaped aliphatic copolyester consisting of a poly(ɛ-caprolactone) ring and two poly( l-lactide) tails was reported for the first time. First, a high molecular weight cyclic PCL macroinitiator ( M n = 31,000) was prepared by intramolecular photocross-linking of “living” chains. Polymerization of l-lactide was resumed by the tin dialkoxide containing macrocycles, thus making the targeted tadpole-shaped copolyester available. A preliminary investigation of the crystallization of these copolyesters was carried out by differential scanning calorimetry and polarized optical microscopy.