The effects of solvent and initiator on anionic ring opening polymerization of ϵ‐caprolactone: synthesis and characterization

Anionic ring opening polymerization of ?‐caprolactone was studied by using different amounts of two potassium activated initiators containing ethylene glycol (EG) and polyhedral oligomeric silsesquioxane (POSS) diol, in tetrahydrofuran and toluene as solvents. The synthesized hydroxyl terminated macromers and linear poly(?‐caprolactone) (PCL) were characterized by proton and carbon nuclear magnetic resonance and gel permeation chromatography (GPC) techniques. Results showed an increase in molecular weight as the monomer/initiator molar ratio increased from 100 to 151 and 202, while the molecular weight distribution (MWD) showed a minimum by monomer concentration increase. Moreover, POSS‐diol‐initiated PCLs showed a higher MWD than the polymers initiated with the EG initiator. This was attributed to the formation of a vesicular structure of POSS diols which was confirmed by optical microscopy. By deconvolution of GPC peaks, the best conditions to synthesize PCL with the narrowest MWD were selected. Finally, the effects of some other parameters were studied in more detail.     

Thermal and mechanical properties of semi‐interpenetrating polymer networks composed of diisocyanate‐bridged,four‐armed,star‐shaped ε‐caprolactone oligomers and poly(ε‐caprolactone)

Semi‐interpenetrating polymer networks (S‐IPNs) were prepared by the reactions of hydroxyl‐terminated four‐armed, star‐shaped ε‐caprolactone oligomers with degrees of polymerization per one oligocaprolactone chain (ns) of 3, 5, and 10 and 2,4‐tolylene diisocyanate (TDI) in the presence of poly(ε‐caprolactone) (PCL). In the dynamic mechanical analysis of the S‐IPN [2,4‐tolylene diisocyanate bridged hydroxyl‐terminated four‐armed, star‐shaped ε‐caprolactone oligomer (TH4CLO)/PCL], only one tan δ peak was observed; its temperature increased with increasing TH4CLO content and with decreasing n value. Differential scanning calorimetric analyses of the TH4CLOs and TH4CLO/PCLs revealed that the TH4CLOs with ns of 3 and 5 were amorphous, whereas TH4CLO with an n of 10 was semicrystalline and that the crystallization of the PCL chain for TH4CLO/PCLs was more strongly disturbed with increasing TH4CLO content and decreasing n value. Although the tensile strength, modulus, and elongation at break of TH4CLO were much lower than those of PCL, those values increased with the n value. Although the tensile strength and modulus of the TH4CLO/PCLs decreased with increasing TH4CLO content, TH4CLO (n = 3)/PCL 50/50 showed the highest elongation at break (314%) among the S‐IPNs because of the suppression of crystallization of the polycaprolactone chain. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4229–4236, 2013     

Poly(ɛ‐caprolactone)/polyhedral oligomeric silsesquioxane hybrids: Crystallization behavior and thermal degradation

Biodegradable organic–inorganic hybrids based on poly(?‐caprolactone) (PCL) and polyhedral oligomeric silsesquioxane (POSS) with 5.3–21.3 wt % POSS were synthesized via ring‐opening polymerization (ROP). Chemical structures of the polymers were characterized by proton nuclear magnetic resonance (¹H NMR), fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC). X‐ray diffraction (XRD) analysis illustrated that both POSS and PCL segment in POSS/PCL hybrids could crystallize and form two well‐separated crystalline phases except in the one with low content of POSS (5.3 wt %). Melting behavior and non‐isothermal crystallization kinetics of POSS/PCL hybrids were studied by differential scanning calorimeter (DSC). The results indicated that the POSS segment suppressed crystallization of the PCL segment to some extent. Polarizing optical microscope (POM) images showed that POSS/PCL hybrids with the highest POSS loading (21.3 wt %) possessed “snowflake” shape crystals whereas the ones with relatively low POSS loading exhibited classic spherulites. Thermogravimetry (TG) measurement revealed that thermal degradation of POSS/PCL hybrids proceeded by four‐step while PCL homopolymers degraded by a single step. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44113.     

Study of a new titanium dual initiator for the synthesis of poly(ε‐caprolactone)‐b‐polystyrene block copolymers by the combination of coordination and nitroxide mediated polymerizations

Poly ε‐caprolactone‐polystyrene block‐copolymers (PCL‐b‐PSt) were synthesized using a modified titanium catalyst as the dual initiator. Alcoholysis of Ti(OPr)₄ by 4‐hydroxy 2,2,6,6 tetramethyl piperidinyl‐1‐oxyl (HO‐TEMPO) gave a bifunctional initiator Ti(OTEMPO)₄. Poly ε‐caprolactone prepolymer end‐capped with the nitroxide group was first prepared by ring opening polymerization of ε‐caprolactone with this initiator at high conversion. The nitroxide‐end‐capped structure and molar mass (M_n) of the polymers were demonstrated by typical UV absorption band. This analytical technique indicates a near‐quantitative nitroxide functionality and a M_n in good agreement with size exclusion chromatography (SEC) ones. This polyester prepolymer was used to further initiate the radical polymerization with styrene and reach the block copolymers (PCL‐b‐PSt). All the prepolymers and block copolymers were characterized by SEC and NMR spectroscopy. Additionally, the preparation of star polymers bearing two kinds of arms (PCL and PSt) was envisaged and a preliminary result was given. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Functionalization of multi‐walled carbon nanotubes with poly(ε‐caprolactone) using click chemistry

Multi‐walled carbon nanotubes (MWNTs) were covalently functionalized with poly(ε‐caprolactone) (PCL) using click chemistry. First, chlorine moiety‐containing PCL was synthesized by the copolymerization of α‐chloro‐ε‐caprolactone with ε‐caprolactone monomer using ring opening polymerization, and further converted to azide moiety‐containing PCL. The alkyne‐functionalized MWNTs were prepared with the treatment of p‐amino propargyl ether using a solvent free diazotization procedure. The covalent functionalization of alkyne‐derived MWNTs with azide moiety‐containing PCL was accomplished using Cu(I)‐catalyzed [3+2] Huisgen dipolar cycloaddition click chemistry. The PCL‐functionalization of MWNTs was confirmed by the measurements of Fourier transform infra‐red, NMR, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Effect of molecular weight on spherulitic growth rate of poly(ϵ‐caprolactone) and poly(ϵ‐caprolactone)‐b‐poly(ethylene glycol)

The spherulitic growth rates of a series poly (?‐caprolactone) homopolymers and poly(?‐caprolactone)‐b‐ poly(ethylene glycol) (PCL‐b‐PEG) block copolymers with different molecular weights but narrow polydispersity were studied. The results show that for both PCL homopolymers and PCL‐b‐PEG block copolymers, the spherulitic growth rate first increases with molecular weight and reaches a maximum, then decreases as molecular weight increases. Crystallization temperature has greater influence on the spherulitic growth rate of polymers with higher molecular weight. Hoffman–Lauritzen theory was used to analyze spherulitic growth kinetics and the free energy of the folding surface (σ_e) was derived. It is found that the values of σ_e decrease with molecular weight at low molecular weight level and become constant for high molecular weight polymers. The chemically linked PEG block does not change the values of σ_e significantly. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Direct grafting of ε‐caprolactone on solid core/mesoporous shell silica spheres by surface‐initiated ring‐opening polymerization

Poly(?‐caprolactone) (PCL) was formed on Solid core/mesoporous shell (SCMS) silica surface by surface‐initiated ring‐opening polymerization (SI‐ROP). The SI‐ROP of ?‐caprolactone was achieved by heating a mixture of SCMS silica, ?‐caprolactone and the tin(II) 2‐ethylhexanoate [Sn(Oct)₂] in a anhydrous toluene for 20 h at different temperatures viz. 40, 60, and 80°C. The PCL grafted SCMS silica was characterized by fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X‐ray, differential scanning calorimetry and scanning electron microscopy (SEM). The FTIR spectroscopic analysis reveals the formation of ester linkage between PCL and hydroxyl terminated SCMS silica. TGA investigation shows increase in PCL content on SCMS silica surface with increase in reaction temperature. The SEM photographs clearly show the formation of PCL polymer on the SCMS silica surface without altering the spherical nature of SCMS silica. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Star-shaped poly(?-caprolactone) with polyhedral oligomeric silsesquioxane core

A readily available octa(3-chloropropyl) polyhedral oligomeric silsesquioxane (POSS) [(ClCH₂CH₂CH₂)₈Si₈O₁₂] framework was used to prepare octa(3-hydroxypropyl) POSS [(HOCH₂CH₂CH₂)₈Si₈O₁₂], which was further used as an initiator to synthesize star poly(?-caprolactone) with POSS core via ring-opening polymerization catalyzed by Stannous (II) octanoate [Sn(Oct)₂]. The organic-inorganic star PCLs were characterized by means of gel permeation chromatograph (GPC), Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The wide-angle X-ray diffraction (WAXRD) experiments indicate that the presence of POSS cores did not alter the crystal structure of PCL. The star PCLs exhibited enhanced melting temperatures in comparison with the linear counterpart. The isothermal crystallization kinetics shows that both the overall crystallization rate and the spherulitic growth rate of the star PCLs increased with increasing the concentration of POSS (or with decreasing the arm lengths of the stars). The fold surface free energy of the star PCLs decreased with increasing the concentration of POSS. These results could be interpreted based on the effect of the heterogeneous nucleation of POSS cores, which accelerates the process of crystallization.     

Poly(ϵ‐caprolactone)‐functionalized carbon nanofibers by surface‐initiated ring‐opening polymerization

Carbon nanofibers (CNFs) were covalently functionalized with biodegradable poly(?‐caprolactone) (PCL) by in situ ring‐opening polymerization (ROP) of ?‐caprolactone in the presence of stannous octoate. Surface oxidation treatment of the pristine CNFs afforded carboxylic CNFs (CNF‐COOH). Reaction of CNF‐COOH with excess thionyl chloride (SOCl₂) and glycol produced hydroxyl‐functionalized CNFs (CNF‐OH). Using CNF‐OH as macroinitiator, PCL was covalently grafted from the surfaces of CNFs by ROP, in either the presence or absence of sacrificial initiator, butanol. The grafted PCL content was achieved as high as 64.2 wt %, and can be controlled to some extent by adjusting the feed ratio of monomer to CNF‐OH. The resulting products were characterized by FTIR, NMR, Raman spectroscopy, TGA, DSC, SEM, TEM, HRTEM, and XRD. Core–shell nanostructures were observed under HRTEM for the PCL‐functionalized CNFs because of the thorough grafting. The PCL‐grafted CNFs showed different melting and crystallization behaviors from the mechanical mixture of PCL and CNF‐OH. This approach to PCL‐functionalized CNFs opens an avenue for the synthesis, modification, and application of CNF‐based nanomaterials and biomaterials. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Tetrahydrosalen‐stabilized,chloride‐containing rare‐earth complexes: Facile initiator precursors for the preparation of hydroxytelechelic poly(ε‐caprolactone)s

End‐capped poly(ε‐caprolactone)s (PCLs) have been prepared elsewhere by various initiators. However, hydroxytelechelic PCLs have been reported less frequently, although there are two hydroxyl end groups in one polymer chain, which allows diversified functionalization. Two tetrahydrosalen‐backboned chlorides containing rare‐earth complexes, YbLCl(DME)₂ and ErLCl(DME) {where L is 6,6′‐[ethane‐1,2‐diylbis(methylazanediyl)]bis (methylene)bis(2,4‐di‐tert‐butylphenol) and DME is dimethoxyethane}, were first synthesized in this study, and they were used as initiator precursors for a ring‐opening polymerization in the presence of NaBH₄ to afford hydroxytelechelic PCLs. The polymerization under different conditions was investigated, and a possible mechanism is proposed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis of star‐shaped poly(ϵ‐caprolactone) by samarium‐based tetrafunctional initiator and its dilute‐solution properties

An in situ–generated tetrafunctional samarium enolate from the reduction of 1,1,1,1‐tetra(2‐bromoisobutyryloxymethyl)methane with divalent samarium complexes [Sm(PPh₂)₂ and SmI₂] in tetrahydrofuran has proven to initiate the ring‐opening polymerization of ?‐caprolactone (CL) giving star‐shaped aliphatic polyesters. The polymerization proceeded with quantitative conversions at room temperature in 2 h and exhibited good controllability of the molecular weight of polymer. The resulting four‐armed poly(?‐caprolactone) (PCL) was fractionated, and the dilute‐solution properties of the fractions were studied in tetrahydrofuran and toluene at 30°C. The Mark–Houwink relations for these solvents were [η] = 2.73 × 10^?2M_w^0.74 and [η] = 1.97 × 10^?2M_w^0.75, respectively. In addition, the unperturbed dimensions of the star‐shaped PCL systems were also evaluated, and a significant solvent effect was observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 175–182, 2006     

Synthesis of poly(ε‐caprolactone)/clay nanocomposites using polyhedral oligomeric silsesquioxane surfactants as organic modifier and initiator

Poly(ε‐caprolactone)/clay nanocomposites were synthesized by in situ ring‐opening polymerization of ε‐caprolactone in the presence of montmorillonite modified by hydroxyl functionalized, quaternized polyhedral oligomeric silsesquioxane (POSS) surfactants. The octa(3‐chloropropyl) polyhedral oligomeric silsesquioxane was prepared by hydrolytic condensation of 3‐chloropropyltrimethoxysilane, which was subsequently quaternized with 2‐dimethylaminoethanol. Montmorillonite was modified with the quaternized surfactants by cation exchange reaction. Bulk polymerization of ε‐caprolactone was conducted at 110°C using stannous octoate as an initiator/catalyst. Nanocomposites were analyzed by X‐ray diffraction, transmission electron microscopy, thermo gravimetric analysis, and differential scanning calorimetry. Hydroxyl functionalized POSS was employed as a surface modifier for clay which gives stable clay separation for its 3‐D structure and also facilitates the miscibility of polymer with clay in the nanocomposites due to the star architecture. An improvement in the thermal stability of PCL was observed even at 1 wt % of clay loading. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Linear‐dendritic copolymers as nanocatalysts

Functionalized poly(ethylene glycol) (PEG) containing four chloride end functional groups (PEG‐Cl₄) was synthesized through reaction between cyanuric chloride and PEG‐(OH)₂. Chloride end functional groups of PEG‐Cl₄ were able to initiate the ring opening polymerization of 2‐ethyl‐2‐oxazoline and star copolymers containing a PEG core, and poly(2‐ethyl‐2‐oxazoline) (POX) arms were obtained. Polymerization was quenched using diethanolamine, and star copolymers containing hydroxyl end functional groups (PEG‐POX‐OH) were obtained. ε‐Caprolactone was then polymerized using the hydroxyl end functional groups of star copolymers and amphiphilic linear‐dendritic copolymers containing PEG and POX, and poly(caprolactone) (PCL) blocks were synthesized. Linear‐dendritic copolymers were able to load the organic and inorganic guest molecules. Application of host‐guest systems such as nanocatalyst for Heck chemical reaction was also investigated. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Self‐assembled micelles prepared from poly(ɛ‐caprolactone)–poly(ethylene glycol) and poly(ɛ‐caprolactone/glycolide)–poly(ethylene glycol) block copolymers for sustained drug delivery

A series of poly(?‐caprolactone)–poly(ethylene glycol) (PCL‐PEG) and poly(?‐caprolactone/glycolide)–poly(ethylene glycol) [P(CL/GA)‐PEG] diblock copolymers were prepared by ring‐opening polymerization of ?‐caprolactone or a mixture of ?‐caprolactone and glycolide using monomethoxy PEG (mPEG) as macroinitiator and Sn(Oct)₂ as catalyst. The resulting copolymers were characterized using ¹H‐NMR, gel permeation chromatography, differential scanning calorimetry, and wide‐angle X‐ray diffraction. Copolymer micelles were prepared using the nanoprecipitation method. The morphology of the micelles was spherical or worm‐like as revealed by transmission electron microscopy, depending on the copolymer composition and the length of the hydrophobic block. Introduction of the glycolide component, even in small amounts (CL/GA = 10), disrupted the chain structure and led to the formation of spherical micelles. Interestingly, the micelle size decreased with the encapsulation of paclitaxel. Micelles prepared from mPEG5000‐derived copolymers exhibited better drug loading properties and slower drug release than those from mPEG2000‐derived copolymers. Drug release was faster for copolymers with shorter PCL blocks than for those with longer PCL chains. The introduction of glycolide moieties enhanced drug release, but the overall release rate did not exceed 10% in 30 days. In contrast, drug release was enhanced in acidic media. Therefore, these bioresorbable micelles and especially P(CL/GA)‐PEG micelles with excellent stability, high drug loading content, and prolonged drug release could be promising for applications as drug carriers. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45732.     

Synthesis,characterization and crosslinking of functional star‐shaped poly(ε‐caprolactone)

Star‐shaped low molecular weight poly(ε‐caprolactone)s (PCLs) were synthesized and functionalized with crosslinkable terminal groups for subsequent crosslinking. The ε‐caprolactone (CL) prepolymers were polymerized by ring‐opening in the presence of polyglycerine (PGL) as an initiator (1, 3 and 5 mol%) and Sn(II)2‐ethylhexanoate as a catalyst. Characterization of the prepolymer by ¹³C/¹H nuclear magnetic resonance (NMR) spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) revealed a six‐armed star‐shaped structure for the prepolymer with the molecular weight controlled by the ratio of PGL and CL. Functionalization of the hydroxyl‐terminated prepolymer was carried out with maleic or itaconic anhydride. In both cases, the characterization of the functionalized prepolymer showed that the hydroxyl groups were completely substituted. The functionalized PCLs were successfully crosslinked through the reaction of double bonds. The crosslinking was induced either thermally with organic peroxide or photochemically with a photosensitive initiator. Characterization of the crosslinked PCLs by Soxhlet extraction, DSC and FTIR showed that the itaconic double bond was much more reactive in thermal crosslinking than the maleic double bond. Thus, the crosslinked prepolymers that were functionalized with itaconic double bonds achieved a gel content of about 90%. A gel content of 100% was achieved with several compositions where crosslinking agents were employed. © 2002 Society of Chemical Industry     

Esterification on solid support by surface‐initiated ring‐opening polymerization of ε‐caprolactone from benzylic hydroxyl‐functionalized Wang resin bead

Biodegradable poly(ε‐caprolactone) (PCL) was formed on benzylic hydroxyl‐functionalized Wang resin surface by surface‐initiated ring‐opening polymerization (SI‐ROP). The SI‐ROP of ε‐caprolactone was achieved first by treating Wang resin with Tin(II) 2‐ethylhexanoate [Sn(Oct)₂] to form Tin(II) complex, and then followed by polymerization of ε‐caprolactone in anhydrous toluene at 60°C. Thus, the polymer‐grafted Wang resin was characterized by Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), optical microscopy (OM), and field‐emission scanning electron microscopy (FE‐SEM). The FTIR spectroscopic analysis of polymer‐grafted Wang resin (Wang‐g‐PCL) reveals the formation of ester linkage between PCL and hydroxyl‐terminated Wang resin. The DSC thermogram shows melting peak corresponding to PCL polymer on Wang resin surface. Thermogravimetric investigation shows increase in PCL content on the Wang resin surface in terms of percentage of weight loss with increase in reaction time. The formation of polymeric layers on the Wang resin surface can be directly visualized from OM and SEM images. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Six‐arm star‐shaped polymer with cyclophosphazene core and poly(ε‐caprolactone) arms as modifier of epoxy thermosets

A six‐arm star‐shaped poly(ε‐caprolactone) (s‐PCL) based on cyclophosphazene core was obtained by presynthesis of a hydroxy‐teminated cyclophosphazene derivative and subsequent initiation of the ring‐opening polymerization of ε‐caprolactone, and its use in different proportions as toughening modifier of diglycidylether of bisphenol A/anhydride thermosets was studied. The star‐shaped polymer was characterized to have approximately 30 caprolactone units per arm. Differential scanning calorimetry revealed a nonsignificant influence on the curing process of the epoxy‐anhydride formulation by the addition of s‐PCL. The s‐PCL‐modified epoxy thermosets exhibited a great improvement in both toughness and strength compared with the neat resin, as the result of a joint effort by the internal rigid core and the external ductile polyester chains of s‐PCL. When the addition of the modifier was 3 wt %, an optimal mechanical and thermomechanical performance was achieved. The impact resistance and tensile strength of the cured epoxy resin were enhanced by 150% and 30%, respectively. The glass transition temperature was also increased slightly. Moreover, the addition of the star‐shaped modifier had little harmful effect on the thermal stability of the material. Thus s‐PCL was proved to be a superior toughening agent without sacrificing thermal and mechanical properties of the thermosets. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44384.     

Synthesis and characterization of novel lipid functionalized poly(ε‐caprolactone)s

A series of novel lipid functionalized poly(ε‐caprolactone)s (PCLs) were synthesized through ROP of ε‐caprolactone in the presence of threo‐9,10‐dihydroxyoctadecanoic acid, synthesized from oleic acid. PCLs with different molecular weights were obtained by controlling the molar ratio of the initiator to the monomer. DSC and XRD analysis indicate that the crystallinity of PCLs decreased when compared to unfunctionalized PCL. The enzymatic degradation study shows that for samples with lower lipid derivatives content, a higher enzymatic degradation rate was observed because the lipase enzymes attack the ester bonds of the polymer; increased lipid content therefore inhibits the action of the lipase enzymes. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis and characterization of polyrotaxanes comprising α‐cyclodextrins and poly(ε‐caprolactone) end‐capped with poly(butyl methacrylate)s

Biodegradable polyrotaxane‐based triblock copolymers were synthesized via the bulk atom transfer radical polymerization (ATRP) of n‐butyl methacrylate (BMA) initiated with polypseudo‐rotaxanes (PPRs) built from a distal 2‐bromoisobutyryl end‐capped poly(ε‐caprolactone) (Br‐PCL‐Br) with α‐cyclodextrins (α‐CDs) in the presence of Cu(I)Br/N,N,N′,N″,N″‐pentamethyldiethylenetriamine at 45 ºC. The structure was characterized in detail by means of ¹H NMR, gel permeation chromatography, wide‐angle X‐ray diffraction, DSC and TGA. When the feed molar ratio of BMA to Br‐PCL‐Br was changed from 128 to 300, the degree of polymerization of PBMA blocks attached to two ends of the PPRs was in the range 382 ? 803. Although about a tenth of the added α‐CDs were still threaded onto the PCL chain after the ATRP process, the movable α‐CDs made a marked contribution to the mechanical strength enhancement, blood anticoagulation activity and protein adsorption repellency of the resulting copolymers. Meanwhile, they could also protect the copolymers from the attack of H₂O and Lipase AK Amano molecules, exhibiting a lower mass loss as evidenced in hydrolytic and enzymatic degradation experiments. © 2013 Society of Chemical Industry     

Non‐monotonic molecular weight dependence of crystallization rates of linear and cyclic poly(epsilon‐caprolactone)s in a wide temperature range

By using a commercial fast‐scan chip‐calorimeter, the effect of molecular weight on the isothermal crystallization rates of linear and cyclic poly(epsilon‐caprolactone) samples (L‐PCLs and C‐PCLs) is investigated. The results confirm the non‐monotonic molecular weight dependence of the crystallization rates of both L‐PCLs and C‐PCLs over a wide temperature range, previously observed only in the high temperature range, in which the medium molecular weight PCLs exhibit the highest crystallization rate. Chain‐end effects of L‐PCL with medium molecular weights induce faster crystallization rates in specific low temperature regions, in comparison to that for counterpart C‐PCL. This phenomenon no longer exists in high molecular weight samples because of ‘diluted’ chain‐end effects in long chain samples. © 2016 Society of Chemical Industry