Synthesis and meticulous molecular, morphological and thermal characterization of linear and star-shaped polycaprolactones

Abstract

In this study, linear and star-shaped polycaprolactones with varying numbers of arms are synthesized by ring opening of ε-caprolactone. The polymers were characterized in detail to obtain information of by-products in the samples. Size exclusion chromatography could not differentiate products with similar molar mass but different architecture and if there are any by-products in the sample. Liquid chromatography at critical conditions (LCCC) on NP and RP columns provided more insight into the product composition with regard to number of arms and possible by-products. MALDI-TOF MS analysis confirmed the structures and successful separation of targeted product from unwanted by-product. 2D-LC by coupling of LCCC to SEC in the second dimension demonstrated the similar size of the products that are separated successfully by LCCC. The purity and analysis of star-shaped precursor is imperative for further synthesis of star-block copolymers that is not given much attention in published literature. Thermal and morphological properties of the polycaprolactones with similar molar mass but different architectures are evaluated by DSC, TGA and AFM.

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Introduction

The architecture of polymers is an important parameter that influences their physical properties and final applications. Polymers with similar molar mass but varying architecture (such as linear to stars with different number of arms) can have distinctly different physical properties. Therefore, characterization of polymers with regard to their architecture is imperative and at the same time very challenging. Size exclusion chromatography (SEC) being the most widely used mode of liquid chromatography of polymers, cannot separate polymers with similar hydrodynamic size but different architectures. Spectroscopic techniques cannot be applied to complex mixtures that contain side products with similar chemical nature but varying architectures. Poly(ε-caprolactone) (PCL) is believed to be the first generation of synthetic aliphatic polyesters. PCL has a semi-crystalline structure with hydrophobic properties, contains one polar ester group and five non-polar methylene groups in its repeating unit. PCL has a low glass transition temperature and shows an elastic behaviour at ambient temperature. High contents of methylene groups in the PCL impart olefin like character while hydrolytically labile polar ester group induces biodegradability.^1–3 Aforementioned properties of PCL render it to be an excellent candidate for drug delivery and other bio-related formulations.^1,4–8 Architectures of these biodegradable polymers have enormous effect on their physical and performance properties.^4,9–17 Star-shaped polymers are claimed to be superior as drug delivery systems than their linear counterparts because of their smaller hydrodynamic volume and low viscosity that are important parameters for drug loading and delivery.^16–18 The star-shaped block copolymers are generally synthesized by ring opening polymerization of ε-caprolactone, using polyols with varying number of hydroxyl groups as initiator. These star-shaped polycaprolactones are then used as precursor for polymerization of second monomer to make star-block copolymers.^{8,10,13–15,17,19–24} Purity of star-shaped PCL precursor is very important for synthesis of pure star-block copolymers, to achieve their maximum efficiency. There can be several side reactions during ring opening polymerization of ε-caprolactone that can lead to by-products whose structures are different from intended (such as linear and cyclic polymers), that may be present in the targeted star-shaped polymer samples. Presence of these by-products in the star-block copolymers has significant influence on their performance properties. Therefore, the purity and comprehensive characterization of precursor is imperative. In this context, we synthesized polycaprolactones by using different polyols as initiators that resulted in star-shaped polycaprolactones with varying number and lengths of arms. Detailed characterization of star-shaped polycaprolactones is carried out by liquid chromatography at critical conditions and MALDI-TOF-MS. Two-dimensional liquid chromatography has been used to demonstrate the heterogeneity of the products. Fractions from HPLC separation were collected and analyzed by MALDI-TOF-MS. The targeted products are separated successfully from unwanted by-products. The physiochemical properties of linear and star-shaped polymers could be different due to their unique molecular architecture.^25–27 Self-assembly phenomena of polycaprolactones with different architectures are particularly interesting.²⁸ Therefore, preliminary insight on this aspect is also presented.

Materials and methods

Materials

Diethylene glycol (Scharlau, Spain), n-hexanol (Scharlau, Spain), glycerol (Merck, Germany), ε-caprolactone (TCI, Japan), stannous octoate (Sigma Aldrich, Germany) and pentaerythritol (Sigma Aldrich, Germany) were used as received. HPLC grade 1,4-dioxane, n-hexane, tetrahydrofuran (THF), and acetonitrile were purchased from Samchun, Korea.

Polymerizations

The polymerizations were carried out in bulk by cationic ring opening of ε-caprolactone, using different polyols (n-hexanol, diethylene glycol, glycerol and pentaerythritol) as initiator to yield linear and star-shaped polymers with varying number of arms while stannous octoate (0.05 wt%) was used as catalyst at 120 °C. Relative amounts of initiators and monomer are varied to yield star-shaped polycaprolactones with varying lengths of arms and total molar mass.

Liquid chromatography

For SEC analyses, a three column set (Agilent Polypore 300 × 7.5 mm, Waters Styragel HR4 300 × 7.8 mm, and Jordi DVB mixed bed 300 × 8.0 mm) was used and column temperature was controlled at 40 °C using a column oven (Eppendorf, TC-50). A triple detector (Viscotek TDA302) was used to record light scattering (LS), and differential refractive index (RI) signals. The eluent was THF (Samchun, HPLC grade) at a flow rate of 0.7 mL min⁻¹ (Bischoff, compact pump 2250). The refractive index increment (dn/dc) value of PCL in THF is 0.098. Sample concentrations were kept between 3.0 and 5.0 mg mL⁻¹ and 100 μL of sample solution was injected.

Uni-dimensional liquid chromatographic analysis was performed on a modular system consisting of Shimadzu LC20AD binary pump, Rheodyne 6-port injection valve equipped with a 100 μL sample loop, and a column oven. The chromatograms were recorded by polymer laboratories PL ELS 2100, an evaporating light scattering detector. Evaporator and nebulizer temperature of detector were set at 55 and 40 °C, respectively. Filtered compressed air was used as carrier gas at a flow rate of 1.0 standard liters per minute (SLM) for all measurements. Sample concentrations were 1.0–5.0 mg mL⁻¹.

For RPLC separations, above described system was used with a Nucleosil C18, 100 Å, 150 × 4.6 mm column. The flow rate was 0.5 mL min⁻¹ and column oven was maintained at 40 °C.

For NPLC separations, above described HPLC system was used with a Nucleosil, 100 Å, 250 × 4.6 mm column. The flow rate was 0.5 mL min⁻¹ and column oven was maintained at 40 °C. For gradient separation on NP column, above mentioned column was used with a binary gradient. Solvent A was critical solvent for polycaprolactone (1,4-dioxane/n-hexane = 52/48, v/v) and solvent B was 1,4-dioxane. The details of gradient are given in Table 1.

Table 1 Solvent gradient for NPLC around critical conditions of PCL for uni-dimensional LC at flow rate of 0.5 mL min⁻¹; solvent A: 1,4-dioxane/n-hexane = 52/48, v/v, solvent B: 1,4-dioxane

Time (min)	Solvent B (%)
0	0
5	0
20	100
25	100

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In 2D-LC, NPLC separation is coupled to SEC. Briefly, 2D-LC system consists of two HPLC pumps, a 6-port injection valve, a 10-port switching valves, two column ovens and an ELS detector. First dimensional setup consists of a Shimadzu LC20AD binary pump, a Rheodyne 6-port injection valve equipped with a 100 μL sample loop, and a column oven. Same NP column was used for first dimension separation at a flow rate of 0.05 mL min⁻¹. The details of gradient are given in Table 2.

Table 2 Solvent gradient for NPLC around critical conditions of PCL for 2D-LC at flow rate of 0.05 mL min⁻¹; solvent A: 1,4-dioxane/n-hexane = 52/48, v/v, solvent B: 1,4-dioxane

Time (min)	Solvent B (%)
0	0
50	0
200	100
250	100

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The second dimension consisted of a Water's HPLC 515 pump connected to a polypore column 250 × 4.6 mm (Agilent). Detection in the second dimension was accomplished by Polymer Laboratories PL ELS 2100, an evaporating light scattering detector. Evaporator and nebulizer temperature of detector were set at 55 and 40 °C, respectively. Filtered compressed air was used as carrier gas at a flow rate of 1.0 SLM for all measurements. The flow rates used in the first and second dimensions were 0.05 mL min⁻¹ and 1.75 mL min⁻¹, respectively. Sample concentrations were kept between 5.0 and 20.0 mg mL⁻¹. THF was used as eluent for the second dimension analysis.

MALDI-TOF MS

Bruker Autoflex speed mass spectrometer was used for MALDI-TOF mass spectrometry. The instrument is equipped with a 2 kHz smart beam-II laser. The irradiations slightly above the threshold laser power were used to generate ions. Positive ion spectra were recorded in reflectron mode. The system was externally calibrated with a suitable mixture of polystyrene. To improve the signal-to-noise ratio, the spectra of 100–150 shots were averaged.

A solution of trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) (30 mg mL⁻¹ in THF), a solution of the analyte (5 mg mL⁻¹ in THF), and a solution of CF₃COONa (2 mg mL⁻¹ in THF), were mixed to prepare sample solutions in a ratio of 1 : 10 : 1 (v/v/v). 0.5 μL of the resulting mixture were deposited on the stainless steel sample plate and allowed to dry in air.

Differential scanning calorimetry (DSC)

Thermal properties of linear and star-shaped PCL were studied by using TA Instruments STD Q600 system in nitrogen atmosphere at 50 kPa pressure. Approximately 10–12 mg of sample were heated from room temperature to 120 °C at a rate of 10 °C min⁻¹. The melting parameters of all polymers were recorded from heating scans such as onset temperature and peak temperature.

Thermogravimetric analysis (TGA)

Thermogravimetric analyses (TGA) of different architecture of PCL were studied by using a TA Instruments STD Q600 system. Approximately 10–12 mg of sample were placed in a crucible and tested with a thermal ramp over a temperature range of 25 to 800 °C, at a heating rate of 10 °C min⁻¹ in nitrogen atmosphere at 50 kPa.

Atomic force microscopy (AFM)

Topographical variations in the polymers at room temperature were investigated with atomic force microscopy AFM 5500 (Agilent, USA). Triangular soft silicon nitride cantilever (Veeco, model MLCT-AUHW) with a nominal value of the spring constant of 0.01 N m⁻¹ and 0.1 N m⁻¹ being used in the non-contact mode for measurements. A 2 μg mL⁻¹ solution of polymer was prepared in chloroform. The solution was vortexed for 1 min and sonicated (KQ 500-DE) for 30 min. About 10 μL solution was transferred onto a freshly cleaved mica surface and allowed to dry. The samples were then analyzed by atomic force microscopy to monitor the self assembly behaviour of the polymers on mica surface.

Results and discussion

Polycaprolactones with similar molar mass but varying architecture were synthesized by using different polyhydric alcohols as initiator (n-hexanol, diethylene glycol, glycerol and pentaerythritol). Hexanol has one hydroxyl end-group, therefore, polymerization proceeds in one direction and resulting polycaprolactone should be linear having a hexyl and a hydroxyl end-group. Diethylene glycol has two hydroxyl end-groups and polymerization should proceed on both sides resulting in linear polycaprolactones with two hydroxyl end-groups. Glycerol is a tri-hydric alcohol and resulting polycaprolactone should be a tri-arm star with three hydroxyl end-groups. Similarly, pentaerythritol has four hydroxyl end-groups and resulting polymer should be a tetra-arm star with similar number of hydroxyl end-groups. The general reaction of cationic ring opening of caprolactones is given in Scheme 1. Fig. 1A–D depicts the targeted structures of polycaprolactones initiated by different initiators. Most probable by-products of cationic ring opening polymerization of caprolactones are given in Fig. 1E and F. Linear polycaprolactone shown in Fig. 1E is the result of polymerization initiated with the moisture content whereas cyclic polycaprolactone (Fig. 1F) is the result of backbiting of a growing chain. These star-shaped polycaprolactones have been used as precursor to synthesize the star block copolymers by several research groups.^{8,10,13–15,17,19–24} The purity of these precursors is very important for synthesis of pure star block copolymers and their practical performance. Above mentioned side products can lead to block copolymers with different structure and influence their performance properties.

Scheme 1 General chemical reaction of ring opening of ε-caprolactones.

Fig. 1 The polymer structures; initiator: red, ε-caprolactone repeat unit: blue.

The first question when synthesizing such polymers that have same chemical nature but differing in architectures is if the polymerization is proceeding from targeted functional end-groups and resulting in desired structures. To answer this question, we took polymers with low number of repeat units and conducted their MALDI-TOF-MS analysis. All the oligomers initiated with different polyols should have different molar mass distribution, however, same molar mass increment of 114 (formula weight of one caprolactone unit). As can be seen for all the products under study, the major series in all cases is the targeted product. All the products have similar molar mass increment but having different molar mass of the oligomers with same number of repeat units. The experimental values obtained for products initiated with hexanol, diethylene glycol and glycerol match well with the theoretical values, Fig. 2A–C. However, the results obtained for tetra-arm stars (initiated by pentaerythritol) vary slightly from theoretical calculations (Fig. 2D). At first look, it seems that products belong to single series of ions with an increment of 114. The molar mass discrepancies obtained for four-arm stars arise from linear polycaprolactones initiated with moisture. The theoretical values of the products initiated with pentaerythritol having ‘n’ number of caprolactone units and polycaprolactones initiated with water having ‘n + 1’ caprolactone units are so close that their isotopic pattern overlaps. Therefore, the experimental values obtained are somewhere in the middle of both values. The LCCC fractionation and their MALDI-TOF-MS analysis confirmed above assumptions, explained later in the manuscript.

Fig. 2 MALDI-TOF MS of polycaprolactones initiated with different polyols having similar increment of 114, formula weight of one caprolactone unit; (A) hexyl-CL; theoretical molecular weight of [n-hexyl-(CL)₂₉ + Na]⁺ (C₁₈₀H₃₀₄O₅₉Na) = 3435.36, (B) diethylene glycol-CL: theoretical molecular weight of [DEG-(CL)₉ + Na]⁺ (C₅₈H₁₀₀O₂₁Na) = 1156.41; (C) glycerol-CL: theoretical molecular weight of [glycerol-(CL)₉ + Na]⁺ (C₅₇H₉₈O₂₁Na) = 1142.39; (D) pentaerythritol-CL: theoretical molecular weight of [pentaerythritol-(CL)₃₆ + Na]⁺ (C₂₂₁H₃₇₂O₇₆Na) = 4268.34.

In this study, targeted molar masses of different architectures are varied, therefore, molar mass analysis of the products is required. Size exclusion chromatography (SEC) is the most widely used method for molar mass determination of polymers.²⁹ SEC separates polymers with regard to their hydrodynamic volume in the dilute solution and their RI trace can be correlated to molar mass of standards in order to obtain relative molar mass distribution. The discrepancies in the hydrodynamic volume due to different architectures might further complicate the situation and results could not be representative. However, absolute molar mass can be obtained by using light scattering detector in addition to RI.³⁰ The combination has been used in this study to obtain the absolute molar mass of the samples. As a typical example, RI traces of the products with similar molar masses but different architectures are shown in Fig. 3A. It is very difficult to differentiate the products and to obtain any information about by-products that can be formed during ring opening polymerization of caprolactone. Fig. 3B depicts the overlay of RI and LS signals. The two signals do not fully overlap which is an indication of broad polydispersity of the sample. The molar mass data of the products shown in Fig. 3 is summarized in Table 3. The products shown in Fig. 3 and Table 3 are taken as focus materials for further studies.

Fig. 3 SEC analysis of star-shaped polycaprolactones on three column set (Agilent Polypore 300 × 7.5 mm, Waters Styragel HR4 300 × 7.8 mm, and Jordi DVB mixed bed 300 × 8.0 mm) using THF as eluent at flow rate of 0.7 mL min⁻¹; (A) overlay of RI signal of polycaprolactones with similar molar mass but different architecture, (B) overlay of RI (blue) and LS (red) traces of (diethylene glycol) DEG-CL and absolute molar mass as a function of elution volume (dn/dc of polycaprolactones = 0.098 in THF).

Table 3 Absolute molar mass data of representative products from each class of architecture as obtained by light scattering detection

Product	Mn	Mw	Mp	Mw/Mn
n-Hexyl-CL	8000	14 000	11 500	1.7
Diethylene glycol-CL	6000	10 500	10 000	1.7
Glycerol-CL	8000	11 500	11 000	1.4
Pentaerythritol-CL	9000	13 000	11 500	1.4

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The above discussion clarifies that it is not possible to differentiate the products with similar molar mass but varying architectures by SEC. In addition, no by-products can be detected in the samples that can influence further processing of the materials. Interaction chromatography is the method of choice for such analysis. Liquid chromatography at critical conditions (also termed as critical adsorption point) is the method that suppresses molar mass dependence on the elution behaviour, and polymers can be separated with regard to other distributions such as other block in case of block copolymers and functionality for functional polymers.^31,32 It has been shown that architecture of polymers^33,34 and their functionality^35–38 influence the chromatographic retention at critical conditions. There are only few reports about critical adsorption point of PCL on reversed phase columns.^39–45 The critical conditions of PCL on RP columns reported earlier were established in THF–water mobile phase system. The system is useful for lower molar mass samples but have solubility problems as the molar mass of PCL exceeds. We established chromatographic critical conditions of PCL on reversed phase column in acetonitrile–THF mobile phase system and applicable molar mass range is extended to fairly high values. The polymers under study have hydroxyl end-groups that should not influence the chromatographic retention on RP column and polymer should elute at the same elution volume irrespective of architecture. It is really the case as can be seen in Fig. 4, the polymers with different architectures eluted at similar elution volume at CAP of PCL. The products initiated with hexanol have a hexyl end-group that evidently has strong interaction with the RP compared to the caprolactone repeat unit and eluted later. An early eluting shoulder of the product initiated by hexanol could be the by-product that can be formed by initiation with the moisture or by back biting of the growing chains. As can be seen, reversed phase LC is not the method of choice for this particular case.

Fig. 4 Elution behaviour of PCL with different number of arms at critical conditions of PCL on RP column (Nucleosil C18, 150 × 4.6 mm, 100 Å) in acetonitrile/tetrahydrofuran = 85/15 (v/v) at a flow rate of 0.5 mL min⁻¹.

Liquid chromatography at critical conditions on NP column should show distinct differences in the products under study because all products have different number of hydroxyl end-groups that should show stronger interactions with the stationary phase compared to caprolactone repeat unit. To the best of our knowledge, there is no literature report on critical adsorption point of PCL on NP columns.^31,44,45 The theoretical background of chromatographic behaviour of star shaped polymers is elaborated excellently by Radke et al.⁴⁶ In this study, we realized critical conditions of PCL on NP column in dioxane–hexane binary mixture. As can be seen in Fig. 5, PCLs initiated with a particular initiator eluted independent of their molar mass at same elution volume. The product initiated with hexanol eluted earlier than others since it has only one hydroxyl end-group. The products initiated with DEG, glycerol and pentaerythritol eluted in the same order irrespective of their molar mass but number of hydroxyl end-groups that can be translated into number of arms of the star-shaped polycaprolactones, corresponds well to the theoretical expectations.⁴⁶ The products contain significant amounts of by-products along with targeted architectures. These by-products can be cyclic polycaprolactones (by backbiting of growing chains) and polycaprolactone-diols (initiated with moisture in the system). The pentaerythritol-based products eluted a bit later as a broad peak compared to others. Therefore, we decided to use a shallow gradient around critical conditions of PCL to push the pentaerythritol-based products to earlier elution volume as a comparatively narrow peak. The pentaerythritol-based products also contain significant amounts of by-products that eluted earlier. As a next step, we mixed the products of similar molar masses (given in Table 3) and separated them by shallow gradient around critical conditions of PCL (see Fig. 6). The peaks in Fig. 6 and 7 are assigned on the basis of MALDI-TOF MS analysis as shown in Fig. 2. As can be seen, the products with similar molar masses (compare Fig. 3 and Table 3) are excellently separated from each other. The separation with regard to number of hydroxyl end groups is achieved that is then translated into number arms of star-shaped polycaprolactones. The cyclic polycaprolactones whose traces were detected in magnified MALDI-TOF MS spectra of almost all the products were not enough to be detected in analysis of individual products (see Fig. 8A). However, in the mixture of four different products, the amount was sufficient to be detected by HPLC.

Fig. 5 Elution behaviour of PCL with different number of arms at critical conditions of PCL on NP column (Nucleosil, 100 Å, 250 × 4.6 mm) in 1,4-dioxane/n-hexane = 52/48 (v/v) at a flow rate of 0.5 mL min⁻¹ (A) n-hexyl-CL, (B) diethylene glycol-CL, (C) glycerol-CL, (D) pentaerythritol-CL; F-1 and F-2 are the fractions collected.

Fig. 6 Separation of mixture of similar molar mass of PCL but differing in number of arms (see, Table 3) around critical conditions of PCL on NP column (Nucleosil, 100 Å, 250 × 4.6 mm) using gradient elution as given in Table 1.

Fig. 7 2D-LC of mixture of PCL of similar molar mass but different architectures; y-axis shows SGIC near CAP of PCL on NP bare silica columns (Nucleosil, 100 Å, 250 × 4.6 mm) at flow rate of 0.05 mL min⁻¹, gradient details are given in Table 2. X-Axis represent SEC analysis in second dimension on Agilent polypore column 250 × 4.6 mm at flow rate of 1.75 mL min⁻¹.

Fig. 8 Magnified MALDI-TOF MS spectra (A) hexyl-CL, major series: [n-hexyl-(CL)₁₀ + Na]⁺ (C₆₆H₁₁₄O₂₁Na) = 1266.62, minor series: [cyclic-(CL)₁₁ + Na]⁺ (C₆₆H₁₁₀O₂₂Na) = 1278.58; (B) pentaerythritol-CL F-1 from Fig. 5D, major series: [water-(CL)₃₄ + Na]⁺ (C₂₁₀H₃₅₂O₇₁Na) = 4036.07, minor series: [water-(CL)₃₄-Na + Na]⁺ (C₂₁₀H₃₅₁O₇₁Na₂) = 4058.05; (C) pentaerythritol-CL F-2 from Fig. 5D, [pentaerythritol-(CL)₃₄ + Na]⁺ (C₂₀₉H₃₅₂O₇₂Na) = 4040.05.

To demonstrate the similar hydrodynamic size of the polymers with different architectures, two-dimensional LC around critical conditions of PCL on NP columns (shown in Fig. 6) is coupled to SEC in the second dimension (Fig. 7). The flow rate of first dimension separation was reduced to 0.05 mL min⁻¹ to cope with high speed SEC analysis in the second dimension. The time required for the analysis in the second dimension must match the time to fill the other loop by eluent from the first dimension for two-dimensional LC analysis in a comprehensive manner. Y-Axis shows the separation of first dimension on NP phase column where as SEC analysis in second dimension is depicted by x-axis. The contour plot clearly demonstrates that hydrodynamic volume of all the PCL polymers is similar (compare x-axis). As expected, the cyclic PCL eluted later in second dimension that indicates its lower molar mass compared to other species in the mixture. This is possible as backbiting can happen at any time during polymerization and terminate further growth of the chain. Traces of cyclic polycaprolactones were detected in the magnified MALDI-TOF MS spectra of the products (see Fig. 8A).

Magnified MALDI-TOF MS spectra of a product and fractions from HPLC separation are shown in Fig. 8. Fig. 8A depicts the magnified MALDI-TOF MS spectrum of hexanol-based polycaprolactones (shown in Fig. 2A). As can be seen, it has two series of peaks. Major series is the targeted product initiated with hexanol whereas minor series is the cyclic polycaprolactones that are also detected by 2D-LC analysis. To get more insight into the product composition and sources of different peaks at critical conditions on NP columns, one of the low molar mass pentaerythritol-based product was fractionated and MALDI-TOF MS analysis of fractions was conducted, limits of fractionation are shown in Fig. 5D. The first observation from the spectra of fractions was that low molar masses are more abundant in fraction-1 compared to fraction-2. Furthermore, magnified spectrum of fraction-1 has two series of peaks (Fig. 8B). The major series corresponds to polycaprolactones initiated with moisture plus one Na. The polycaprolactones initiated with water have one hydroxyl and one carboxyl end-groups. The hydrogen of carboxyl end group have tendency to be replaced by cation from the MALDI-TOF MS sample solution.⁴⁰ Therefore, it shows two series of peaks that are basically same homologues series. The major series has one Na whereas minor series has two Na included. This phenomenon was further confirmed by using different salts (K, Li and Ag) for MALDI-TOF MS analysis. Fig. 8C demonstrates the magnified MALDI-TOF MS spectra of fraction-2, collected from the same product. This fraction is very clean with only single series of peaks with an increment of 114. The MALDI-TOF MS analysis of fractions confirmed that the separation obtained for pentaerythritol-based products was with regard to number of hydroxyl end-groups. The experimental values obtained for fractions exactly match the theoretical calculations.

The physiochemical properties of linear and star-shaped polymers could be different due to their unique molecular architecture. The disparity in thermal and morphological properties of polymers with similar molar mass but varying architecture is conducted with the focused products given in Table 3. DSC measurements were performed to analyze the deviance in the melting temperatures of linear and star-shaped polycaprolactones. The DSC curves obtained by the heat scan from 20 to 120 °C are shown in Fig. 9A. Melting temperature of star-shaped polycaprolactones shifted slightly towards lower temperature compared to their linear counterparts, corresponds well to the previous studies.²⁵ Thermogravimetric analysis was conducted to analyze the effects of architectures on the thermal stability. Thermogravimetric analyses reveal single step degradation of all the polymers under study. General thermal degradation trends seem to be similar, however, magnified TGA profiles reveal that pentaerythritol-based polycaprolactones have slightly more initial thermal stability compared to its linear analogues whereas glycerol-based polycaprolactone showed least initial thermal stability (Fig. 9B and C).

Fig. 9 Thermal properties of linear and star shaped polycaprolactones (A) DSC heating curves to obtain melting points, (B) thermogravimetric profile, (C) magnified thermogravimetric profile.

Polymers having similar molar mass but different architecture (such as linear to stars with different number of arms) revealed different behaviour in self assembly process along with morphological variations as evident from AFM analysis of earlier reports.^47–50 Therefore, investigations of factors affecting the self assembly and morphology of polycaprolactones with different architecture have attracted attention of scientists recently.⁵¹ Preliminary results with regards to self-assembly of polycaprolactones are presented here. The aggregation tendency of linear and star-shaped polycaprolactones was investigated by atomic force microscopy (AFM). Irregular aggregation and folding of polymeric strands in case of glycerol-CL is evident probably due to tri-armed structure while somewhat more regular folding and aggregation in case of pentaerythritol-CL was observed (see Fig. 10C and D). The self-assembly of DEG-CL and hexyl-CL generated regular spherical particles of different sizes probably due to extensive folding of the linear polymer (Fig. 10A and B). Interestingly, circular architectures of self-assembled polymeric particles are observed for both linear polymers (i.e. DEG-CL, hexyl-CL). Further investigation to explore the forces behind geometrical circular arrangements of the particles is under process.

Fig. 10 Aggregation and morphological analysis of linear and star-shaped polycaprolactones via AFM; (A) n-hexyl-CL, (B) diethylene glycol-CL, (C) glycerol-CL, (D) pentaerythritol-CL.

Conclusions

In this study, linear and star-shaped polycaprolactones with varying number of arms are synthesized. The products with similar molar mass but different architectures are separated from each other successfully by LCCC on NP and RP columns. It is demonstrated that products could have several types of by-products that were not detected by SEC. Detailed liquid chromatographic characterization and coupling of LCCC to SEC and to MALDI-TOF MS provided much detailed insight into the product composition. The current study provides analysis method for the precursors of star-shaped block copolymers that are gaining great interest due to their improved properties as drug delivery systems. Melting temperature of star-shaped polycaprolactones shifted towards lower value compared to their linear counterparts, whereas, deviations in the thermal degradation behaviour are also quite evident. Linear polymers exhibited extensive folding while star-shaped polymers revealed regular morphology.

Acknowledgements

MIM acknowledges support of Prof. Taihyun Chang of Pohang University of Science and Technology for this project.

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