Poly(lactide-co-ε-caprolactone) copolymers prepared using bis-thioetherphenolate group 4 metal complexes: synthesis, characterization and morphology

Abstract

Titanium and zirconium complexes 1–3 (1 = (^t-BuOS)₂Ti(O-i-Pr)₂; 2 = (^t-BuOS)₂Zr(O-t-Bu)₂; 3 = (^CumOS)₂Zr(O-t-Bu)₂) supported by two phenolate bidentate ligands (^t-BuOS-H = 4,6-di-tert-butyl-2-phenylsulfanylphenol and ^CumOS-H = 4,6-di-cumyl-2-phenylsulfanylphenol) promoted the copolymerization of L-lactide with ε-caprolactone. The reactivity displayed by the two monomers during the copolymerization experiments and the microstructure disclosed by ¹³C NMR analysis indicated a gradient distribution of the two monomers along the polymer chain. Copolymers with high ε-caprolactone content showed a large scale formation of crystalline spherulites prone to perfection of the crystallinity upon thermal annealing at 50 °C. Differently L-lactide rich copolymers revealed a thin film morphology consisting of small rigid domains of L-lactide segments of about 15 nm embedded in a soft matrix of the counterpart. Copolymers with comparable mole fractions of the two monomers were entirely amorphous.

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Introduction

Aliphatic polyesters, such as polylactide (PLA), polycaprolactone (PCL) and their copolymers are among the most promising and attractive polymers for pharmaceutical and biomedical applications.¹ PCL is a semicrystalline polymer with remarkable drug permeability and good thermal features but modest mechanical behavior.² Conversely, PLA has poor elasticity but is hardly permeable to most drugs. Moreover, the PCL's degradation rate (half-life of about one year in vivo)³ is much slower than that of PLA (half-life of few weeks in vivo).⁴ In such scenario, the copolymerization of lactide (LA) and ε-caprolactone (CL) offers the advantage of combining PCL permeability and the rapid biodegradation of PLA by simply controlling the content and the distribution of the two monomers along the polymer chain. The election method for the fine-tuning of the microstructure of the polymer chain is the ring-opening polymerization (ROP) of cyclic esters.⁵ Although CL is much more reactive than LA for a given catalyst, in the copolymerization reaction LA is generally consumed first, and consequently block (PLA-b-PCL) or gradient poly(LA-grad-CL) copolymers are obtained. In few cases, copolymers with “randomized” structure were produced as a result of transesterification reactions occurring at high conversion and/or high temperature.⁶ A variety of catalysts have been used for the copolymerization of CL and LA, ranging from the traditional stannous octanoate,⁷ to aluminium compounds,⁸ to zinc catalysts,^6b,9 to rare earth complexes.¹⁰ In this context, notwithstanding the low toxicity of group 4 metals renders them particularly attractive for the synthesis of copolymers with potential biomedical application, very few examples of copolymers prepared by discrete group 4 initiators have been reported so far.^6a,11

In the framework of our interest in the ROP of cyclic esters,¹² we recently reported on the performance of new group 4 metal complexes supported by two bidentate thioetherphenolate ligands, namely 1–3 (1 = (^t-BuOS)₂Ti(O-i-Pr)₂; 2 = (^t-BuOS)₂Zr(O-t-Bu)₂; 3 = (^CumOS)₂Zr(O-t-Bu)₂; ^t-BuOS-H = 4,6-di-tert-butyl-2-phenylsulfanylphenol and ^CumOS-H = 4,6-di-cumyl-2-phenylsulfanylphenol) (Scheme 1).¹³ The polymerizations were well-controlled, giving polymers with predictable molecular weights and narrow molecular weight distributions. Herein we explore the possibility for efficient synthesis of copolymers of L-lactide with ε-caprolactone in a wide range of composition and give a full account of their characterization through ¹³C NMR and DSC studies. Furthermore the morphology of these copolymers was investigated by atomic force microscopy (AFM).

Scheme 1 Titanium and zirconium complexes 1–3.

Experimental section

All experiments that required inert atmosphere were performed under nitrogen using a MBraun drybox or standard Schlenk line techniques. Solvents were distilled over sodium (toluene), sodium/benzophenone/diglyme (hexane, pentane) or sodium/benzophenone (diethyl ether). When not pointed out the reagents were used as received from the supplier (Sigma-Aldrich). Ligands used for the synthesis of the complexes were anhydrificated in vacuum with P₂O₅. ε-Caprolactone (CL) was distilled over CaH₂. L-Lactide was purified by crystallization from toluene and then dried over P₂O₅. Titanium and zirconium complexes 1–3 (1 = (^t-BuOS)₂Ti(O-i-Pr)₂; 2 = (^t-BuOS)₂Zr(O-t-Bu)₂; 3 = (^CumOS)₂Zr(O-t-Bu)₂) were synthesized according to the previously reported procedure.¹³

Instruments and measurements

NMR spectra were recorded on a Bruker AVANCE 400 spectrometer operating at 400 MHz for ¹H using CDCl₃ as the NMR solvent. ¹H and ¹³C NMR spectra are referenced to the residual solvent peak (7.27 ppm and 77.23 ppm, respectively). Chemical shifts (δ) are listed as parts per million and coupling constants (J) in hertz.

The molecular weights (M_n and M_w) and the molecular weights distributions (M_w/M_n) of polymer samples were measured by gel permeation chromatography (GPC) at 30 °C using THF as the solvent, a flow rate of the eluent of 1 mL min⁻¹ and narrow polystyrene standards as the references. The measurements were performed on a Waters 1525 binary system equipped with a Waters 2414 RI detector using four Styragel columns (range 1000–1 000 000 Å).

Glass transition temperatures (T_g) and melting points (T_m) of the polymers were measured by differential scanning calorimetry (DSC) using a DSC 2920 (TA Instruments) in nitrogen flow with a heating and cooling rate of 10 °C min⁻¹ in the range −100 to 200 °C. Glass transition temperatures and melting temperatures were reported for the second heating cycle.

Atomic force microscopy images of polymer films were collected in tapping mode (TM-AFM) using a Nanoscope Dimension 3100 from Bruker. Commercial probe tips with nominal spring constants of 20–100 Nm⁻¹, resonance frequencies in the range of 200–400 kHz and tip radius of 5–10 nm were used. Topographic and phase contrast AFM images were acquired in air at room temperature. The samples were prepared by deposition at room temperature of 30 μL of a chloroform polymer solution (0.2 wt%) onto glass slides and analysed by AFM soon later their preparation.

Synthesis of poly(ε-caprolactone-co-L-lactide)

In a typical experiment (Table 1, entry 3), a Schlenk-tube equipped with a magnetic stirrer was charged sequentially with L-lactide (0.180 g, 1.25 mmol), ε-caprolactone (0.14 mL, 1.25 mmol) and the pre-catalyst (12.5 μmol) dissolved in 2.4 mL of dry toluene. The mixture was thermostated at 100 °C. After the required polymerization time (8 hours for complex 1; 2 hours for complex 2 or 3), an aliquot of the crude material was sampled by a pipette and quenched in wet CDCl₃ to evaluate the yields. Conversions were determined by integration of the monomer vs. polymer resonances in the ¹H NMR spectrum of the crude product (in CDCl₃). The reaction mixture was quenched with wet n-hexane. The precipitates collected from the bulk mixture were dried in air, dissolved with dichloromethane, and sequentially precipitated into methanol. The obtained polymer was collected by filtration and further dried in a vacuum oven at 40 °C for 16 h. The polymer was characterized by NMR spectroscopy, GPC and DSC analysis.

Table 1 Copolymerization of L-lactide with ε-caprolactone promoted by 1, 2 and 3

Entry^a	Initiator	Feed compos.^b (molLA/molCL)	Time (h)	Conversion^c (%)		Pol. compos.^d (molLA/molCL)	Sequence lengths^e		Mn^f (kDa)	Mn/Mw^f	Tg^g^,^h (°C)		Tm^h (°C)
				LA	CL		LLA	LCL			Exp.	Theor.	CL	LA
a Copolymerization conditions: [initiator] = 5.2 mM; ([LA] + [CL]) = 1.04 M; ([LA] + [CL])/[initiator] = 200; toluene = 2.4 mL; 100 °C.b Monomer composition in the copolymerization feed.c Monomer conversion determined by ^1H NMR.d Copolymer composition.e Average sequence lengths of the lactidyl unit and of the caproyl unit in the polymer backbone determined by ^13C NMR.f Determined by GPC.g Experimental (DSC) and calculated (Fox equation) values.h Determined by DSC from the second heating cycle (10 °C min^−1).
1	1	10 : 90	8	99	94	8 : 92	1.0	30.2	38.3	1.38	−51	−52	49	—
2	1	30 : 70	8	99	90	29 : 71	1.7	4.3	30.3	1.55	−38	−31	—	—
3	1	50 : 50	8	99	64	58 : 42	2.9	1.7	26.4	1.41	−2	2	—	—
4	1	70 : 30	8	96	62	79 : 21	6.1	1.6	29.2	1.29	31	28	—	—
5	1	90 : 10	8	89	66	96 : 4	21.1	1.0	31.5	1.16	54	51	—	157
6	2	10 : 90	2	99	89	8 : 92	1.5	14.5	63.1	1.52	−49	−52	52	—
7	2	30 : 70	2	99	65	32 : 68	2.1	5.0	34.8	1.55	−45	−27	−39	116
8	2	50 : 50	2	99	64	52 : 48	2.8	2.2	37.2	1.48	7	−5	—	127
9	2	70 : 30	2	99	37	82 : 18	7.0	1.6	47.1	1.36	37	32	—	152
10	2	90 : 10	2	80	31	96 : 4	19.2	1.0	53.1	1.31	54	54	—	163
11	3	10 : 90	2	99	91	7 : 93	1.2	11.0	61.4	1.46	−53	−53	52	—
12	3	30 : 70	2	99	63	33 : 67	2.3	4.2	44.4	1.46	−38	−26	—	—
13	3	50 : 50	2	99	70	59 : 41	3.6	2.2	36.3	1.54	4	3	—	135
14	3	70 : 30	2	99	43	84 : 16	8.4	1.3	47.5	1.42	38	35	—	153
15	3	90 : 10	2	99	41	95 : 5	12.1	1.0	50.8	1.44	52	50	—	163

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Results and discussion

The behaviour of titanium and zirconium complexes 1 and 2 in the LA/CL copolymerization was investigated in toluene solution at 100 °C at various molar ratios of L-lactide and ε-caprolactone. In all cases the conversion of lactide was high (80–99%), while the conversion of ε-caprolactone ranged from 31 to 94% (Table 1). Complex 1 gave CL incorporation higher than 2 under similar experimental conditions.

The mole fraction of CL in the polymer was always lower than that in the feed, apparently in contrast with the homopolymerization rates of CL and LA, nevertheless it is frequently reported in the literature and seems to be a common feature reported for the copolymerization of CL and LA.¹⁴ Gel permeation chromatography analysis of these copolymers showed that the molecular weight distributions were monomodal, consistent with the material being copolymeric in nature.

The microstructural features of the copolymers were investigated by ¹H and ¹³C NMR spectroscopy. In particular, in the proton spectra, the integrals relative to the resonances assigned to ε-caprolactone and lactide linkages increased by increasing the percentage of L-lactide in the feed. Accordingly, in the ¹³C NMR spectra, the peaks due to all the possible monomer sequences were observed (see ESI†).^8d,15 As expected the triads composition paralleled the LA/CL molar ratio in the feed: by decreasing the LA/CL molar ratio in the feed, the amount of the CL–CL homosequences and CL–LA heterosequences raised. The average lengths of the blocks of the lactidyl (L_LA) and caproyl (L_CL) sequences were calculated (see Fig. S4†). Notably, for the copolymers obtained by the titanium initiator 1, L_LA ranged from 1 to 21 monomer units and L_CL ranged from 1 to 30 monomer units (see Table 1). Lactide block lengths of about 1 to 19 monomer units and caprolactone block lengths of about 1 to 14 monomer units were found in the copolymers prepared by 2. In the copolymers obtained with a LA/CL molar ratio of 50/50 (entry 3 and 8, Table 1) L_LA and L_CL were 2.9 and 1.7 for the copolymer by 1 and 2.8 and 2.2 for the copolymer by 2 (an ideal random copolymer will have L_LA = L_CL = 2). On the basis of these data and considering that the LA conversion was higher than the corresponding CL conversion, we suggest the formation of a gradient copolymer comprised of LA-enriched sequences transiting to CL-enriched sequences.

It is worth observing that complex 3 featuring encumbered cumyl group in proximity of the metal centre could result in a compensation of the inherent reactivity imbalance between the two monomers. Therefore we also investigated the LA/CL copolymerization by using the latter complex. Significantly the lactide block lengths were slightly shorter than those obtained with complex 2, as an example, in the copolymerization with LA/CL molar ratio of 90/10 in feed, the L_LA obtained with complex 2 was 19 monomer units while that obtained with 3 was 12 monomer units.

Transesterification reaction can affect the average block length producing a randomization of the copolymer structures.

The process is evidenced by the presence of a resonance at 171.0 ppm in the ¹³C NMR spectrum, relative to the carbonyl group of a triad in which a single “lactic” ester unit is flanked by two units of caprolactone; that triad cannot result from the insertion of the monomer into the growing chain. Fig. 1 displays the ¹³C NMR spectra of the copolymers obtained by 1–3 (runs 3, 8 and 13 of Table 1). The inspection of these spectra revealed that the transesterification process was active only in case of polymerization initiated by the zirconium complexes 2 and 3.

Fig. 1 Carbonyl range of ¹³C NMR spectra (CDCl₃, 25 °C) of copolymers of entries 3 (a), 8 (b), and 13 (c) in Table 1.

The thermal properties of the copolymers were analysed by differential scanning calorimetry (DSC); the glass transition temperatures and the melting points are listed in Table 1. DSC thermograms of the copolymers of varying composition obtained by 1 are displayed in Fig. 2. Remarkably, the thermal properties are strictly dependent on the microstructure of the polymer chains.

Fig. 2 DSC thermograms of the LA/CL copolymers obtained with complex 1 (entries 1–5 of Table 1, second heating cycle, 10 °C min⁻¹).

On one hand, all the copolymers for which the NMR analysis disclosed long average block homosequences showed a melting endotherm clearly related to the presence of crystalline microdomains. On the other hand the copolymers with comparable mole fractions of the two monomers resulted amorphous. In all cases, the copolymers displayed unique glass transition temperature with values intermediate between those of the corresponding homopolymers (T_g(PCL) = −60 °C, T_g(PLA) = 57 °C) and decreasing as the percentage compositions of CL in the copolymer increased. The experimental values of T_g were in good agreement with those calculated by the Fox equation (Fig. S8, ESI†).¹⁶

In order to study the nanoscale morphology of thin films of the CL/LA copolymers we used atomic force microscopy. In particular this technique, when operating in tapping mode (TM-AFM), allows the disclosure of the topographic morphology of surfaces, as well as the phase distribution in the specimen by means of the detection of the different mechanical properties, including stiffness, elasticity and adhesion of the different phases.¹⁷ The topographic TM-AFM images of a thin film of the copolymer in run 1 of Table 1, synthesized by means of the initiator 1, with a high CL content (92 mol%), showed the large scale formation of crystalline polymer spherulites due to the PCL segments (Fig. 3a). The corresponding phase contrast image revealed a greater stiffness of the spherulite cores, as an evidence of the crystalline nature of these regions (Fig. 3a). Copolymers with a similar CL content of entry 6 and 11 of Table 1, synthesized respectively with the initiator 2 and 3 showed a similar formation of PCL spherulites. The treatment of these thin films at 50 °C for 15 minutes affords the formation of fully crystalline spherulites, as pointed out by the phase contrast images (Fig. S9 and S10†). For the thin films of copolymers with intermediate composition (sample of run 8 and 12 of Table 1) a dewetting process was observed: i.e. the polymer during the casting fails to cover the entire glass slide and large empty areas were formed. However, no phase segregation was observed for the polymer region, as pointed out by the phase contrast micrographs (Fig. 3b and S11†). Although LA-rich samples were prone to crystallize¹⁸ (see the DSC thermograms in Fig. S5–S7 in the ESI†), the formation of well-defined crystalline spherulites was not observed for these samples. On the contrary, a clear nanoscale segregation of phases was observed. Thin films of sample 5 of Table 1 showed the formation of rigid small domains, with average dimension around 15 nm (Fig. 3c on the right). Based on the polymer composition, the polymer microstructure and the mechanical properties of the different constituents of this copolymer, one can argue that the rigid domains consist of PLA segments and the soft matrix consists of the random copolymer counterpart of the polymer backbone. The annealing of the specimen at 100 °C, carried out for affording the eventual crystallization of the PLA segments, did not produced significant modification of the thin film morphology (Fig. S12†). The observed segregating phases, respectively for the CL or LA rich copolymer, could be ascribed to the presence of long homosequences in the polymer chain.

Fig. 3 Height (left) and phase contrast (right) TM-AFM micrographs of LA/CL copolymers of entries 1 (a), 12 (b) and 5 (c) in Table 1.

Conclusions

In conclusion, titanium and zirconium thioetherphenolate complexes 1–3 turned out to be very efficient initiators for copolymerization of caprolactone with lactide in a wide range of compositions. Although homopolymerization rates of lactide and caprolactone were substantially different, copolymers of LA and CL were easily prepared by mixing the two monomers in appropriate proportion. Transesterification reaction that could contribute to the redistribution of the monomer sequence was observed only for polymerization initiated by the zirconium complexes 2 and 3. The reactivity displayed by the two monomers during the copolymerization runs and the microstructure disclosed by ¹³C NMR analysis suggested the formation of gradient copolymers characterized by a continuous change in composition along the polymer chain.

The obtained copolymers were analysed by differential scanning calorimetry (DSC) and atomic force microscopy operating in tapping mode (TM-AFM). In all cases, a unique glass transition temperature was observed with values intermediate between those of the corresponding homopolymers and decreasing as the percentage compositions of CL in the copolymer increased. Finally the thin film morphology of these copolymers resulted strongly dependent on the composition. Copolymer with a high LA or CL content produced phase segregations ascribable to the domains of PLA or PCL segments. Differently copolymers with comparable mole fractions of the two monomers resulted entirely amorphous.

Acknowledgements

The authors thank Dr Patrizia Oliva for NMR assistance, Dr Ilaria D'Auria for GPC measurements and Dr Vito Speranza for AFM assistance. For the financial support of this research the Universita' degli Studi di Salerno (FARB 2012), Ministero dell'Universita' e della Ricerca Scientifica (MIUR, Roma – Italy; PRIN-2010: “Materiali Polimerici Nanostrutturati con strutture molecolari e cristalline mirate, per tecnologie avanzate e per l'ambiente.”) and Regione Campania (POR CAMPANIA Rete di Eccellenza FSE. Progetto “MAteriali e STrutture Intelligenti”, MASTRI, Codice 4-17-3, CUP B25B09000010007) are gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: NMR spectra, DSC thermograms, height and phase contrast TM-AFM micrographs. See DOI: 10.1039/c4ra10255j

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