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DOI: 10.1039/B9PY00310J (Paper) Polym. Chem., 2010, 1, 333-338

Triblock copolymers composed of soft and semi-crystalline segments—synthesis and characterization of poly[(n-butyl acrylate)-block-(ε-caprolactone)-block-(L-lactide)]

Anbanandam Parthiban *, Asawin Likhitsup , Foo Ming Choo and Christina L L. Chai
Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), 1, Pesek Road, Jurong Island, Singapore 627833. E-mail: aparthiban@ices.a-star.edu.sg; Fax: +65 63166184; Tel: +65 67963910

Received 21st October 2009 , Accepted 22nd November 2009

First published on 4th January 2010

Abstract

Poly(n-butyl acrylate) terminated with benzyl alcohol units (PnBA-MI) of varying molecular weights were synthesized by polymerizing n-butyl acrylate using substituted benzyl alcohol based initiators under atom transfer radical polymerization (ATRP) conditions. Between the ligands bipyridine and PMDETA, the latter ligand showed better control for the polymerization of n-BA. High molecular weight diblock copolymers of poly[(n-butyl acrylate)-block-(ε-caprolactone)] were synthesized by ring opening polymerization (ROP) of ε-caprolactone using PnBA-MI which formed thin, flexible and opaque films which are soft to touch. Triblock copolymers of poly[(n-butyl acrylate)-block-(ε-caprolactone)-block-(L-lactide)] were synthesized in a one pot, two step reaction by the sequential addition of monomers. The molecular weights of PCL and PL segments in the triblock copolymers were comparable. Due to this, the triblock copolymer showed exothermic transition upon cooling well above the Tm of PCL block.

Introduction

New polymeric materials are needed for the rapidly evolving areas of biomedicine like tissue engineering and many other applications in order to meet requirements such as biodegradability, biocompatibility, mechanical properties, cellular adhesion and proliferation.1 These materials are also expected to be processable by a few selected techniques like photo crosslinking over high temperature vacuum methods and processes like electrospinning.2,3 Among the polymers made from various vinyl monomers, polyacrylates are widely used in many biomedical applications owing to their biocompatibility even though polyacrylates are non-biodegradable. The highly reactive nature of acrylates under photo crosslinking conditions make them attractive candidates for such applications.4 Of the polyacrylates, poly(n-butyl acrylate) is unique in that the low glass transition temperature (Tg), results in the formation of a soft, rubbery segment when present in a copolymer.

Poly-(ε-caprolactone) and polylactide are well known biodegradable and biocompatible polymers used in applications like tissue engineering, carriers for drug delivery systems, absorbable sutures, engineering of cartilage, bone, skin, smooth muscle cells, heart valve etc.5,6 Recently it has been reported that the cellular interaction of fibrous meshes of polylactide and polycaprolactone are poor and improves upon blending with biopolymers.7–11 In comparison to blending covalent bonding may offer some advantages by way of increased compatibility. Indeed, copolymerization is one of the most often employed strategies to improve the properties of polymers.12

A copolymer composed of poly(n-butyl acrylate), poly(ε-caprolactone) and polylactide can be potentially biocompatible. Poly(ε-caprolactone) and polylactide are also semi-crystalline and this could confer new property variations into the soft poly(n-butyl acrylate) polymer. Such a triblock copolymer could also be semi-biodegradable. Interestingly, there is no report on the synthesis of such triblock copolymers possessing a soft block and two semi-crystalline and biodegradable blocks. Even among the diblock copolymer of poly[(n-butyl acrylate)-block-(ε-caprolactone)] there are only three reports available in the literature, one of the reports by the authors of this article13 and the others by Zerroukhi et al.14 and Zheng and Xu.15 A related report by Rodriguez-Hernandez et al. deals with copolymers where n-butyl acrylate is present as a pendant group.16

Controlled radical polymerization techniques like atom transfer radical polymerization (ATRP) enables the polymerization of monomers polymerizing under contrasting conditions to make copolymers by way of employing modified initiators such as bifunctional initiators.17,18 Two synthetic protocols are commonly employed in such cases viz., ATRP followed by ROP or vice versa leading to the formation of macroinitiators in the first step and the block copolymers in the subsequent step. There are also few instances of simultaneous polymerization by ATRP and ROP under one pot conditions to make copolymers.19,20 Copolymers thus made are partially biodegradable because of the presence of cleavable ester linkages in the backbone and also by suitable choice of monomers could be biocompatible. This article describes the synthesis of poly(n-butyl acrylate) based macroinitiators of varying molecular weights derived from novel bifunctional initiators designed and developed by us. The synthesis and characterization of di- and triblock copolymers are also reported.

Experimental

Materials and methods

Materials. ε-caprolactone (99+%), N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA) (99%) and L-lactide (98%) were purchased from the Aldrich Chemical Co. Tin(II) 2-ethyl hexanoate (tin octoate) (95%) was obtained from Sigma. n-butyl acrylate (nBA) was passed through neutral alumina to remove radical inhibitor and stored under argon at low temperature. 2,2′-Bipyridine (bpy) was recrystallized from hexane. Toluene was purified in a Solvent Purification system (Glass Contour). All other reagents were used as received from the supplier without further purification.
Characterizations. IR spectra were obtained from Digilab Excalibur Series FTS 3000 infrared spectrophotometer fitted with a horizontal Attenuated Total Reflection (ATR) cell. NMR spectra were recorded on a Bruker AC-400 spectrometer. Thermal analysis of the polymers was carried out in a Universal TA thermal analyzer. Gel permeation chromatography (GPC) was used to determine molecular weights and molecular weight distributions, Mw/Mn, of polymer samples with respect to polystyrene standards using tetrahydrofuran as eluent on a system equipped with a series of two Water Styragel HR 5E columns (300 × 7.8 mm) connected to Waters 2414 Refractive Index detector. Wide angle X-ray diffraction patterns were recorded on a D8 Advance Bruker AXS instrument using Cu-Kα radiation. Scanning electron microscopic images were observed using JEOL JSM-6700F Field Emission Scanning Electron Microscope.
Preparation of 3-(hydroxymethyl)phenyl 2-bromo-2-methylpropanoate), BFI. BFI was prepared by reacting α–bromoisobutyryl bromide with 3-hydroxybenzyl alcohol as reported previously by us.18
Preparation poly(n-butyl acrylate) macroinitiator (PnBA-MI) by ATRP. To a suspension of copper(I) bromide (72 mg, 0.50 mmol) and small pieces of copper granules (32 mg, 0.50 mmol) in toluene (5.0 mL) was added n-butyl acrylate (6.41 g, 50 mmol) and BFI (68 mg, 0.25 mmol), sequentially. The mixture was degassed by three freeze-pump-thaw cycles. PMDETA (87 mg, 0.50 mmol) was added and the mixture was degassed again. The reaction mixture was then stirred at 300 rpm under argon at 80 °C for 17 h. The temperature was lowered to 0 °C before it was opened to air. The mixture was passed through a short plug of neutral alumina eluted with THF to remove the copper salt. After the solvent was removed, the polymer was re-dissolved in acetone and precipitated into methanol–water 5[thin space (1/6-em)]:[thin space (1/6-em)]1. Yield 6.18 g (96%). GPC: Mn = 24,942; Mw = 35,964; PDI = 1.44. IR (ATR mode) cm−1: 2959, 2875, 1733, 1456, 1259, 1163, 1118, 1065, 943, 841, 750. 1H-NMR (CDCl3) δH (ppm): 0.9–1 (–CH3 of butyl side chain), 1.3–1.5, 1.6–1.8, 1.8–2, 2.2–2.4, 3.9–4.2 (–OCH2 of butyl side chain), 4.7 (s, terminal benzylic CH2). NMR end-group analysis was carried out by comparing the benzylic CH2 signal (2H) at 4.7 ppm with the –OCH2– protons on the PnBA backbone (398H) at 3.9–4.2 ppm; Mn(NMR) = 25,779.
Preparation of diblock copolymer, poly[(n-butyl acrylate)-block-(ε-caprolactone)], PnBA-PCL. PnBA-MI (0.2164 g) and ε-caprolactone (1 g) were weighed in to oven dried single neck round bottom flask and dissolved in dry toluene (10 mL). Tin octoate (0.05 g) was then added and the reaction mixture was refluxed for 24 h. After cooling the reaction mixture, the viscous solution was added drop wise to methanol (150 mL). The white solid precipitated was filtered, washed repeatedly with methanol and dried. Yield 1.2 g. GPC: Mn = 75,966; Mw = 119,465; Polydispersity = 1.57. IR (ATR mode) cm−1: 2946, 2866, 1725, 1471, 1368, 1296, 1244, 1192, 1168, 1108, 963, 733. 1H-NMR (CDCl3) δH (ppm): 0.847–0.884 (t, –CH3 of n-butyl side chain), 1.275–1.351, 1.544–1.64, 2.2–2.26 (t), 3.98–4.01 (t). 13C-NMR (CDCl3) δC (ppm): 13.71, 19.07, 24.56, 25.51, 28.5, 30.59, 34.1, 41.39, 64.41, 64.53, 173.54, 174.44. The composition of the diblock copolymer, PnBA199PCL1476 was calculated from the 1H-NMR spectroscopy by comparing the intensities of signal at δH = 0.847–0.884 ppm (–CH3 group of PnBA side chain) with that of ε–CH2 of PCL block appearing at 3.975–4.008 ppm after deducting the protons corresponding to the –OCH2 unit of PnBA side chain.
Preparation of triblock copolymer, poly[(n-butyl acrylate)-block-(ε-caprolactone)-block-(L-lactide)], PnBA-PCL-PL. To an oven dried single neck flask were added PnBA-MI (0.3578 g ), ε-caprolactone (1 g, 0.00876 mol), tin octoate (0.1 g, 0.000247 mol) followed by dry toluene (10 mL) and refluxed for 18 h. Then L-lactide (1 g, 0.00694 mol) and tin octoate (0.1 g, 0.000247 mol) in dry toluene (10 mL) was added and refluxed for an additional 24 h. After cooling the reaction mixture, the viscous solution was added drop wise into large excess of methanol (500 mL). The white solid thus precipitated was filtered, washed repeatedly with methanol and dried. The solid was dissolved in THF and reprecipitated in methanol. Yield 1.7 g. IR (KBr) cm−1: 2956, 2870, 1760, 1730, 1363, 1183, 1094, 1046, 960, 872, 734. 1H-NMR (CDCl3) δH (ppm): 0.768–0.804 ((t, –CH3 of n-butyl side chain), 1.195–1.271 (m), 1.424–1.442 (d, –CH3 of lactide), 1.46–1.514 (m), 2.141–2.179 (t), 3.896–3.93 (t), 4.987–5.04 (q, CH of lactide). 13C-NMR (CDCl3) δC (ppm): 13.71, 16.61, 19.06, 24.54, 25.49, 28.31, 30.58, 34.08, 41.34, 64.12, 64.32, 69.25, 169.58, 173.54, 174.43. The composition of the triblock copolymer, PnBA199PCL714PL536 was calculated from the 1H-NMR spectroscopy by comparing the intensities of signal at δH = 0.768–0.804 ppm (–CH3 group of PnBA side chain) with that of ε–CH2 of PCL block appearing at 3.896–3.93 ppm after deducting the protons corresponding to the –OCH2 unit of PnBA side chain and the –CH of lactide appearing at 4.987–5.04 ppm.

Results and discussion

Preparation of poly(n-butyl acrylate) macroinitiator (PnBA-MI)

The bifunctional initiator (BFI) was prepared by reacting hydroxy methyl phenol and α–bromoisobutyryl bromide by exploiting the reactivity difference between a phenol and an aliphatic alcohol present in the same compound as reported by us previously.18 Among the various methods of preparing bifunctional initiators, the one involving small organic compounds may be preferred because of the ease of purification and characterization. Scheme 1 shows the preparation of PnBA-MI from nBA. ATRP of n-butyl acrylate (nBA) was attempted under different conditions. In the presence of bpy as a ligand at 90 °C, the polymerization was slow and the yield was also low (46% after 24 h). When PMDETA was used as a ligand at 110 °C, ATRP was rapid yielding a moderately narrow disperse polymer (PD = 1.34) in 2.5 h. Since there were some reports that excess of PMDETA could be involved in chain-transfer in the ATRP of nBA, the polymerization was repeated with equimolar amounts of Cu(I)Br and PMDETA. When two equivalents of CuBr and PMDETA were used as compared to the initiator, the polymerization proceeded smoothly to yield 96% with moderately good control (PD = 1.44) after 17 h. The initiator efficiency (Ieff), which is the ratio between theoretical molecular weight and observed molecular weight, was found to be the highest for the reactions employing PMDETA as ligand. Also, the molecular weights estimated from 1H-NMR spectroscopy and the theoretical molecular weights were in good agreement in polymerizations where PMDETA was employed as ligand. The presence of benzylic methylene units at 4.7 ppm in the 1H-NMR spectrum establishes the suitability of the polymers as macroinitiators for ring opening polymerizations. The polymerization results and the molecular weight details of macroinitiators are summarized in Table 1.
Preparation of macroinitiator, PnBA-MI.
Scheme 1 Preparation of macroinitiator, PnBA-MI.
Table 1 Polymerization conditions and molecular weights of PnBA-MI
I[thin space (1/6-em)]:[thin space (1/6-em)]Cu[thin space (1/6-em)]:[thin space (1/6-em)]La Yield (%) Mn (GPC) PD Mn (NMR) Mn (theor) Ieff
a mole ratio of M/I = 200, in toluene. b bpy, 90 °C, 24 h. c PMDETA, 110 °C, 2.5 h. d PMDETA, 80 °C, 17 h.
1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3 46b 15[thin space (1/6-em)]574 1.50 16[thin space (1/6-em)]935 12[thin space (1/6-em)]065 0.71
1[thin space (1/6-em)]:[thin space (1/6-em)]1[thin space (1/6-em)]:[thin space (1/6-em)]3 60c 17[thin space (1/6-em)]222 1.34 16[thin space (1/6-em)]679 15[thin space (1/6-em)]654 0.93
1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]2 96d 24[thin space (1/6-em)]942 1.44 25[thin space (1/6-em)]779 24[thin space (1/6-em)]882 0.97

Preparation of di- and triblock copolymers, PnBA-PCL and PnBA-PCL-PL

The polymer synthesized under conditions of entry 3 in Table 1 was used as macroinitiator for ring opening polymerization of ε–caprolactone. Fig. 1 shows the 1H-NMR spectra of the di- and triblock copolymers. Using a high monomer to macroinitiator ratio, high molecular weight diblock copolymer, poly[(n-butyl acrylate)-block-(ε-caprolactone)], PnBA-PCL was obtained which forms a colorless, free standing, flexible and opaque film which is soft to touch when cast from a 10 wt% chloroform solution. Unlike the previously reported diblock copolymers of PnBA-PCL available in literature,14,15 the molecular weight of the diblock copolymer reported here is very high. The observed molecular weight of diblock copolymers calculated by 1H-NMR spectroscopy is higher than the theoretically estimated molecular weight. PnBA-PCL is made up of about 12% of soft segment in the form of poly(n-butyl acrylate) and 88% of semi-crystalline segment present as poly(ε-caprolactone) based on the estimation by 1H-NMR spectroscopy. The triblock copolymer, poly[(n-butyl acrylate)-block-(ε-caprolactone)-block-(L-lactide)], PnBA-PCL-PL was prepared in a one pot two stage process where ε–caprolactone was polymerized first followed by L-lactide. Scheme 2 shows the preparation of di- and triblock copolymers. Our attempts to synthesize the copolymers by using a mixture of monomers as well as the step wise addition of monomers where L-lactide was the first monomer did not succeed propably because of catalyst deactivation as reported in the case of ABC type triblock copolymers of poly[styrene-block-(ε-caproalctone)-block-lactide].21 In the case of triblock copolymer, PnBA-PCL-PL, the observed molecular weight was higher than the theoretical molecular weight. Based on 1H-NMR spectroscopic analysis, PnBA-PCL-PL is composed of approximately 14% of soft segment, 49% of soft semi-crystalline segment in the form of poly(ε-caprolactone) and 37% of hard semi-crystalline segment represented as polylactide. The polymer composition and observed molecular weight details are given in Table 2. The efficiency of macroinitiator in both cases were comparable and moderately high.
1H-NMR spectra of di- (top) and triblock (bottom) copolymers.
Fig. 1 1H-NMR spectra of di- (top) and triblock (bottom) copolymers.

Preparation of block copolymers.
Scheme 2 Preparation of block copolymers.
Table 2 Composition and molecular weights of di- and triblock copolymers
Copolymer Mn (NMR) Mna (theor) Polymer Composition Ieffb
a determined as reported before.21 b ratio between MW (theor) and Mn (NMR).
PnBA-PCL 194[thin space (1/6-em)]270 144[thin space (1/6-em)]633 PnBA199PCL1476 0.75
PnBA-PCL-PL 184[thin space (1/6-em)]529 150[thin space (1/6-em)]504 PnBA199PCL714PL536 0.82

Thermal characterization of di- and triblock copolymers

Since the block copolymers are composed of semi-crystalline segments, their thermal transitions like melting and crystallization were studied by low temperature differential scanning calorimetric (DSC) technique. Fig. 2 shows the DSC curves of di- and triblock copolymers. The diblock copolymer showed a melting transition, Tm, upon heating, at about 56 °C. It also showed a strong exothermic transition at about 32 °C corresponding to the crystallization temperature, Tc, upon cooling. The Tm and Tc of these diblock copolymers are higher than that of similar diblock copolymers of poly[styrene-block-(ε-caprolactone)].21 No glass transition temperature was observed at the temperature range studied at a heating rate of 10 °C min−1.
DSC curves of di- and triblock copolymers.
Fig. 2 DSC curves of di- and triblock copolymers.

In the case of triblock copolymers, the Tm of both caprolactone block and lactide block appear as a slightly split signal. This is most likely caused by the interaction of compatible, soft, PnBA segment of the triblock copolymer with both caprolactone and lactide blocks. It is significant to note here that ABC type triblock copolymers of poly[styrene-block-(ε-caprolactone)-block-lactide]21 showed a simple endo and exothermic transitions in the DSC analysis. The interaction and compatibility of soft and rubbery PnBA segment with caprolactone and lactide blocks may have been far greater than that of the hard, glassy unit like polystyrene (PS). Also the dynamics of polymer chain having the low Tg block like PnBA at one termini would be completely different from that of a high Tg block like PS. In the triblock copolymer, the Tm of caprolactone block appeared at 51 °C and that of lactide was observed at 149 °C. The Tc of the caprolactone block occurred at 23 °C. The lactide block did not show any crystallization transition. However, an exothermic transition was observed at 88 °C upon cooling which is most likely due to some phase changes which is common in block copolymers.22,23 Depending upon the nature of components involved in making block copolymers, they undergo phase changes like lamellar, cylindrical, spherical etc. Similar observation was also made in a triblock copolymer of poly[styrene-block-(ε-caprolactone)-block-lactide].21 Interestingly, in both classes of triblock copolymers, the Tm of lactide block is similar and lower than normal (<160 °C). The block lengths of PCL and PL are also comparable within the respective block copolymers. This phenomenon has been observed in both classes of triblock copolymers even when the PCL and PL content is different. For example, in the case of poly[styrene-block-(ε-caprolactone)-block-lactide] the degree of polymerization of PCL and PL are 198 and 151 (PS-PCL-PL 2)21 respectively whereas in the case of PnBA-PCL-PL the degree of polymerization of PCL and PL are 714 and 536 respectively. However, the change in enthalpy (ΔH) is different. The molecular weight of these blocks based on the degree of polymerization is 81[thin space (1/6-em)]496 g mol−1 (714 × 114.14) for PCL and 77[thin space (1/6-em)]254 g mol−1 (536 × 144.13) for PL and the chain lengths are comparable. It has been reported that compatibility of polymers is dependent on the molecular weight and at low molecular weights even incompatible polymers could be compatible and as a result show a single glass transition temperature.24 Previously, we have observed that the melting transitions were completely supressed when the block copolymers were intentionally admixed with homo- and copolymers in order to verify the purity of block copolymers.21Table 3 summarizes the thermal characteristics of di- and triblock copolymers. The enthalpy changes presented in Table 3 are those observed in DSC analysis. The lower enthalpy changes observed for the triblock copolymer is most likely due to the lesser crystallinity of the PCL block in the triblock copolymer. The lesser extent of crystallinity of the PCL block in the triblock copolymer is also indicated by the lower melting and crystalline transitions of the triblock copolymer as compared to the diblock copolymer. The lowered ordering observed in the triblock copolymer is due to the restricted mobility of PCL chains since it is capped at both ends by long polymer chains whereas in the diblock copolymer, one end of the chain is free which can facilitate long range ordering of the PCL block.

Table 3 Thermal transitions of di- and triblock copolymers
Copolymer TmCL/°C ΔH/Jg−1 TcCL/°C ΔH/Jg−1 TmL/°C ΔH/Jg−1
a exotherm upon cooling at 87.94 °C with ΔH = −16.01Jg−1.
PnBA-PCL 55.78 59.36 31.59 −60.3 — —
PnBA-PCL-PLa 51.08 30.01 22.91 −26.3 149 19.57

Thermal stability of polymers

Block copolymers comprising of segments whose thermal stability differs greatly show step wise degradation in the thermogravimetric analysis (TGA).18,25 A single degradation step was observed for PnBA-MI. The diblock copolymer, PnBA-PCL also shows a single step degradation with a minor deviation around 350 °C. The absence of step wise degradation is most likely due to the comparable thermal stability of both blocks. However, the thermal stability of diblock copolymer was lower by about 30 °C than the PnBA-MI as indicated by the 10% weight loss in the TGA analysis. The triblock copolymer shows a two step degradation curve with the lower step corresponding to the degradation of lactide block. The 10% weight loss of triblock copolymer, PnBA-PCL-PL was observed at about 100 °C lower than that of the diblock copolymer. The differential thermal analysis (DTA) of the polymers make a clear distinction of the degradation of various blocks. In DTA, the macroinitiator shows a single peak associated with the disintegration PnBA unit. For the diblock copolymer, this peak shifts by about 25 °C to a lower temperature with a minor shift at the low temperature side of the peak. In the case of triblock copolymer, three peaks were observed with the first peak by about 110 °C lower than the diblock copolymer and another two closely spaced peaks at a gap of about 20 °C. Fig. 3 shows the differential thermograms of macroinitiator, di- and triblock copolymers. Table 4 gives the results of TGA and DTA of the macroinitiator and the block copolymers.
Differential thermograms of MI, di- and triblock copolymers.
Fig. 3 Differential thermograms of MI, di- and triblock copolymers.
Table 4 Thermal stability of macroinitiator and block copolymers
Copolymer 10% wt loss/°C Exotherm (in DTA)/°C
PnBA-MI 361.52 400.83
PnBA-PCL 338.26 374.78
PnBA-PCL-PL 252.11 263.98, 302.65, 323.29

Morphology of di- and triblock copolymers

The morphology of block copolymers were studied by wide angle X-ray diffraction (WAXRD) and scanning electron microscope (SEM). The macroinitiator was a gum-like mass and hence it was not included for the analysis. In the WAXRD analysis, the diblock copolymer showed amorphous halo pattern at scattering angles of about 13° (d = 6.83) and 16° (d = 5.66) corresponding to the PnBA unit. Peaks corresponding to the PCL block were observed at scattering angles of about 21° (d = 4.15) and 24° (d = 3.75). This may be due to the spherulite morphology of the caprolactone block.26 Additional peaks were observed at scattering angles of about 17° (d = 5.34) and 19° (d = 4.69) associated with PL segment of the triblock copolymer corresponding to the indices (200, 110) and (203) respectively which belong to the orthorhombic unit cell,27 apart from the peaks corresponding to PCL unit. The presence of PL unit not only lowers the intensity of peaks corresponding to the PCL segment but also a causes a minor split at scattering angle 22°. Fig. 4 shows the WAXRD of block copolymers.
WAXRD of block copolymers.
Fig. 4 WAXRD of block copolymers.

Fig. 5 shows the scanning electron micrographs of di- and triblock copolymers. The diblock copolymer, PnBA-PCL showed a dispersed structure with ill-defined microscopic separation of PCL blocks which may be due to the semi-crystalline nature of the polycaprolactone unit. However, the scanning electron micrographs of triblock copolymers are featureless probably because of the enhanced compatibility of the blocks.


Scanning electron micrographs of di- (top) and triblock copolymers (bottom) [left, ×500 and right, ×10,000 magnifications].
Fig. 5 Scanning electron micrographs of di- (top) and triblock copolymers (bottom) [left, ×500 and right, ×10,000 magnifications].

Conclusions

Benzyl alcohol terminated macroinitiators (PnBA-MI) based on poly(n-butyl acrylate) was prepared by ATRP. The ATRP of n-BA proceeded in a controlled manner using PMDETA as ligand. High molecular weight diblock copolymers of PnBA-PCL were prepared by the ring opening polymerization of ε-caprolactone by PnBA-MI. Triblock copolymers were prepared by a two step one pot reaction by adding ε-caprolactone and L-lactide monomers sequentially. The diblock copolymers showed melting and crystallization transitions corresponding to the caprolactone block in the DSC studies. The triblock copolymer showed only melting transition and also an exothermic transition well above the melting point of caprolactone block associated with phase changes in the copolymer. The thermal stability of the polymers decreased in the order, PnBA-MI > PnBA-PCL > PnBA-PCL-PL. The block copolymer morphology was studied by WAXRD and SEM techniques. Further characterization of the block copolymers reported here and others reported previously by AFM and polarized optical microscopy is underway.

Acknowledgements

The authors thank Ms Lim Kai Shuang for her assistance in the laboratory. This work was supported by the Science and Engineering Research Council (SERC) of A*STAR (Agency for Science, Technology and Research), Singapore.

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