Polym. Bull. DOI 10.1007/s00289-017-2089-9 ORIGINAL PAPER
Use of samarium(III)–amino acid complexes as initiators of ring-opening polymerization of cyclic esters Dimas A. Medina1 • Jesu´s M. Contreras1 • Francisco J. Lo´pez-Carrasquero1 • Eduardo J. Cardozo2 • Ricardo R. Contreras2
Received: 24 March 2017 / Revised: 2 June 2017 / Accepted: 5 June 2017 Ó Springer-Verlag GmbH Germany 2017
Abstract Poly(e-caprolactone) (PCL) and poly(L-lactide) (PLA) were prepared by ring-opening polymerization (ROP) using samarium(III) acetate (SmAc3) and amino acid complexes of samarium(III) as initiators. The catalytic behavior of samarium(III) acetate and their respective amino acid complexes, Sm(2,20 bipyridine)(Ln)3 (L1 = L-aspartic acid; L2 = L-glutamic acid and L3 = glycine), was studied. It could be observed that the amino acid structure and chemical bond type have great influence on the catalytic activity. Polymerization reaction temperature was 125 °C and the effect of time reaction on the conversion of monomers to polymers and the molecular weight were studied. The results indicate that the initiators induce the polymerization of e-caprolactone (e-CL) and L-lactide (L-LA). Polymers were characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (1H-NMR) and size exclusion chromatography (SEC). Polyesters with average-number molar mass of 1.50 9 103 - 104 Da were obtained. Based on the 1H-NMR end group analysis of low molecular weight of polymers, a coordination–insertion mechanism is proposed for the polymerization of lactones. Kinetics study indicated a polymerization rate of first order with respect to monomer concentration. Differences in the rates of polymerization in the four initiators appeared to be governed by the different degrees of steric hindrance in the initiator structure and monomer.
& Jesu´s M. Contreras
[email protected] 1
Grupo de Polı´meros, Departamento de Quı´mica, Facultad de Ciencias, Universidad de Los Andes, Me´rida 5101, Venezuela
2
Laboratorio de Organometa´licos, Departamento de Quı´mica, Facultad de Ciencias, Universidad de Los Andes, Me´rida 5101, Venezuela
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Keywords Samarium(III) acetate Amino acid–samarium(III) complex Ring-opening polymerization e-Caprolactone Poly(e-caprolactone) L-Lactide Poly(L-lactide)
Introduction There is great interest in the field of biodegradable polymers in replacing conventional synthetic materials (e.g., derived from petroleum) [1]. In the last two decades, many biodegradable polymers have been developed [1–3], using methods based on ring-opening polymerization (ROP) [4–8], also known as pseudo-anionic polymerization. The ROP of lactones and lactides is an attractive and simple method for the preparation of aliphatic polyesters that exhibit a high molecular weight with narrow molecular weight distribution [4–9]. These kinds of polymers (PCL, PLA and polyglicolide) and their copolymers are very interesting because of their use in biomaterial syntheses and high biodegradability. Biocompatibility has great scope in medicine, pharmacology and agriculture [5–10]. The use of derivatives of metals of the block ‘‘f’’ of the Periodic Table has been reported as initiators, especially the lanthanides series, on many polymerizations of heterocyclic compounds [11]. Lanthanide compounds have a high reactivity in polymerization without a tendency to show intramolecular and intermolecular transesterification reactions, allowing a high catalytic performance and polymerization control. Guillaume et al. showed extensive development in this area, focusing on Ln(BH4)3(THF)3 (Ln = La, Nd, Sm) and Cp*2Sm(BH4)(THF) for the ROP of e-CL [12–14]. The results revealed that the performance of these systems is very similar to ‘‘La(OiPr)3’’ under comparable experimental conditions [15]. A series of guanidine-supported alkoxide complexes [Ln{(Me3Si)2NC(N-iPr)2}2(OR)] (R = OtBu, Ln = Y, Nd, Sm, Lu; R = OiPr, Ln = Y, Nd, Lu), another series of dimeric aryloxide complexes [LLn(OAr) (THF)]2 (Ln = Nd, Sm, Yb, Y, and L = ligand) and some of the lanthanide complexes have been used as initiators for the ROP with different heterocyclic compounds [16]. Several studies have been carried out on lanthanide complexes as initiators for the preparation of PLA and PCL [14, 16]. The studies aim to find: (i) how the ‘‘spectator’’ ligands would affect the polymerization dynamics and (ii) the relative catalytic efficiency of lanthanide(II) and (III) toward ROPs. Additionally, strong efforts to develop new initiator systems for ROP of L-LA and e-CL, as well as the applications of these materials in photoluminescence and antibacterial drugs have been undertaken [17]. Besides, new trends in this area lead to design synergistic organolanthanide systems, e.g., organolanthanide complexes induce e-CL and L-LA polymerization and also have bactericidal activity [16]. This tendency is reinforced due to the innovative development of ‘‘bioinorganic chemistry’’ which studies the chemical reactivity of elements and inorganic complexes in biological systems [18]. This paper describes the polymerization of e-CL and L-LA initiated by samarium(III) acetate (SmAc3) and their respective amino acid complexes type: Sm(2,20 -bipyridine)(Ln)3 (L1 = L-aspartic acid; L2 = L-glutamic acid and
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L3 = glycine). The results are compared with those obtained when SmAc3 is used as an initiator. The relationship between the structure and the chemical bond amino acids/samarium(III) and the catalytic activity of the complexes is discussed, and based on the 1H-NMR end group analysis of low molar mass of PCL a possible reaction mechanism is proposed.
Experimental Materials e-CL (Aldrich Chemical Co.) was dried for 48 h over calcium hydride, then distilled at 4 mmHg collecting the 86–88 °C fraction. L-LA (Aldrich Chemical Co.) was used without further purification. Samarium (III) acetate (SmAc3) (Aldrich Chemical Co.) was dried at 100 °C for 1 h. The complexes C1, C2 and C3 were synthesized according to previous report by Cardozo et al. [17]. Polymerization All polymerizations were carried out in bulk in ampoule-like flasks equipped with a magnetic stirrer. In a typical experiment, quantities previously established of the monomer and initiator were added into the flask and successively purged by dry nitrogen or argon. The flasks were capped and then kept at the pre-established temperature for the required length of time. Then it was opened at room temperature and the reaction was quenched by the addition of a small amount of methanol required to precipitate the initiator. The product was then dissolved in chloroform and the initiator residues were removed by centrifugation. The polymers were isolated by precipitation in excess methanol. Finally, the purified polymer samples were dried at 40 °C in vacuum for 24 h. Measurements Infrared spectra were recorded on a Perkin-Elmer 2000 instrument from KBr disc samples. All the spectra were performed with 32 scans and a spectral resolution of ±4. 1 H-NMR spectra were obtained with a Bruker Avance DRX 400 spectrometer operating at 400 MHz at room temperature. NMR spectra were recorded (16 scans) in benzene-d6 and referenced to residual protons in the deuterated solvent (d = 7.15 ppm). Size exclusion chromatography (SEC) was performed with a Waters Breeze 2 HPLC System, operated at 40 °C and equipped with four columns connected in series and packed with Ultrastyragel, tetrahydrofuran (THF) as a mobile phase and polystyrene as a standard. Polymerization kinetics was performed using the weight-measuring method, taking into account the values of conversion from monomer to polymer at different reaction times.
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Results and discussion In a work previously developed in our laboratory [19], where samarium acetate was used as initiator of the polymerization of CL, it was possible to determine that the reaction conditions that give rise to polymers with high conversions of monomer to polymer and with greater molar mass were 125 °C and the reaction time between 18 and 24 h. At this point, it is important to mention that several experiments were performed to determine whether both monomers could polymerize thermally in the absence of the initiator. For that, the temperature was varied between 60 and 200 °C, and it was observed that in the case of the CL the formation of PCL occurred at T C 150 °C, while in the case of LA it was only possible to observe the formation of PLA at 200 °C. Therefore, the efficiency of each of the samarium complex in the polymerization of both monomers was studied at 125 °C and a reaction time of 18 h, and the results are shown in Table 1. All the obtained polymers were solid and in general it may be seem that the dispersity index (ÐM) is low; also, SmAc3 was more efficient in the polymerization than complexes C1, C2 and C3. Even more, this effect is more evident with the e-CL than L- LA, where the yields among the SmAc3 and the complexes are closer (Fig. 1). The IR (not shown here) and 1H-NMR spectra exhibit the characteristic signal of both polymers indicating that these systems allow obtaining the expected products. As an example, Fig. 2 shows the 1H-NMR spectrum of PCL obtained with the respective assignations, which can be consider representative of all the PCL obtained here [19]. The signals a, b, c, d and e were assigned to the PCL repeating unit, while the multiplet peak at 3.4 ppm (signal f and g) was attributed to methylene protons adjacent to the hydroxyl end group and the methoxide end group [20–24]. Both end groups were generated by hydrolysis of the growing chain when methanol was added [6, 7, 19, 24–33]. These data are consistent with a coordination–insertion mechanism which involves: coordination of the monomer with the initiation species followed by the cleavage of acyl-oxygen bond in the coordinated monomer molecule and the simultaneous insertion of the cloven monomer residue into the Sm–O bond. In Fig. 3, this mechanism is shown for CL polymerization.
Table 1 Yield and molar weight of the polymers synthesized with different samarium complexes Initiator
M/Ia
Yield to PCL (wt%)b
Mn 9 10-3 (Dalton)c
ÐcM
M/Ia
Yield to PLA (wt%)b
Mn 9 10-3 (Dalton)c
ÐcM
SmAc3
5800
16.10
3.84
1.50
4950
17.35
1.64
1.15
C1
7982
9.17
3.43
1.39
6141
12.35
2.00
1.13
C2
8351
8.97
3.00
1.38
6426
10.45
1.83
1.08
C3
6397
9.95
1.66
2.65
4922
15.90
1.55
1.14
Without initiatord
–
0.00
–
–
–
–
–
0.00
a Molar ratio monomer/initiator; b based on the initial monomer (1 g); c determined by SEC. Reaction time = 18 h; reaction temperature = 125 °C; d thermal polymerization at 125 °C
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Fig. 1 Structure (neutral form) of the samarium complex: (a) C1 = 2,20 -bipyridine-tris(L-aspartic acid)˚ 3. (b) C2 = 2,20 -bipyridinej3COO–samarium(III), Sm(bipy)(L1)3, MW = 882.98 g/mol, V = 686.08 A ˚ 3. tris(L-glutamic acid)-j3COO–samarium (III), Sm(bipy)(L2)3, MW = 925.06 g/mol, V = 728,77 A (c) C3 = 20 -bipyridine-tris(glycine)-j3COO–samarium(III), Sm(bipy)(L3)3, MW = 708.87 g/mol, ˚ 3. (Molecular weight (MW) calculated by Sheffield ChemPuter and the volume V = 468.48 A (V) occupied by each of them was determined by calculations using the programs MOPAC2009 and Gabedit 2.3.0.) [17]
Fig. 2
1
H-NMR spectrum of the PCL obtained (Mn = 3.00 9 103 Da)
In the case of lanthanide complex initiators, and especially in the case of samarium complexes (III), it is necessary to point out that the typical number of lanthanide coordination is eight and nine (Fig. 1). For this reason, initially, it is not
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Fig. 3 Reaction mechanism for the polymerization of CL using C2 as the initiator (L = L2 or bipy)
necessary to labilize any of the bonds in the initial coordination sphere of the complex; simply, a monomer molecule is inserted in an oxidative addition process. The affinity between the carboxylate and the samarium (III) groups is quite high as it is known to be highly oxophilic; however, the monomer molecule shown in Fig. 3 must be linked by means of one of the oxygen present, preferably the one belonging to the group C=O. Then the polarization of the contiguous bond CO would facilitate the opening of the ring. To study how the reaction time influences the conversion and molar mass on the bulk polymerization of e-CL and L-LA initiated by samarium(III) acetate and the Sm(bipy)(Ln)3 (L1 = L-aspartic acid, L2 = L-glutamic acid and L3 = glycine) complexes, a series of polymerizations at 18, 24, 36 and 48 h were carried out under the same condition as used before in Table 1. The results are shown in Fig. 4. These results indicate a notorious influence of reaction time on polymerizations and the conversion of monomer to polymers increased with reaction time and is slightly greater for the L-LA than e-CL. On the other hand, when comparing the results obtained with the different initiators used, it can be observed that SmAc3 is more effective as initiator in polymerization, possibly owing to the fact that it is less bulky; so the steric effect will be less and therefore the addition of the monomer
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(a) 70
(b)
50
50
Yield (%)
Yield (%)
60
C1 C2 C3 SmAc3
60
C1 C2 C3 SmAc3
40 30 20
40 30 20 10
10 0
0 0
10
20
30
Time (hours)
40
50
0
10
20
30
40
50
Time (hours)
Fig. 4 Effect of time on the conversion in the polymerization of: a e-CL and b L-LA
molecules to the chain ends active during the polymerization process will be less impeded, resulting in high yields, as compared to the other initiators [9, 17, 34, 35]. Additionally, the samarium(III) compounds are more efficient initiators for L-LA polymerization, due to the smaller size of the L-LA against e-CL. Taking into account that CL is the most voluminous monomer and structures of the different complexes (C1, C2 and C3), it is possible to think that the steric hindrance would be making the interaction between the monomer with these initiators very difficult to make the centers active and give rise to polymerization processes. This is confirmed by analyzing the results shown in Fig. 4. Finally, although the molecular volume and its steric effect is important, it is not the only factor affecting initiation, since the cyclic monomers have less degrees of freedom and volume than the open chain. It is clear that those more voluminous initiators could hinder the entry of the monomer, but here is also a problem of thermal stability affecting initiation. In principle, the bulky complexes are thermally more stable than the less voluminous ones, so that at the working temperature, in the case of the smaller complexes, one or more amino acid ligands may very well have decomposed, generating more vacant sites, or in its absence a smaller volume, which facilitates the coordination of more monomer units. The typical SEC chromatograms of the resulting polymers are shown in Fig. 5. In general, all the curves (PCL and PLA) showed a clear tendency to move toward the region of high molar mass as the reaction time is increased. This fact and also the increase of the conversions with the reaction time suggest some living character for these polymerizations [19, 22, 34–40]. A very important point to highlight with respect to these latter types of initiators, C1, C2 and C3, is that even when it was not possible to obtain polymers with very high molar mass or when the yields were not quantitative, the results obtained indicate that they are active toward the polymerization of CL and L-LA even though they were used in relatively small amounts (molar ratios M/I greater than 2000). This is an advantage over works developed by other authors, in which other types of initiators have been used in higher amounts (M/I range of 150–1500) [27, 30]. In addition, taking into account the fact that most of these polymerizations are made in solution using solvents such as THF or toluene, whereas in this work the
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Fig. 5 Typical size exclusion chromatograms (SEC) of polymers obtained at different reaction times: a PCL, b PLA
polymerizations does not involve the use of solvents (bulk polymerizations), as a result we have that these complexes are effective initiators and are suitable for the new trend and search for ‘‘green chemistry’’. Kinetics of polymerization The bulk polymerization kinetics of both monomers with samarium(III) derivatives was studied by the weight-measuring method at 125 °C. In this kinetic study it should be assumed that the final grams of monomer correspond to the difference in mass of the initial amount of monomer added and the grams of polyester obtained [19]. Figure 6 shows a straight-line relationship for ln[(M)0/(M)t] with respect to reaction time for both monomers. This indicates that polymerization is first order with respect to monomer concentration [19, 22, 26–28, 36–41]. It also allows the calculation of the apparent rate constant (kapp) for e-CL and L-LA polymerization initiated by the samarium complex and SmAc3, which are shown in Table 2. In the polymerization of CL, it can be seen that the kapp depends on the initiator and can vary according to the following order: SmAc3 [ C1 [ C2 [ C3. Comparing the kapp the polymerization initiated with SmAc3 with the other kapp,
(b)
C1 C2 C3 SmAc3
1,0 0,8
ln M0/ln Mt
ln M0/ln Mt
(a)
0,6 0,4 0,2
C1 C2 C3 SmAc3
1,0 0,8 0,6 0,4 0,2
0,0 15
20
25
30
35
40
Time (hours)
45
50
15
20
25
30
35
Fig. 6 First-order plot polymerization initiated by samarium derivatives: a CL, b LA
123
40
Time (hours)
45
50
Polym. Bull. Table 2 Apparent rate constants (kapp) of polymerizations of CL and L-LA initiated by the samarium complex Initiator
kapp CL (h-1)
kapp L-LA (h-1)
SmAc3
0.0309
0.0237
C1
0.0133
0.0231
C2
0.0093
0.0193
C3
0.0082
0.0283
it is observed that this value is much higher; this is associated with greater steric hindrance on the complex. Also, the molar amount of SmAc3 present in the reaction medium is greater and hence an increased number of active species propagate the polymerization and therefore there is a greater constant value. On the other hand, for the polymerization of LA, the kapp has the same magnitude order for each initiator. The size and geometric arrangement allow the monomer insertion onto the coordination sphere of the samarium. Comparing the kapp, the polymerization initiated with SmAc3 with the other kapp, it is observed that this value is much higher; this is associated with greater steric hindrance on the complex.
Conclusion Samarium(III) acetate (SmAc3) and amino acid complexes of samarium(III) (Sm(2,20 -bipyridine)(Ln)3 (L1 = L-aspartic acid; L2 = L-glutamic acid and L3 = glycine) showed activity on the ring-opening polymerization of e-CL and L-LA. The polymerization reaction was a first-order one with respect to monomer in both cases. The end group analysis of low molar mass polyester using NMR spectroscopy indicates that the reaction proceeds by a coordination insertion mechanism, in which the monomer is inserted onto the metal oxygen bond with selective acyl-oxygen bond scission of the monomer. The molar mass distribution curves range from a lower molecular weight zone to a higher molar mass zone as the reaction time increases. The kapp to polymerization of e-CL is initiator dependent, in the following order: SmAc3 [ C1 [ C2 [ C3. In the cases of the polymerization of L-LA, the kapp has the same order of magnitude for each of the initiator used. Acknowledgements The authors thank the Consejo de Desarrollo Cientı´fico, Humanı´stico, Tecnolo´gico y de las Artes of the Universidad de Los Andes (CDCHTA-ULA) for financial support (Grants C-1752 and C-1744). The authors also wish to thank Lic. Freddy Rojas and Dr. Marı´a Luisa Arnal of Simo´n Bolı´var University, for their help with the SEC experiments.
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