Introduction

Poly(L-lactide) has become the most widely produced biodegradable and biosourced polymer with a worldwide production of approximately 700 000 ton per year estimated for the year 2021 [1]. The technical production of almost all commercial poly(L-lactide) is based on ring-opening polymerization (ROP) of L-lactide catalyzed by Sn(II) 2-ethylhexanoatee (SnOct2). The predominant role of this catalyst is a consequence of its excellent performance, its low costs and the fact that it has been admitted as food stabilizer by the American Food and Drug Administration (FDA). This decision considers that SnOct2 combines a high toxicity for a broad variety of microbes and fungi with a rather low toxicity for humans.

Over the past 50 years the toxicity of tin(II) ions has intensively been studied, because billions of cans were produced from steel with an interior wall plated by tin [2,3,4]. For this reason, the term “tin” became a synonym for can in British English. Therefore, it is not surprising that also other tin(II) salts containing nontoxic anions were admitted as food stabilizers, for example SnCl2. Since the chloride ions belong to the human metabolism in contrast to 2-ethylhexanoate, SnCl2 may be even less toxic than SnOct2. Its broad application as food stabilizers is highlighted by the fact, that almost all tomato soups, tomato sauces and tomato juices are stabilized against fouling by SnCl2. Considering the hundred thousand of liters of tomato juice served on board of airlines every year, millions of people were exposed to SnCl2 consumption. After a careful review of previously published data Blunden and Wallace concluded that less than 200 mg of SnCl2 per kilogram human body may be considered to be harmless [3,4,5]. Nonetheless several research groups have recommended highly reactive tertiary amines (incl. amidines) as nontoxic substitute of tin(II) salts [6,7,8,9,10,11]. As example, the LD50 doses of pyridine has been found to be 500 mg Kg−1 and 4-N,N-dimethylamino pyridine is even ten times more toxic than pyridine [12, 13]. On the other hand, the increasing delivery of Sn(II) salts to the environment due to decomposition of polylactides or their hydrolysis in oceans, does not cause concern about environmental pollution. Sn(II) is a reducing agent, which is oxidized by oxygen or Fe(III) ions to SnO2. This extremely insoluble and harmless tin salt has been known as Cassiterite to the mankind for more than 5 000 years.

Whereas numerous patents and papers deal with application and polymerization mechanism of SnOct2, rather little is known about SnCl2 as polymerization catalyst. A first short study of SnCl2 (and SnCl4) catalyzes ROPs of L-lactide was published by Kricheldorf and Sumbel in 1989 [14]. At that time MALDI-TOF mass spectrometry did not exist yet, and SEC was not used. Tin(II)chloride hydrate in combination with toluene sulfonic acid was used by Kimura and coworkers as catalyst for polycondensation of L-lactic acid [15]. Three papers describing the oligomerization of l-lactide with isosorbide as initiator followed by in situ polycondensation with isophthalic or terephthalic acid dichloride were recently published by the authors [16,17,18]. Quite recently, the authors have unexpectedly discovered that neat SnOct2 catalyzes the formation of high molar mass cyclic polylactides and not of linear chains as believed by many researchers (incl. the first author) for almost 50 years [19]. In this context, the present work had the purpose to study ROPs of l-lactide catalyzed by neat SnCl2 and by combinations of SnCl2 with either difunctional cocatalysts or with ethyl lactate as initiator under conditions allowing for direct comparison with analogous polymerizations catalyzed by SnOct2. Furthermore, a comparison with ε-caprolactone (CL) was also intended.

Experimental

Materials

L-Lactide, a product of Corbion Purac, was kindly supplied by Thyssen-Uhde AG (Berlin) and recrystallized once from “Toluene 99.89%, extra dry” purchased from ACROS Organics (Geel, Belgium). Tin(II)acetate and anhydrous SnCl2 were purchased from Alfa Aesar (Karlsruhe, Germany) and used as received. SnOct2 (purity > 96%) was also purchased from Alfa Aesar and used as received, since no water was detected upon azeotropic drying with toluene. Salicylic acid, 4-tert.butylcatechol, 1,1′-diphenol, ethyl L-lactate, anhydrous zinc chloride, magnesium chloride, ferrous(III)chloride, bismuth chloride and hafnium chloride THF complex were all purchased from Sigma-Aldrich and used as received.

Polymerizations of L-lactide

With Neat SnCl2 at 180, 160 or 140 °C (Tables 1, 2, and 3)

Table 1 Polymerizations catalyzed with neat SnCl2 in bulk at 160 °C
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Table 2 Polymerizations catalyzed with neat SnCl2 at 180 °C
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Table 3 Polymerizations catalyzed by SnCl2 in bulk at 140 °C without and with addition of ethyl L-lactate (Lac/Cat = 1000/1, Lac/In = 200/1)
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Anhydrous SnCl2 was weighed into a flame-dried 50 mL Erlenmeyer flask, l-lactide (50 mmol) and a magnetic bar were added under a blanket of argon. The reaction vessel was the immersed into an oil bath preheated to 140 or 160 °C. The resulting polylactide was isolated by means of spatula and pincer and characterized without purification. For lac/cat ratio of 2 000/1 100 mmol of l-lactide was used.

With SnCl2 and Difunctional Cocatalyst (Table 4)

Table 4 Polymerizations catalyzed by SnCl2 at 160 °C in bulk with addition of ortho-diphenols or ortho-hydroxy acids (Lac/Cat = 1000/1, Time 1 h)
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The cocatalyst (0.05 mmol) and SnCl2 (0.05 mmol) were weighed into a flame-dried Erlenmeyer flask, l-lactide (50 mmol) and a magnetic bar were added under a blanket of argon. The reaction vessel was immersed into a preheated oil bath and the resulting polylactide was finally isolated by means of spatula and pincer.

With SnCl2 in Combination with Ethyl Lactate as Initiator (Table 5)

Table 5 ROPs catalyzed with SnCl2 + EtLac at 120 °C (Lac/Cat = 1 000/1)
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SnCl2 (0.05 mmol) and neat ethyl L-lactate (1 or 0.5 mmol) were weighed into a flame dried 50 mL Erlenmeyer flask, l-lactide and a magnetic bar were added under a blanket of argon. The reaction vessel was immersed into an oil bath thermostated at 20 °C and the polylactide was isolated approx. 0.5 h after its crystallization. For the Lac/Cocat ratios > 60/1 the L-lactate was added in form of a 2 M solution in toluene.

With Neat Bu2SnCl2 at 160 °C (Table 6)

Table 6 Comparison of ROP of l-lactide catalyzed by neat SnCl2 or Bu2SnCl2 in bulk at 160 °C/4 h
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Bu2SnCl2 (0.1, 0.2 or 0.4 mmol) was weighed into a flame-dried 50 mL Erlenmeyer flask, l-lactide (40 mmol) and a magnetic bar were added under a blanket of argon. The reaction vessel was immersed into an oil bath preheated to 160 °C and the reaction product was isolated by means of spatula and pincer.

Comparison with Other Metal Chlorides

The catalyst (SnCl2, ZnCl2, FeCl3, BiCl3 HfCl4 + 2THF, 0.1 mmol) was weighed into a 50 mL Erlenmeyer flask, and L-lactide (40 mmol) was added under a blanket of argon. After addition of a magnetic bar, the reaction vessel was immersed into an oil bath thermostated at1 60 °C. In the case of SnCl2, part of the colorless reaction product was removed with a spatula after 1 h. In the case of MgCl2 80 mmol of L-lactide was polymerized with 0.2 mmol of catalyst.

Polymerizations of ε-Caprolactone (Table 7)

Table 7 SnCl2- and SnOct2-catalyzed polymerizations of ε-caprolactone in bulk at160°C
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SnCl2 (0.1 mmol) was weighed into a flame-dried 50 mL Erlenmeyer flask, a magnetic bar and CL (40 mmol) were added under a blanket of argon. The reaction vessel was immersed into an oil bath thermostated at 160 °C.Finally part of the viscous reaction product was removed by means of a spatula and characterized in the virgin state. Analogous experiments were conducted with SnOct2 as catalyst, so that a 1.0 M or a 0.4 M solution in toluene was injected into the reaction vessel.

Measurements

The 400 MHz 1H NMR spectra were recorded with a Bruker Avance 400 in 5 mm sample tubes. CDCl3 containing TMS served as solvent and shift reference. The 150 MHz 13C NMR spectra were recorded with a Brucker Avance 600 MHz spectrometer again in 5 mm sample tubes with CDCl3 as solvent. The DSC measurements were conducted with a Mettler-Toledo DSC-1 equipped with Stare Software version 11.00. Matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass spectrometry was performed using an Autoflex Max (Bruker Daltonik GmbH, Bremen) equipped with a Nd-YAG laser (355 nm). Trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) dissolved in chloroform (20 mg mL−1) was doped with potassium trifluoroacetate and served as matrix. Matrix solution was premixed with the analyte solution (chloroform, 4 mg mL−1) in a ratio of 50/20 µL. A droplet (1 µL) of this solution was pipetted on the sample target. At least 6 000 single spectra recorded at 3 different spot positions were accumulated in the positive linear positive mode. The instrument was previously calibrated with PEO standards.

For the GPC experiments a modular LC system kept at 40 °C (isocratic pump, 1 mL min−1, refractive index detector, RI-501 -Shodex) was applied. Samples were manually injected using a Rheodyne valve (100 µL, 2–4 mg mL−1). For instrument control and data calculation Clarity software (GPC extension, DataApex) was used. The calibration was performed using polystyrene standard sets (Polymer Standards Service – PSS, Mainz, Germany). For the determination of the Mark-Houwing-Sakurada (MHS) relationship a viscometer (Viscostar, Wyatt, Germany) and a multiangle laser light scattering (MALS) detector (Dawn EOS, Wyatt, Germany) were used. Astra 6.1 software (Wyatt) served for calculating the MHS curves.

Results and Discussion

Polymerizations with Neat SnCl2 at 160 and 180 °C

A first series of polymerizations was performed at 160 °C in bulk with variation of Lac/Cat ratio. This temperature was selected, because reaction times of several hours do not cause degradation, racemization or crystallization, and because this standard temperature was also used in numerous previous studies, so that these results were comparable. The initially insoluble catalyst dissolved in the course of 5–20 min at Lac/Cat ratios around 200 and higher. At lower Lac/Cat ratios the rapid polymerization stops stirring before the catalyst has completely dissolved, and therefore the evaluation of the experiments summarized in Table 1 begins with a Lac/Cat ratio of 200/1. The polymerizations were fast, so that the melt viscosity raised dramatically within 5–15 min. at Lac/Cat ratios < 2 000/1.

The SEC measurements revealed four trends. First, the weight average molecular weights (Mw) vary little, when the Lac/Cat ratio increases by a factor of 10. Second, the number average molecular weights (Mn′s) obtained at Lac/Cat rations of 1 000/1 and below are significantly higher than expected from a stoichiometric initiation of ROPs. As demonstrated in Table 1, this deviation is also true when the measured Mn value are modified by a correction factor of 0.68 (relevant for chloroform solutions [22] but 0.58 for THF [23]), which is based on the fact that calibration of SEC measurements with polystyrene overestimates the real molecular weights of polylactide and other aliphatic polyesters due to its lower hydrodynamic volume. A third trend is a significant decrease of the molecular weight with longer polymerization times and the fourth trend consists in increasing dispersities with longer times. This pattern is typical for a polymerization process, where an initial rapid and kinetically controlled propagation is followed by slower equilibration reactions. In this connection it should be mentioned that ring chain equilibration resulting from reversible “back-biting” automatically involves ring-ring and chain-chain equilibration.

From the preparative point of view, it is a positive result that Mw values above 150 000 g mol−1 were achieved, values which are satisfactory for any application as engineering plastic with thermal processing from the melt. Commercial poly(L-lactide)s produced for injection molding typically have (uncorrected) Mw values in the range of 100 000 – 150 000 g mol−1 [24]. Mw′s in the range of 150–200 000 g mol−1are even too high for injection molding but good for extrusion of films. However, it is also noteworthy that under identical conditions higher molecular weights were achieved (Mw′s up to 320 000 g mol−1) when SnOct2 was used as catalyst [19]. One more positive result is the observation that the polylactides isolated after 2 h at 160 °C were colorless, whereas after extension up to 4 h (see Table 2) a slight brownish discoloration was detectable.

The MALDI-TOF spectra revealed two general trends. First, peaks of three species were detectable, namely peaks of cycles (labeled C in the mass spectra) and peaks of two linear types of chains (Lx and Lz) having masses of 16 Da below that of the next cycle and 18 Da above. The authors have observed these peaks in numerous mass spectra of polylactides prepared by various tin catalysts at temperatures around or above 140 °C. Hence, the Lx and Lz peaks are not characteristic for SnCl2 catalyzed ROPs. By repetition of several MALDI-TOF measurements with Na-doping it was confirmed that the Lx peaks in the K-doped spectra originate from doping with coexisting Na ions. However, in all mass spectra of samples listed in Table 1 (and Table 2) the Lz chains were the predominant species, and in this regard the results obtained with SnCl2 are different from those obtained with SnOct2 or Sn(II)acetate. Unfortunately, the authors were not able to identify the end groups of the Lz peaks. At Lac/cat ratios > 400/1 those latter catalysts yielded polylactides almost exclusively consisting of cycles.

The second trend observed for SnCl2 is a predominance of even numbered chains (Fig. 1a), which indicates that equilibration by transesterification was not complete after a reaction time of 1 h. This phenomenon was not observed for SnOct2-catalyzed ROPs under identical conditions and suggests that the transesterification activity of SnOct2 is higher than that of SnCl2. Prolonged heating favors increasing equilibration and after 8 h complete leveling of even and odd-numbered species was achieved (Fig. 1b).

Fig. 1
figure 1

MALDI TOF mass spectra of poly(L-lactide)s prepared with SnCl2 (Lac/Cat = 1 000/1) at 160 °C: a after 1 h (No. 4A, Table 1), b after 8 h (No. 4D, Table 1)

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For several tin chlorides, such as Bu3SnCl and Ph2SnCl2 it was previously demonstrated [25] that the poly(L-lactide)s prepared in bulk at 160 °C almost exclusively consist of cycles when relatively low Lac/Cat ratios (< 600/1) were used, and the polymerization mechanism outlined in Scheme 1 was proposed. Such a ROPPOC mechanism (ring-opening polymerization combined with simultaneous polycondensation, for definition and review see ref. [26]) was also proposed for polymerizations catalyzed by neat SnOct2. The mass spectra of the SnCl2-catalyzed polymerizations obtained in this study suggest that the polymerization catalyzed by SnCl2 also follows this ROPPOC mechanism but involves more side reactions so that a nearly complete formation of cyclic polylactides cannot be achieved.

Scheme 1
scheme 1

Simplified reaction mechanism of Bu3SnCl catalyzed polymerization of l-lactide

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With regard to discoloration the following trend was found. At a reaction time of 1 h the polylactides were colorless regardless of the catalyst concentration. Slight discoloration became observable at a reaction time of 2 h, but this discoloration was so weak that the samples still looked white after crystallization. As expected, discoloration intensified with longer reaction times.

For two reasons several polymerizations were performed at 180 °C (Table 2). First, this temperature is needed at the end of a technical polymerization to avoid crystallization and to enable extrusion of the melt from the reactor (whereupon the temperature may even be raised to 200 °C). Second, it should be found out, if the short time needed for complete conversion at this temperature involves partial racemization and/or discoloration. The high reactivity of SnCl2 in combination with the higher temperature had the consequence, that a reaction time of 0.5 h sufficed for complete conversion, meaning 97% according to the thermodynamic equilibrium concentration of L-lactide. At this short time the resulting polylactides were still colorless, whereas a slight brownish discoloration was observable for the amorphous product after 1 h. After crystallization this product was white. However, the molecular weights achieved at 180 °C were all lower than those obtained tat 160 °C and thus, polymerization at 180 °C proved not attractive.

Concerning the optical purity of the poly(L-lactide)s prepared at 160 or 180 °C it should be mentioned, that the technical production of poly(L-lactide)s and numerous publications showed that tin salts and compounds are less prone to cause racemization than salts of all other metal with exception of bismuth. However, when chloride ions are not solvated by hydrogen bonds or small cations (e. g. Li or Na), the are rather strong bases above all at high temperatures. Therefore, the samples Nos. 4B and 5B of Table 1and Nos. 2–4 of Table 2 were subject to high-resolution 13C NMR measurements (150 MHz) with a s/n ratio > 200/1. All spectra were identical, and the CO or CH signals, which are particular sensitive to the presence of different stereosequences were perfectly symmetrical and did not display a presence of L-D/D-L tetrads. Two CO signals are shown in Fig. 2.

Fig. 2
figure 2

CO signals of the 13C NMR spectra of poly(L-lactide)s prepared with SnCl2 a at 160 °C/2 h (No. 4B, Table 1) and b at 190 °C/1.5 h (No.3, Table 2)

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Polymerizations at 140 °C

Polymerizations at 140 °C were conducted for two reasons. Firstly, it should be found out, if Mw values around 150 000 g mol−1 can also be achieved at this lower temperature which further reduces the risk of discoloration. Secondly, at this temperature poly(L-lactide) crystallizes within a few hours, and for the following reason, the melting temperature (Tm) of the resulting crystallites was of interest. The standard Tm of optically pure poly(L-lactide) falls into the range of 172–176 °C with little annealing (e.g. at 120 °C for 1 h) and rises to 176–180 °C after intensive annealing [27,28,29,30,31]. However, quite recently the authors have shown that these Tm values only represent the kinetically controlled type of crystallites, whereas the thermodynamically controlled counterparts have a Tm in the range of 193–196 °C [32, 33]. The crystal modification (α-modification) is the same, but the high Tm crystallites possess a smooth homogeneous surface resulting from thermodynamically controlled transesterification reaction across the flat surfaces of the crystallites typically combined with thickness growth. Formation of such high Tm crystallites was preferentially observed for catalysts and reaction conditions yielding cyclic polylactides [33]. Therefore, it was of interest to see if and to what extent SnCl2 is capable of yielding poly(l-lactide)s having a Tm above 180 °C.

The results summarized in Table 3 indicate, firstly, that molecular weights (uncorrected Mw′s) close to 150 000 g mol−1 can indeed be achieved, and up to a polymerization time of 4 h the polylactides were colorless. Secondly, at 4 h the beginning of crystallization was observed and after 16 h a polylactide having a Tm of 188 °C was obtained. Further annealing at 140 °C finally yielded a polylactide with a Tm of 190 °C and 76% crystallinity (Fig. 3a). These results prove that SnCl2 is capable to produce high Tm poly(L-lactide) in analogy to the previously studied tin(II) or dibutyltin(IV) catalysts. Comparable experiments with SnOct2 have not been published yet, but a pertinent study is in progress. Finally, an experiment with addition of ethyl L-lactate as initiator was conducted (No. 5, Table 3) which gave the unexpected result that even an alcohol-initiated polymerization catalyzed by SnCl2 yields a Tm > 180 °C (Fig. 3b), although similar experiments with SnOct2 as catalyst had failed to succeed in this regard [34].

Fig. 3
figure 3

DSC heating traces of polylactides prepared with a SnCl2 + ethyl lactate at 140 °C/16 h (No. 5, Table 3), b neat SnCl2 at 140 °C /48 h (No. 4, Table 3)

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Attempts to Prepare Cyclic Polylactides

Since a predominant formation of cyclic polylactides was not observed, when neat SnCl2 was used as catalyst, further experiments were conducted with certain diphenols as cocatalysts. It has been previously reported that addition of salicylic acid or catechols or 2,2′-dihydroxybipheyl to solutions of SnOct2 yields cyclic tin(II) compounds (illustrated in Scheme 2) [24, 35, 36]. These cyclic tin compounds catalyzed ring-expansion polymerizations (REPs) and yielded cyclic poly(L-lactide)s. Those results prompted the authors to study in the present work, if combinations of SnCl2 with afore mentioned difunctional cocatalysts also yield cyclic polylactides (Table 4). The MALDI TOF mass spectra displayed a significant increase of the molar fraction of cyclics as compared with polylactides prepared with neat SnCl2 (Fig. 4). However, none of these experiments yielded a product with total absence of linear chains. Hence, it may be concluded that SnCl2 is inferior to SnOct2, when a homogeneous cyclic topology is the goal of the experiments.

Scheme 2
scheme 2

Formation of cyclic Sn(II)bisphenoxide from SnOct2

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Fig. 4
figure 4

MALDI TOF mass spectra of the poly(L-lactide) prepared with SnCl2 (Lac/Cat = 1 000/1) at 140 °C/48 h, (No. 4, Table 2)

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Polymerizations with Addition of Ethyl L-Lactate as Initiator

If no maximum molecular weights are needed, as it is desirable for fiber spinning, alcohols are used as initiators (cocatalysts) in the technical production of polylactide. Addition of alcohols accelerates the polymerization and allows for a rough control of the average molecular weights via the Lac/alcohol (Lac/In) ratio, because the alcohol is quantitatively incorporated as dead chain end (Scheme 3). When such ROPs are performed at relatively low temperatures (e.g. 80 °C) in solution, the alcohol-initiated ROPs catalyzed by SnOct2 even have the character of living polymerizations [37,38,39]. However, at temperatures around 130 °C (and higher [40]) transesterification reactions occur, which modify the initially kinetically controlled polymerization pathway.

Scheme 3
scheme 3

Ethylactate initiated ROP of l-lactide catalyzed by SnOct2

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A first series of alcohol-initiated polymerizations was performed with SnOct2 at 120 °C for comparison. These experiments were not listed in detail, because such polymerizations have already been reported by several other research groups [37,38,39, 41,42,43]. This temperature was selected for two reasons. First, formation of cycles by back-biting is negligible under these conditions. Second at 120 °C (and slightly above) a relatively rapid crystallization of polylactides occurs with formation of the orthorhombic α-modification which is the thermodynamically most stable crystal modification of poly(L-lactide)) [34]. As expected, the Mn′s parallel the Lac/Cat ratios and after correction with a factor of 0.68 [22] the agreement with the theoretical values is satisfactory. The MALDI-TOF mass spectra display a considerable predominance of even-numbered chains, a result which has already been reported by several groups over the past twenty years for polymerizations at temperatures < 130 °C (Fig. 5a). [37,38,39,40,41,42,43]

Fig. 5
figure 5

MALDI-TOF mass spectra of polylactides prepared at 120 °C with ethyl L-lactate as initiator (Lac/In = 100/1): a SnOct2 as catalyst (Lac/Caz = 1 000/1); b SnCl2 as catalyst (No. 3, Table 6)

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When SnCl2 was used as catalyst (Table 5) under identical conditions the ROPs initiated with L-ethyl lactate proved to be slower by a factor of 2–3 influenced by the fact that dissolution of the solid catalyst required up to 30 min. After 1.5 h, when all the SnOct2-catalyzed polylactides had almost completely crystallized, all the reaction mixtures containing SnCl2 were still liquid, and times of 3–5 h were required to achieve crystallization (Table 5). The Mn′s were almost identical with those obtained from SnOct2-catalyzed ROPs up to a Lac/In ratio of 100/1 but fell below the theoretical values at higher Lac/In ratios, partially due to side reactions including cyclization as indicated by the MALDI TOF mass spectra (Fig. 5b). Nonetheless the mass spectra suggest that the mechanism catalyzed by SnCl2 is the same as that discussed for SnOct2 and a rough control of the molecular weight can be achieved with addition of an alcohol as initiator. Furthermore, the dispersities are considerably lower than those obtained in the absence of an initiator Scheme 4.

Scheme 4
scheme 4

Alternative mechanism of SnOct2 catalyzed-initiated ROPs of caprolactone

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DSC measurements of the polylactides prepared with SnCl2 were recorded for comparison with sample No.5, Table 3, the DSC heating trace of which is presented in Fig. 3b. The DSC hating traces of the samples listed in Table 6 display melting endotherms (Tm′s) far below the Tm measured for sample No. 3, Table 5 (Fig. 6). This is a result that underlines the effect of long annealing at 140 °C in the presence of the catalyst on the properties of sample No. 5. Table 3.

Fig. 6
figure 6

DSC heating traces of polylactides prepared at 120 °C with SnCl2 and ethyl L-lactate as initiator: (a) Lac/In = 100/1 (No. 3, Table 6); b Lac/Cat = 200/1 (No. 4, Table 6)

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Comparison with Various Metal Chlorides

In order to see how SnCl2 compares to other metal chlorides two series of polymerizations were conducted. At first, a comparison of SnCl2 and Bu2SnCl2 was studied. From a couple of previously published polymerization conducted with Bu2SnCl2 (and other butyltin(IV) chlorides) at 160 °C [25] it was expected that the SnCl2-catalyzed polymerizations will be faster, and identical reaction conditions without interference of crystallization or thermal degradation would be difficult to find. After preliminary experiments, not described here in detail, low Lac/Cat ratios, at a temperature of 160 °C and a reaction time of 4 h were selected as reaction conditions for both catalysts (Table 6). Under these conditions the SnCl2-catalyzed reaction mixtures turned viscous within a few minutes, although the SnCl2 did not dissolve completely at a Lac/Cat ratio of 100/1, whereas the Bu2SnCl2 containing mixtures required 2–3.5 h to reach a high viscosity. The consequences of these different catalytic activities were considerably lower Mn and Mw values of Bu2SnCl2 catalyzed polylactides. The MALDI mass spectra proved to be rather complex with a pattern of five peaks, four peaks of different kinds of linear chains and the peak of cycles. In other words, Bu2SnCl2 is even worse than SnCl2, when a homogeneous topology is desired.

The second series of polymerizations was performed with the purpose to compare SnCl2 with the chlorides of other metals such as Mg, Zn, Fe(III), Bi or Hf. These chlorides were selected, because the toxicity of Mg(II), Zn(II) and Bi(III) is certainly lower than that of Sn(II). Since preliminary experiments had shown that BiCl3 is a sluggish catalyst, the temperature was raised to 160 °C and the Lac/Cat ratio was lowered to 400/1. As expected on the basis of experiment No. 3, Table 1, the SnCl2-catalyzed polylactide melt turned viscous within 15 min, and when the product was isolated after 1 h a Mn of 102 000 g mol−1 and a Mw of 161 000 g mol−1 were found in good agreement with the results of No. 3, Table 1. However, the reaction mixtures of all other five experiments did not show a significant increase of the viscosity within 4 h and therefore, these experiments were stopped at that time. Hence, these experiments document that the effectiveness of SnCl2 as catalyst for ROPs of lactide is far superior to any other metal chlorides studied in this work.

Polymerizations of ε-Caprolactone

CL is more basic and less electrophilic than lactide as demonstrated by the fact that 4-tolulene sulfonic acid catalyzes a cationic polymerization of CL, whereas in the case of lactide trifluoromethane sulfonic acid or fluorosulfonic acid are needed as catalysts. Furthermore, it was found that BiCl3 (and BiBr3 and BiI3) catalyze polymerizations of CL at 140 °C in bulk [44], whereas the experiments listed in Table 4 demonstrate that BiCl3 is an extremely poor catalyst for lactide. Since SnCl2 is a stronger Lewis acid than SnOct2, it may be hypothesized that the negative reactivity ratio of the pair SnCl2/SnOct2 found for ROPs of lactide might turn into a positive ratio when CL is used as monomer. To test this hypothesis several polymerizations were conducted at 160 °C in bulk, (i.e. under the conditions of Table1) with variation of the CL/Cat ratio (Table 7). Since the solid SnCl2 needs more time for dissolution in CL than SnOct2, which dissolves immediately, an exact kinetic comparison did not make sense. Therefore, the time when the increasing viscosity of the reaction mixture stopped stirring of the magnetic bar was used for a rough comparison of both catalysts. This comparison indicated that SnCl2 is more reactive by a factor of 1.5–2.0 than SnOct2. However, the SEC measurements revealed that the molecular weights produced by SnCl2 are significantly lower than those obtained with SnOct2, in close analogy to the results obtained from ROPs of lactide. The MALDI-TOF mass spectra (exemplarily illustrated in Fig. 7) showed that in addition to peaks of cycles two peaks of two kinds of linear chains were present, one of which was identified, namely the Ly peaks. These peaks represent polylactone chains having one ClCH2- and one COOH end group. In other words, a Cl transfer from the catalyst to the CH2-O group of monomeric or polymeric CL units has occurred. The formation of a ClCH2 end group was also confirmed by a triplet signal at 3.6 ppm in the 1H NMR spectra (not shown here). Such halogen transfer reaction was already reported for polymerizations of CL with other metal halides and their mechanisms were discussed, so that a more detailed discussion in this work is not necessary [45, 46]. This Cl transfer, which does not occur in ROPs of lactide, is the main reason why the molecular weights obtained from the monomer/catalyst combination CL/SnCl2 are lower than those obtained from the combinations CL/SnOct2 or Lac/SnCl2.

Fig. 7
figure 7

MALDI TOF mass spectrum of the poly-CL prepared at 160 °C with SnCl2 (No. 2B, Table 7)

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Conclusion

The results obtained in this work and previous publications allow for the following conclusions. SnCl2 is inferior to SnOct2 with regard to polymerization rates and maximum molecular weights that can be achieved in ROPs of L-lactide, However, uncorrected Mw values above 150 000 g mol−1 can be obtained which suffice for applications of polylactides as engineering plastics for injection molding, because commercial polylactides designed for this purpose typically have Mw′s in the range of 100 000–150 000 g mol−1. Furthermore, SnCl2 is far more reactive than all other metal chlorides studied in this work, above all more reactive than the less toxic chlorides MgCl2, ZnCl2 and BiCl3, which proved to be useless. Moreover, due to its higher Lewis acidity relative to SnOct2, SnCl2 is useful as catalyst for polycondensations and can be used to catalyze ROPs and polycondensation in an “one-pot” procedure.