Dichloro(2,2′-bipyridine)copper/MAO: An Active and Stereospecific Catalyst for 1,3-Diene Polymerization

Dichloro(2,2′-bipyridine)copper was synthesized by reacting copper dichloride with bypyridine, and its behavior, in combination with methylaluminoxane (MAO), in the polymerization of butadiene, isoprene, 2,3-dimethyl-1,3 butadiene, and 3-methyl-1,3-pentadiene was examined. The purpose of this study is to find catalytic systems that are more sustainable than those currently used for the polymerization of butadiene and isoprene (e.g., Co and Ni), but that are comparable in terms of catalytic activity and selectivity. Predominantly, syndiotactic 1,2 polybutadiene, crystalline syndiotactic 3,4 polyisoprene, crystalline syndiotactic 1,2 poly(3-methyl-1,3-pentadiene), and crystalline cis-1,4 poly(2,3-dimethyl-1,3-butadiene) were obtained in a manner similar to that observed with the analogous iron complex. As far as we know, the investigated catalytic system represents the first example of a copper-based catalyst in the field of stereospecific polymerization. Given the great availability of copper, its extremely low toxicity (and therefore high sustainability), and the similarity of its behavior to that of iron, the result obtained seems to us of considerable interest and worthy of further investigation.

On the basis of this observation, we have focused our attention on copper. Until now, there had been no reports in the literature on the stereospecific polymerization of 1,3-dienes with copper-based catalysts. Our interest in copper arises from the need to find and test new catalytic systems that have a lower environmental impact (and are consequently more sustainable) and that are capable of replacing, by exhibiting comparable catalytic activities and selectivities, the catalytic systems currently used, based on metals characterized by high toxicity such as cobalt, chromium, and nickel.
To begin, we synthesized the bipyridyl copper dichloride complex and examined its behavior as a precatalyst in the polymerization of various 1,3-dienes. The results obtained turned out to be quite interesting and are shown in the present paper.

Synthesis and Characterization of CuCl 2 (bipy)
The reaction of 2,2 -bipyridine with CuCl 2 or CuCl 2 (H 2 O) 2 (bipy/Cu molar ratio = 1) to give pure Cu(bipy)Cl 2 was initially performed in boiling toluene, but we observed that the formation of the complex occurs smoothly at room temperature when using ethanol as a reaction medium [28,29]. The compound is the same either starting from anhydrous or from hydrated copper chloride.
The obtained Cu(bipy)Cl 2 compound was a stable turquoise microcrystalline solid and was characterized using analytical and infrared data. The Infrared spectrum is characterized by strong absorptions in the 1600−1500 cm −1 range due to the C=N stretching vibrations ( Figure S1-S3 in the Supplementary Materials section), which is shifted about 20 cm −1 towards the lower wavenumbers with respect to the uncoordinated species. The copper compound is soluble in polar solvents, such as acetonitrile or dichloromethane, and is substantially insoluble in hydrocarbons.

Polymerization of 1,3-Dienes
The results obtained in the polymerization of 1,3-dienes with the CuCl 2 (bipy)/MAO catalytic system are summarized in Table 1 and can be summarized as follows. The polymerization of 1,3-butadiene gives polymers with a predominantly 1,2 structures (1,2 content about 65%), a rather low syndiotacticity (percentage of syndiotactic pentads (rrrr) around 30% (Figure 2)), and consequently very low melting points (around room temperature), very high molecular weights (up to 1,900,000 g×mol −1 ), and narrow molecular weight distributions (particularly at room temperature, M w /M n from 1.3 to  (Table 1, entry 3 vs. 4). The polymerization rate is rather low since several hours are needed to reach appreciable monomer conversion.
The polymerization of 1,3-butadiene gives polymers with a predominantly 1,2 structures (1,2 content about 65%), a rather low syndiotacticity (percentage of syndiotactic pentads (rrrr) around 30% (Figure 2)), and consequently very low melting points (around room temperature), very high molecular weights (up to 1,900,000 g×mol −1 ), and narrow molecular weight distributions (particularly at room temperature, Mw/Mn from 1.3 to 2.3). The syndiotacticity and the polymer molecular weight decrease with the increase in polymerization temperature from 22 °C to 60 °C (Table 1, entry 3 vs. 4). The polymerization rate is rather low since several hours are needed to reach appreciable monomer conversion. The polymerization of isoprene catalyzed by CuCl2(bipy)/MAO, contrary to what has been observed with most catalytic systems based on transition metals and lanthanides, is instead much faster than the polymerization of 1,3-butadiene. Crystalline polymers with an essentially 3,4 syndiotactic structure ( Figure 3) (percentage of syndiotactic pentads (rrrr) around 55%) and a melting temperature (Tm) of up to 115 °C are obtained. The molecular weight of the resultant polymers is very high, and the molecular weight distribution rather narrow (Mw/Mn from 1.4 to 2). The narrow molecular weight distribution, also observed in the case of the polymerization of butadiene, seems to suggest the presence of single-site catalysts.
The Al/Cu molar ratio has some influence on the catalyst activity, which was found to decrease with a decrease in the Al/Cu molar ratio, while a negligible effect was observed on the polymerization selectivity. An increase in the polymerization temperature determines a decrease in the polymerization stereoselectivity as well as a decrease in the polymer molecular weight accompanied by an increase in the molecular weight distribution (Table 1, entry 7 vs. 6). The polymerization of isoprene catalyzed by CuCl 2 (bipy)/MAO, contrary to what has been observed with most catalytic systems based on transition metals and lanthanides, is instead much faster than the polymerization of 1,3-butadiene. Crystalline polymers with an essentially 3,4 syndiotactic structure ( Figure 3) (percentage of syndiotactic pentads (rrrr) around 55%) and a melting temperature (T m ) of up to 115 • C are obtained. The molecular weight of the resultant polymers is very high, and the molecular weight distribution rather narrow (M w /M n from 1.4 to 2). The narrow molecular weight distribution, also observed in the case of the polymerization of butadiene, seems to suggest the presence of single-site catalysts. tion rate is rather low since several hours are needed to reach appreciable monomer conversion. The polymerization of isoprene catalyzed by CuCl2(bipy)/MAO, contrary to what has been observed with most catalytic systems based on transition metals and lanthanides, is instead much faster than the polymerization of 1,3-butadiene. Crystalline polymers with an essentially 3,4 syndiotactic structure ( Figure 3) (percentage of syndiotactic pentads (rrrr) around 55%) and a melting temperature (Tm) of up to 115 °C are obtained. The molecular weight of the resultant polymers is very high, and the molecular weight distribution rather narrow (Mw/Mn from 1.4 to 2). The narrow molecular weight distribution, also observed in the case of the polymerization of butadiene, seems to suggest the presence of single-site catalysts.
The Al/Cu molar ratio has some influence on the catalyst activity, which was found to decrease with a decrease in the Al/Cu molar ratio, while a negligible effect was observed on the polymerization selectivity. An increase in the polymerization temperature determines a decrease in the polymerization stereoselectivity as well as a decrease in the polymer molecular weight accompanied by an increase in the molecular weight distribution (Table 1, entry 7 vs. 6). The Al/Cu molar ratio has some influence on the catalyst activity, which was found to decrease with a decrease in the Al/Cu molar ratio, while a negligible effect was observed on the polymerization selectivity. An increase in the polymerization temperature determines a decrease in the polymerization stereoselectivity as well as a decrease in the polymer molecular weight accompanied by an increase in the molecular weight distribution (Table 1, entry 7 vs. 6).
On the other hand, the polymerization of (E)-3-methyl-1,3-pentadiene is rather slow. Again, a polymer insoluble in ortho-dichlorobenzene and C 2 D 2 Cl 4 is obtained, preventing GPC and NMR analysis in solution. However, in this case the melting point (241 • C) and the FT-IR spectrum in the solid state also perfectly correspond to those observed in the case of the poly(3-methyl-1, 3-pentadiene) with a syndiotactic 1,2 structure obtained with the catalytic system FeCl 2 (bipy) 2 /MAO [16,17]. The fact that different polymer structures are obtained from butadiene, isoprene, 2,3-dimethyl-1,3 butadiene, and (E)-3-methyl-1,3pentadiene with the same catalytic system only confirms once more the importance of the monomer structure in the stereoselectivity in the polymerization of conjugated dienes [1,11].
On the other hand, the polymerization of (E)-3-methyl-1,3-pentadiene is rather slow Again, a polymer insoluble in ortho-dichlorobenzene and C2D2Cl4 is obtained, preventin GPC and NMR analysis in solution. However, in this case the melting point (241 °C ) and the FT-IR spectrum in the solid state also perfectly correspond to those observed in th case of the poly(3-methyl-1, 3-pentadiene) with a syndiotactic 1,2 structure obtained with the catalytic system FeCl2(bipy)2/MAO [16,17]. The fact that different polymer structure are obtained from butadiene, isoprene, 2,3-dimethyl-1,3 butadiene, and (E)-3-methyl-1,3 pentadiene with the same catalytic system only confirms once more the importance of th monomer structure in the stereoselectivity in the polymerization of conjugated diene [1,11].
The results obtained in the polymerization of 1,3-dienes with the catalys FeCl2(bipy)2/MAO were interpreted by admitting the formation of a catalytic center hav ing the structure shown in Figure 4A, i.e., only one bipyridyl ligand coordinated to th iron atom, the monomer coordinated cis-η 4 , and the growing chain linked to the iron atom by means of an anti η 3 -allyl bond [16,17].
One could imagine a similar structure for the catalytic center in the case of coppe ( Figure 4B), but, unfortunately, this is not possible as copper would end up with a highe number of electrons (3 excess electrons) than those it can accept (8 electrons) according t the 18 electron rule. It is indeed necessary to hypothesize a different situation. A plausible one is tha shown in Figure 4C: the bipyridyl ligand coordinated to the copper atom by means of onl one nitrogen and the monomer trans-η 2 and the growing chain coordinated to the coppe atom through a syn η 3 -allylic bond as this is the allyl unit that originates from a trans-η coordination of the monomer (an anti η 3 -allylic bond originates from a cis-η 4 coordinatio One could imagine a similar structure for the catalytic center in the case of copper ( Figure 4B), but, unfortunately, this is not possible as copper would end up with a higher number of electrons (3 excess electrons) than those it can accept (8 electrons) according to the 18 electron rule.
It is indeed necessary to hypothesize a different situation. A plausible one is that shown in Figure 4C: the bipyridyl ligand coordinated to the copper atom by means of only one nitrogen and the monomer trans-η 2 and the growing chain coordinated to the copper atom through a syn η 3 -allylic bond as this is the allyl unit that originates from a trans-η 2 coordination of the monomer (an anti η 3 -allylic bond originates from a cis-η 4 coordination of the monomer). With such a structure, copper would receive 7 electrons, thereby respecting the 18 electron rule (a 1 electron deficit). However, the structure shown in Figure 3 is not yet able to explain the polymerization data obtained: the polymer obtained from 2,3-dimethyl-1,3-butadiene has a cis-1,4 structure, and in the polymers from 1,3-butadiene and isoprene, having respectively a predominant 1,2 and 3,4 structure, the remaining units have only a cis-1,4 structure, but while a 1,2 unit and a 3,4 unit can derive from both a syn and an anti allyl unit, a cis-1,4 unit is formed solely from an anti allyl unit [1,4,5,11]. It is therefore necessary to hypothesize that the allyl unit of the syn type isomerize to an anti allyl unit to allow the formation of a cis-1,4 unit, and that the occurrence of such isomerization and the frequency with which it occurs may be a function of the type of monomer polymerized.
An alternative explanation is shown in Figure 5. The diene monomer can coordinate with both double bonds (cis-η 4 ) favoring the displacement of the ligand and its complete migration onto MAO, thus allowing the formation of an anti allyl unit which in turn can lead to the formation of a cis-1,4 unit through the insertion of the incoming monomer into C1 of the butenyl group. A sort of equilibrium can be hypothesized between the form (I), with the monomer trans-η 2 and the ligand coordinated with only one nitrogen atom, and the form (II), with the monomer cis-η 4 coordinated and the ligand having migrated onto MAO, with the formation of 1,2 (3,4) units through the insertion of the incoming monomer into C3 of the butenyl group rather than cis-1,4, depending on whether the equilibrium is more shifted towards form (I) or form (II), respectively. with both double bonds (cis-η ) favoring the displacement of the ligand and its complete migration onto MAO, thus allowing the formation of an anti allyl unit which in turn can lead to the formation of a cis-1,4 unit through the insertion of the incoming monomer into C1 of the butenyl group. A sort of equilibrium can be hypothesized between the form (I) with the monomer trans-η 2 and the ligand coordinated with only one nitrogen atom, and the form (II), with the monomer cis-η 4 coordinated and the ligand having migrated onto MAO, with the formation of 1,2 (3,4) units through the insertion of the incoming monome into C3 of the butenyl group rather than cis-1,4, depending on whether the equilibrium i more shifted towards form (I) or form (II), respectively.
Obviously, the above interpretations represent only working hypotheses, both plau sible in our opinion, but certainly to be further explored through additional computationa studies.

General Procedures and Materials
Anhydrous copper dichloride (Merck, 99.9% pure), copper chloride dihydrate (Merck, reagent grade), 2,2′-bipyridine (Supelco, ACS reagent), methylaluminoxane (MAO) (Merck, 10 wt% solution in toluene), and deuterated solvent for NMR measure ments (C2D2Cl4) (Merck, >99.5% atom D) were used as received. Commercial ethyl alcoho (Merck, 96% pure) was degassed under vacuum, the flask was then filled with dry dini trogen, and the solvent was stored over molecular sieves. Diethylether (Merck, 99% pure was refluxed over Na/K alloy for ca. 8 h, distilled, and stored over molecular sieves unde dry dinitrogen. Toluene (Merck, 99.8% pure) was refluxed over Na for ca. 8 h, then dis tilled and stored over molecular sieve under dry dinitrogen. Prior to each run, 1,3-Buta diene (Merck, ≥99%) was evaporated from the container, dried by passing through a Obviously, the above interpretations represent only working hypotheses, both plausible in our opinion, but certainly to be further explored through additional computational studies.

Synthesis of CuCl 2 (bipy)
The compound was prepared according to a slight modification of the procedure outlined in the literature [28]. An ethanol solution (25 mL) of bipy (2.330 g, 0.015 mmol) was added to a solution of copper chloride dihydrate (2.532 g, 0.015 mol) in ethanol (45 mL), and the resulting solution was stirred for 60 min. During the stirring, a large amount of solid precipitated from the solution. The mixture was stirred at room temperature for

Polymerization of 1,3-Dienes
Polymerizations were carried out in a 25 mL round-bottomed Schlenk flask. A standard procedure is reported. Prior to starting the polymerization, the reactor was heated to 110 • C under vacuum for 1 h and backfilled with nitrogen. The 1,3-Butadiene was condensed into the Schlenk flask kept at −20 • C, toluene was added, and the solution was brought to the desired polymerization temperature. MAO and a toluene solution (2 mg/mL) of the copper complex were then added in that order. The polymerization was stopped with methanol containing a small amount of hydrochloric acid. The polymer obtained was then coagulated by adding 40 mL of a methanol solution containing 4% Irganox ® 1076 antioxidant and HCl, repeatedly washed with fresh methanol, and finally dried under vacuum at room temperature to a constant weight. The polymerizations with isoprene, 2,3-dimethyl-1,3-butadiene, and 3-methyl-1,3-pentadiene were carried out in the same way.

Polymer Characterization
Attenuated total reflectance (ATR)-Fourier transform infrared spectroscopy (FTIR) spectra were recorded at room temperature in the 600-4000 cm −1 range with a resolution of 4 cm −1 using a Perkin Elmer Spectrum Two spectrometer. NMR spectra were recorded on a Bruker NMR advance 400 Spectrometer operating at 400 MHz (1H) and 100.58 MHz ( 13 C) in the PFT mode at 103 • C. NMR samples were prepared by dissolving from 60 mg to 80 mg of polymer in about 3 mL of C 2 D 2 Cl 4 in 10 mm probes, and hexamethyldisiloxane (HMDS) was referred to as the internal standard. The relaxation delay was 16 s. The molecular weight average (M w ) and the molecular weight distribution (M w /M n ) were obtained using a high temperature Waters GPCV2000 size exclusion chromatography (SEC) system equipped with a refractometer detector. The experimental conditions consisted of three PL Gel Olexis columns, ortho-dichlorobenzene (DCB) as the mobile phase, a flow rate of 0.8 mL/min, and a temperature of 145 • C. The calibration of the SEC system was achieved using eighteen narrow M w /M n PS standards with molar weights ranging from 162 g/mol to 5.6 × 10 6 g/mol. For the SEC analysis, about 12 mg of polymer was dissolved in 5 mL of DCB with 0.05% of BHT as antioxidant. The microstructure of the resultant polymers (i.e., cis-1,4 unit content (%) and 1,2 (3,4 in the case of isoprene) unit content (%); syndiotactic index (rrrr%) of the 1,2 poly(1,3-butadiene)s and of the 3,4 poly(isoprene)s) was determined by 1 H and 13 C NMR, in accordance with the literature [17,[32][33][34][35][36].
Furthermore, syndiotactic 1,2 poly(1,3-butadiene) and syndiotactic 3,4 poly(isoprene) are polymers of potential industrial interest, and these new copper-based catalysts, in light of the natural abundance of copper, its low toxicity, and low environmental impact, could represent a valid alternative to other catalytic systems, such as those based on cobalt, currently used for their production.