Article pubs.acs.org/Macromolecules
An Investigation of the Pathways for Oxygen/Sulfur Scramblings during the Copolymerization of Carbon Disulfide and Oxetane Ming Luo,†,‡ Xing-Hong Zhang,*,† and Donald J. Darensbourg*,‡ †
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States S Supporting Information *
ABSTRACT: The catalytic coupling of oxetane, the symmetric isomer of propylene oxide, with carbon disulfide has been investigated utilizing (salen)CrCl in the presence of various onium salts. Oxygen and sulfur atom exchange was observed in both the polymeric and cyclic carbonate products. The coupling of oxetane and CS2 was selective for copolymer formation over a wide range of reaction conditions. Five different polymer linkages and two cyclic products were determined by 1H and 13C NMR spectroscopy, and these results were consistent with in situ infrared spectroscopic monitoring of the process. The major cyclic product produced in the coupling process was trimethylene trithiocarbonate, which was isolated and characterized by single crystal X-ray crystallography. Upon increasing the CS2/oxetane feed ratio, a decrease in the O/S scrambling occurred. The reaction temperature had the most significant effect on the O/S exchange process, increasing exchange with increasing temperature. The presence of the onium salt initiator both accelerated the coupling process and promoted O/S scrambling. COS (observed), and CO2 intermediates are proposed in the reactions leading to various polymeric linkages.
■
INTRODUCTION While the copolymerization of epoxides and carbon dioxide to synthesize polycarbonates has garnered widespread attention, the analogous reaction of epoxides and carbon disulfide, CS2, to synthesize polythiocarbonates has received little consideration.1 The greatest challenge for studying these reactions is the oxygen/sulfur atom scrambling process which widely occurs in this copolymerization reaction. This atom exchange process will disconnect the coupling reaction, and as a result, complicated linkages will be generated in the polymer chain. In 2007, Nozaki and co-workers investigated the coupling of propylene sulfide with CS2 to yield poly(propylene trithiocarbonate) and cyclic propylene trithiocarbonate.2 This process was optimized to provide a high molecular weight copolymer with 92% selectivity. As there was no oxygen-containing monomer in this reaction, O/S scrambling was not an issue, and this polymer was completely alternating. Because of the high sulfur content of the copolymer, it possessed a very high refractive index. In 2008, one of us investigated the coupling of propylene oxide and CS2 using a heterogeneous zinc−cobalt double metal cyanide complex (Zn−Co DMCC) catalyst with the aim of synthesizing copolymers.3 This catalyst system afforded O/S scrambling which was noted in both the polymer and cyclic byproducts. The important scrambling intermediate, carbonyl sulfide (COS), was identified by GC-MS. Following this work, the copolymerization of COS and epoxides was examined to further study the mechanism of the O/S scrambling.4,5 © 2015 American Chemical Society
In 2009, the copolymerization of cyclohexene oxide with CS2 was investigated utilizing the well-defined (salen)CrCl/PPNCl system in hope of elucidating the mechanistic aspects of this process with more clarity.6 In this instance, O/S scrambling was observed as well in both the polymeric and cyclic products, with the copolymer being oxygen enriched and the cyclic carbonate being sulfur enriched. Nevertheless, the pathway for O/S scrambling was inadequately understood. More recently, the catalytic coupling of cyclopentene oxide with carbon disulfide has been studied utilizing (salen)CrCl in the presence of added onium salts.7 Both polymeric and cyclic materials were produced, with O/S scrambling observed in both instances. (salen)CrCl was shown to be necessary for the scrambling process, and onium salts impacted this process. Mixspecies scrambling resulted in very complicated products, and the proposed mechanism for scrambling was somewhat better assessed. In all these aforementioned reports, the structure of the produced polythiocabonates was not well-defined, and the mechanism of the O/S scrambling still remained questionable. In this report, we have investigated the copolymerization of carbon disulfide with oxetane catalyzed by binary (salen)CrCl/ onium salt catalyst (Figure 1). Oxetane is an analogue of propylene oxide, but oxetane is symmetric and the binary Received: June 9, 2015 Revised: July 17, 2015 Published: August 7, 2015 5526
DOI: 10.1021/acs.macromol.5b01251 Macromolecules 2015, 48, 5526−5532
Article
Macromolecules
Figure 1. Structure of (salen)CrCl (1).
symmetric copolymerization has provided very informative results. The various thiocarbonates linkages and cyclic products are not only qualitatively determined but also quantitatively determined for the first time, providing a reliable reference to test the factors effecting the O/S scrambling. The mechanism for O/S scrambling has been more extensively defined.
■
Figure 2. 1H NMR spectrum of the product of CS2/oxetane copolymerization (Table 1, entry 1).
RESULTS AND DISCUSSION Copolymerization of Carbon Disulfide and Oxetane. Initially, oxetane and carbon disulfide in a 1:1 mole ratio were copolymerized in the presence of complex 1 along with the cocatalyst bis(triphenylphosphine)iminium chloride (PPNCl) at 80 °C (eq 1). After 12 h, the oxetane was almost completely
the cyclic monothiocarbonate (C2) was generated in only a few high temperature runs, and its molar percentage was less than 1%. Hence, C2 was not included in the product distribution. All of the linkages and the cyclic product were simultaneously confirmed by 13C NMR spectroscopy of the crude product as seen in Figure 3.16 Fortunately, all 1H NMR peaks for the
consumed as evidenced by the disappearance of the 1H NMR signals at 4.75 and 2.69 ppm. As the aforementioned epoxide/ CS2 systems, in this instance, scrambling of oxygen and sulfur atoms was observed. That is, five different thiocarbonate linkages (Scheme 1) and two distinct cyclic products were Scheme 1. Different Linkages and Cyclic Products in Reaction
Figure 3. Carbonyl region of the product (Table 1, entry 1).
13
C NMR spectrum of the crude
various linkages did not overlap with each other; thus, the ratio of polymer product versus cyclic product and the composition of the polymer backbone could be approximately defined on the basis of integration of the 1H NMR signals. The molar percentages of cyclic product and various linkages are listed in Tables 1 and 2. This represents the first time that the structure of a copolymer formed from the copolymerization of a cyclic ether and CS2 has been described in detail. Notably, the cyclic product C1 (trimethylene trithiocarbonate) (TMTTC) could be separated and purified from the crude product. The crude product was first precipitated in hot cyclohexane, followed by a filtration, and the clear yellow liquid was collected. Upon slow cooling down in temperature, TMTTC crystallized as a yellow crystalline product, which was suitable for X-ray analysis. The crystallographic data of TMTTC are reported in Figure 4 and Tables S1 and S2). The 1 H NMR spectrum of the TMTTC is shown in Figure S1. Effects of Monomer Feed Ratio. The effect of monomer’s feed ratio was investigated, and the results are shown in Table 1 and Figure 5. The feed ratio did not have a big effect on the formation of the L5 and C1, the molar ratio of these two species did not vary significantly in the copolymer, remaining stable around 45% and 30%, respectively. However, when the
identified by 1H NMR spectroscopy. On the basis of the reported 1H NMR assigned peaks for the model species shown in Scheme 1, we determined linkages L1−L5 and C1 present in the copolymer derived from eq 1 as illustrated in the 1H NMR spectrum of the crude reaction product in Figure 2. 1 H NMR signals a1 (4.66 ppm), a2 (4.50 ppm), a3 (4.29 ppm), a4 (4.22 ppm), and a5 (3.45 ppm) have previously been assigned to the protons in the linkages of −S(CS)O−,8 −O(CS)O−,8 −S(CO)O−,9 −O(CO)O−,10−12 and −S(CS)S−,13 respectively. In this instance, the linkage −S(CO)S− 14 was not observed in the 1H NMR spectrum of the crude copolymer. Two cyclic products (C115 and C2 in Scheme 1) were observed in the 1H NMR spectrum; however, 5527
DOI: 10.1021/acs.macromol.5b01251 Macromolecules 2015, 48, 5526−5532
Article
Macromolecules Table 1. Effect of Monomer Feed Ratio entrya
CS2/OX (molar ratio)
convb (%)
L1c (%)
L2c (%)
L3c (%)
L4c (%)
L5c (%)
C1d (%)
Mne (kg/mol)
PDIe (Mw/Mn)
1 2 3
1/1 2/1 4/1
>99 >99 >99
3 6 17
8 8 16
40 34 20
5 5 2
44 48 45
24 36 29
7.3 6.5 13.7
2.56 1.76 1.70
a
The reaction was performed in the neat oxetane (OX) (7.7 mmol; (Salen)CrCl and PPNCl were employed, catalyst/cocatalyst = 1/1, catalyst/ epoxides = 1/1000, temperature is 80 °C) in a 10 ml stainless steel reactor for 12h. bThe conversion of the oxetane was determined by 1H NMR. c The molar ratio of different linkages on polymer backbone which was determined by 1H NMR. dThe molar ratio of C1 in crude product which was determined by 1H NMR. eDetermined by gel permeation chromatography in THF, calibrated with polystyrene standards.
Table 2. Effects of Reaction Temperature entrya
T (°C)
convb (%)
L1c (%)
L2c (%)
L3c (%)
L4c (%)
L5c (%)
C1d (%)
Mne (kg/mol)
PDIe (Mw/Mn)
1 2 3 4 5 6 7 8
RT 40 50 60 80 90 110 120
0 58 70 90 >99 >99 >99 >99
48 41 15 3 1 0 0
4 14 7 8 1 0 0
26 18 39 40 55 55 50
99 >99 >99 >99 >99 98 80 0
3 0 0 2 3 4 4
8 4 1 4 10 14 47
40 57 60 55 42 37 2
5
