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Nov 21, 2016 - Poly(trimethylene monothiocarbonate) from the Alternating. Copolymerization of COS and Oxetane: A Semicrystalline Copolymer. Hai-Lin Wu...
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Poly(trimethylene monothiocarbonate) from the Alternating Copolymerization of COS and Oxetane: A Semicrystalline Copolymer Hai-Lin Wu,† Jia-Liang Yang,† Ming Luo,† Rui-Yang Wang,† Jun-Ting Xu,† Bin-Yang Du,† 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: A semicrystalline poly(trimethylene monothiocarbonate) (PTMMTC) has been synthesized via the selective and alternating copolymerization of carbonyl sulfide and oxetane. This reaction was catalyzed by (salen)CrCl accompanied by organic bases over a wide range of temperatures from 40 to 130 °C. PTMMTC is shown to exhibit similar crystallization behavior to high-density polyethylene (HDPE), i.e., being spherulite and possessing melting temperatures (Tm) up to 127.5 °C and a degree of crystallinity (Xc) of up to 71%. Moreover, PTMMTC has a wide processing temperature window of ca. 100 °C.

P

(PCL), which is synthesized by the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) and exhibits a low Tm of about 60 °C.16 In general, enantiopure catalysts or monomers are not required for the synthesis of such polymers which greatly simplifies their large-scale production. Additionally, upon increasing the number of successive methylene groups, higher Tm can be achieved. For instance, polypentadecalactone, with 14 methylene units sandwiched between ester groups, exhibits a Tm of around 100 °C.17 It is well-established that the introduction of sulfur in place of oxygen atoms in the backbone of polymers can enhance the thermal properties of polymers.18−20 In general, sulfurcontaining polymers have good to excellent thermal and mechanical properties, electrical and optical functions, and other outstanding properties such as adhesion to metals, bacteria, and biocompatibility.21 In polyesters, the replacement of an oxygen atom in the ester linkage by sulfur results in an improvement of its thermal properties.18,19 For example, poly(ε-thiocaprolactone) has a Tm of 45 °C higher than PCL.22 Unfortunately, polythioesters are not biodegradable.23 However, poly(monothiocarbonate)s are potentially biodegradable because they possess ester linkages in their backbone.20,24 Traditional preparation of poly(monothiocarbonates)s generally involves the polycondensation of thiols and phosgene or the ROP of cyclic thiocarbonates which are generally derived

olyolefins, a class of nondegradable polymers with excellent physical and thermal properties, are widely utilized in an array of applications. Prominent among these materials are polyethylene (PE) and polypropylene (PP), where consumption in China alone in 2015 was over 42 million tons.1 Importantly, since polyolefins are not biodegradable, they represent a major environmental hazard for humans and wildlife.2 There are efforts to develop biodegradable polymers which exist for a short time in nature to replace polyolefins.3,4 Typically, polymers with hydrolyzable backbones, such as ester linkages, are susceptible to biodegradation.2,3,5 Although numerous types of biodegradable polymers have been developed, it is still challenging to synthesize biodegradable polymers with good thermal and mechanical properties and a satisfying trade-off between physical properties during processing/service and biodegradability.6−9 It is useful to balance physical properties and biodegradability of polymers by synthesizing polymers with tunable crystallinity. The formation of crystalline polymers depends on intermolecular interactions of polymer chains with different chain conformations. Basically, there are two types of biodegradable crystalline polymers. One is a polymer containing chiral carbon atoms, i.e., the formation of polymer chains with chiral carbon atoms which strengthens their intermolecular interactions for crystallization.10 Furthermore, groups such as esters are introduced into the polymer chains for biodegradability.8,11−15 An example of this type polymer is polylactide (PLA), which has a melting point (Tm) of 175−185 °C.14,15 The other type of biodegradable crystalline polymer does not contain chiral carbon centers. A well-studied example is polycaprolactone © XXXX American Chemical Society

Received: October 20, 2016 Revised: November 9, 2016

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Macromolecules from thiols and phosgene.25−29 Recently, we synthesized poly(monothiocarbonate)s directly from the selective and completely alternating copolymerization of carbonyl sulfide (COS) and epoxides via metal catalysis.20,24,30,31 Carbonyl sulfide is a potentially low-cost sulfur-containing C1 monomer, which upon copolymerization with epoxides provides a “greener” route to supplement or supplant current processes for producing sulfur-containing polymers. However, the synthesis of crystalline COS-based copolymers presents a significant challenge.32 Herein, we report the catalytic synthesis of poly(trimethylene monothiocarbonate) (PTMMTC), a sulfur-containing crystalline copolymer, from the completely alternating copolymerization of COS with oxetane (OX) (Scheme 1). The analogous

copolymerization reaction of COS with the asymmetric monomer, propylene oxide, occurs via the sulfur anion selectively ring-opening at the methylene position of propylene oxide.30 Hence, the coupling of COS with a symmetric cyclic ether is expected to provide a copolymer with a well-defined structure. The copolymerization of COS and oxetane was initially performed using the (salen)CrCl/[PPN]Cl catalyst system at 40 °C for 12 h (entry 1, Table 1). The resulting crude solid brown product was insoluble in most common organic solvents such as chloroform, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, and trichlorobenzene at ambient temperature. The crude PTMMTC product was purified by dissolution in DMSO or TCB at 120 °C followed by precipitation with methanol to afford a pale yellow or white polymeric material (Figure S1-C). Using high temperature GPC with trichlorobenzene as elution at 150 °C, the numberaverage molecular weight (Mn) of this sample was determined to be 4900 g/mol with a polydispersity index (PDI) of 2.22 (Table 1). In addition, the differential scanning calorimetry (DSC) curve of PTMMTC displayed a melting temperature (Tm) of 127.5 °C (Figure S2), a value close to that of HDPE.33 The 1H NMR spectrum of the crude PTMMTC product obtained in a 50:1 mixture of CDCl3 and TCB is shown in Figure 1A. The chemical shift values at 2.04, 2.92, and 4.31 ppm are readily assignable to the protons of a, b, and c methylene units in the monothiocarbonate polymer chain, respectively.25 Similarly, proton signals for the small quantity of cyclic monothiocarbonate produced (6%) are displayed at positions labeled a′, b′, and c′ (see Supporting Information). For a reaction examined in the early stages of the process, only copolymer was observed. Hence, we feel that the copolymer results primarily from the copolymerization of oxetane and COS, as opposed to ROP of the cyclic carbonate. Figure 1B depicts the 13C NMR spectrum of the purif ied copolymer where the three methylene carbon resonances appear at 27.4, 29.0, and 65.7 ppm and a single carbon signal for the monothiocarbonate at 170.3 ppm. No other 13C resonances are observed which is indicative of no oxygen/sulfur exchange processes occurring during the copolymerization reaction.30,34 Hence, the symmetric nature of the oxetane monomer coupled with its selective ring-opening by the sulfur anion of the growing polymer chain assures the formation of a well-defined PTMMTC copolymer.

Scheme 1. Synthesis of PTMMTC from COS/OX Copolymerization Catalyzed by the (Salen)CrCl [Salen = N,N- Bis(salicylidene)cyclohexanediimine] Complex and Cocatalystsa

a

PPNCl: bis(triphenylphosphoranylidene)ammonium chloride; DMAP: 4-(dimethylamino)pyridine; DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene; TBD: 1,5,7-triazabicyclo[4.4.0]dec-5-ene.

Table 1. Fully Alternating COS/OX Copolymerization Catalyzed by (Salen)CrCl/[PPN]Cl Catalytic Systema entry

temp (°C)

t (h)

copolymer selectivityb

TOFc (h−1)

Mnd (g/mol)

PDId (Mw/Mn)

Tme (°C)

ΔHme(J/g)

1 2 3 4 5 6f 7 8g

40 80 100 110 120 120 130 140

12 4 4 4 4 4 4 4

96/4 100/0 99/1 99/1 99/1 91/9 98/2 95/5

4 44 61 58 56 52 31 32

4900 13900 17400 37200 19100 26500 23700 34200

2.22 2.19 2.42 2.36 1.81 1.75 1.71 1.88

127.5 122.4 122.4 117.5 121.9 118.5 123.0 110.5

47.6 50.0 51.0 50.5 48.5 42.8 50.4 30.9

a

The reaction was performed in neat OX (0.3 mL, 4.8 mmol; [Cr]/[PPN]Cl = 1/1, [Cr]/OX = 1/250, COS/OX = 2/1, all in molar ratio) in a 10 mL autoclave. Alternating degrees of all samples were 100% based on high temperature 1H (13C) NMR spectra. bThe molar ratio of the copolymer to the cyclic product, determined by using 1H NMR spectroscopy of the CH2Cl2-soluble part and weighing method (see Supporting Information). c (Mol of epoxide consumed)/(mol Cr h). dDetermined by high-temperature GPC in TCB, calibrated with polystyrene standard (Figure S4). e Determined by DSC with a heating rate of 10 °C/min, N2 atmosphere. Samples were annealed at 140 °C for 10 min and then kept at 110 °C for 8 h under a N2 atmosphere. f0.5 mL of TCB was used. g26% of insoluble part. B

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Macromolecules

Figure 1. (A) 1H NMR spectrum of the crude PTMMTC (500 MHz, CDCl3) (entry 1, Table 1). (B) 13C NMR spectrum of the purified PTMMTC (100 MHz, 1,2-dichlorobenzene-d4, 130 °C) (entry 1, Table 1).

Table 2. Effects of Cocatalyst and Temperature on the OX/COS Copolymerization Catalyzed by (Salen)CrCl Complexa entry

temp (°C)

cocatalyst

copolymer selectivityb

TOFc (h−1)

Mnd (g/mol)

PDId (Mw/Mn)

Tme (°C)

ΔHme (J/g)

1 2 3 4 5f

120 120 120 130 140

DMAP DBU TBD TBD TBD

90/10 100/0 95/5 98/2 99/1

61 58 62 58 34

27900 55700 40900 22700 29300

1.68 1.76 1.79 1.84 1.74

117.7 116.4 108.4 114.1

48.0 24.6 31.3 26.5

a

The reactions were performed in a 10 mL autoclave at proper temperature for a certain time (0.3 mL, 4.8 mmol; [Cr]/[PPN]Cl = 1/1, catalyst/OX = 1/250, COS/OX = 2/1, all in molar ratio), alternating degree of all samples was 100%. bThe molar ratio of the copolymer to the cyclic product, determined by using 1H NMR spectroscopy of the CH2Cl2-soluble part and weighing method (see Supporting Information). c(Mol of epoxide consumed)/(mol Cr h). dDetermined by high-temperature GPC in TCB, calibrated with polystyrene standard (Figure S4). eDetermined by DSC with a heating rate of 10 °C/min, N2 atmosphere. Samples were annealed at 140 °C for 10 min and then kept at 110 °C for 8 h under a N2 atmosphere. fDetermined by GPC in THF, calibrated with polystyrene standard (Figure S8).

insoluble portion was 34 200 g/mol as determined by hightemperature GPC. The effect of the nature of the cocatalyst on the COS/OX copolymerization reaction catalyzed by (salen)CrCl/cocatalyst at 120 °C was investigated, and the results are summarized in Table 2. All afforded copolymers possessed a fully alternating structure as revealed by their 13C NMR spectra. The utilization of 4-(dimethylamino)pyridine (DMAP), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as cocatalysts provided similar TOFs of 58−62 h−1, which are close to that observed for [PPN]Cl (entry 5, Table 1). Of note, polymerization reactions catalyzed by the (salen)CrCl/DBU system led to a higher Mn (55 700 g/mol) with a PDI of 1.76 and 100% copolymer selectivity. Also employing TBD as cocatalyst, no O/S ERs were observed for copolymerization reactions carried out at 130 °C. However, upon raising the temperature to 140 °C led to severe O/S ERs (Figure S7). Hence, the resulting copolymer was soluble in common organic solvents at ambient temperature. As mentioned earlier, Endo and co-workers have prepared PTMMTC via the ROP of the corresponding cyclic monothiocarbonate where selective C−S bond cleavage was observed.25 It was reported at that time that the polymer obtained in a reaction carried out in THF at 0 °C displayed a Tm of 116 °C. Herein, we have provided a greener, higher-yield route to poly(monothiocarbonate)s with high Tm values (up to 127.5 °C) without the necessity of first synthesizing the corresponding cyclic monomers. Similar to HDPE, PTMMTC can be rapidly crystallized from the melt or solution upon cooling. The Tm of a polymer strongly depends on its structure and physical state. In this instance, the melting temperature of

The effect of temperature on the COS/OX copolymerization reaction was investigated, and the results are summarized in Table 1. The reaction time was optimized at 4 h for all processes carried out at or above 80 °C. Upon increasing the temperature from 80 to 120 °C, the selectivity for copolymer formation remained high at 99% based on 1H NMR spectra, and no O/S ERs were observed as indicated by high temperature 13C NMR spectra (Figure S3). It should be noted that the corresponding copolymerization reaction between COS/PO exhibited severe O/S exchange at 60 °C using the same catalyst system.30 In general, Mn of the resulting PTMMTC increased with increasing temperature, reaching a maximum value of 37 200 g/mol at 110 °C (Figure S4-A). In contrast to the copolymerization of COS in pure oxetane at 120 °C, the introduction of trichlorobenzene solvent resulted in an increase of Mn (26 500 g/mol, entries 5 and 6, Table 1). The addition of the inert solvent TCB probably depressed the crystallization process of the in situ produced copolymer and thus enhanced polymer growth and molecular weight. Concomitantly, the resulting copolymer’s molecular weight distribution is more narrow in the presence of TCB and at higher reaction temperatures (entries 5−7, Table 1). The higher PDI values, particularly at lower temperatures, are thought to be due to the heterogeneous nature of the reaction system where the precipitation of PTMMTC led to diffusioncontrolled reaction kinetics, especially in the later stages of the process. A further increase in the reaction temperature to 140 °C (entry 8, Table 1) resulted in severe O/S exchange reactions as confirmed by 13C NMR spectroscopy (Figure S5). Approximately 74 wt % of the crude product produced at 140 °C was soluble in CHCl3 and DMSO at ambient temperature (Mn: 6500 g/mol, Figure S6), while Mn of the C

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Macromolecules PTMMTC was observed to decrease with increasing molecular weight. For example, the Tm values of PTMMTC with Mns of 4900−19 100 g/mol were 121.9−127.5 °C (entries 1−3 and 5, Table 1), whereas those of higher Mns of 26 500−55 700 g/mol were 108.4−118.5 °C (entries 4 and 6 in Table 1; entries 1−3 in Table 2). It is possible that at higher temperatures O/S ERs result in the production of trace quantities of undetectable carbonate units which decreases the crystallinity of the copolymer. The melting enthalpy (ΔHm) of PTMMTC was observed in the range of 40−50 J/g and was significantly affected by the choice of cocatalyst employed in the copolymerization process. Of significance, PTMMTC readily crystallizes in bulk since the crystallization peaks were observed upon cooling (Figure 2 and Figure S10) with the crystallization

Figure 3. Powder XRD profiles of annealed PTMMTCs (the same annealing process as in Figure 2).

Figure 4. POM images of melt-crystallized PTMMTC observed at (A) 96 °C (entry 1, Table 1) and (B) 110 °C (entry 4, Table 1). The growth of spherulites is seen in Figure S13. Each scale bar shows 5 μm.

Figure 2. Selected DSC curves of PTMMTC with different Mns: (A) 4900 g/mol (entry 1, Table 1) and (B) 55 700 g/mol (entry 2, Table 2). Samples were held at 140 °C for 10 min and then at 110 °C for 8 h under a N2 atmosphere.

which might result from periodic distortion of the crystal lamellae during crystal growth. The immediate crystallization of the COS/OX copolymer upon cooling represents an improvement over known crystalline polycarbonates produced from CO2 and epoxides. In the latter instances, crystallinity was observed from solution, rather than from the melt, after heating above the Tm value.35,36 Furthermore, all crystalline CO2-based copolymers were synthesized using chiral catalysts or meso-epoxide monomers.8,11−13 In addition, these crystalline polycarbonates have high Tm (e.g., 180−275 °C)35,36 which are close to or higher than their decomposition temperature (Td). This in turn will result in high temperature consumption or decomposition during bulk processing. Coates and co-workers have reported another example of a biodegradable crystalline polymer by mixing isotactic, regioregular chains of poly(propylene succinate) produced by the copolymerization of the corresponding enantiopure epoxide and cyclic anhydride to afford a polyester with a Tm of 120 °C.37 Pertinent to the present study, poly(trimethylene carbonate) produced by the copolymerization of CO2 and oxetane catalyzed by a similar (salen)CrX catalyst was completely amorphous.38 Reasons for the stark difference in PTMC and PTMMTC may be due to different conformations arising from differences in the bond lengths of C−O (1.43 Å) and C−S (1.815 Å) as well as van der Waals radii of O (1.52 Å) and S (1.85 Å) atoms.39 In addition, the asymmetrical monothiocarbonate units could sharply enhance intermolecular interactions of the PTMMTC chains, being partly responsible for its crystallizability. Nevertheless, the crystallization mechanism of PTMMTC is of significant interest and worthy of further investigation.

temperature (Tc) from 62.4 to 95.8 °C. The Tg of PTMMTC was estimated to be between −17 and −25 °C by DSC measurements (Figure S10). However, when there are significant O/S exchange processes occurring, e.g. at 140 °C, during copolymerization, the resultant copolymers display low Tgs of around −41 °C. Powder wide-angle X-ray diffraction (XRD) analysis also confirmed the crystalline nature of this poly(monothiocarbonate). Basically, these copolymers exhibited five diffraction peaks at 16.8°, 19.0°, 22.9°, 29.5°, and 40.8°, demonstrating a typical semicrystalline structure (Figure 3 and Figure S12). The main diffraction peaks at 19.0° and 22.9° are clearly resolved and relatively strong; hence, the degree of crystallinity (Xc) of annealed PTMMTC could be estimated by the XRD profile to be as high as 71%. As might be anticipated, Xc of the copolymer was strongly affected by the reaction temperature of the copolymerization process. Xc was also shown to decrease with an increase in molecular weight of the copolymer (Figure 3). In the case where O/S exchange reactions were observed at 140 °C, Xc of the insoluble portion (entry 8, Table 1) sharply decreased due to the production of other linkages resulting from the O/S ERs. Figure 4 depicts typical polarized optical microscopy (POM) images of melt-crystallized PTMMTC. Growth of spherulites with negative birefringence was clearly observed at 96 °C for low molecular weight (Mn, 4900 g/mol) PTMMTC (Figure 4A). Whereas, higher molecular weight (Mn, 37 200 g/mol) copolymer exhibited concentric banded spherulites (Figure 4B) D

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Macromolecules Purified PTMMTC was found to be thermally stable with only a 5% weight loss at 228.5 °C (Figure S14). Therefore, it has a wide bulk processing temperature window of around 100−110 °C, i.e., the difference between Td and Tm. The temporal change of the remaining mass was isothermally investigated at 170 and 190 °C (Figure 5). The weight loss of

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Synthesis and spectral details of copolymers; polymer characterization, including GPC, DSC, powder XRD, and POM images of crystal growth (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (X.-H.Z.). *E-mail [email protected] (D.J.D.). ORCID

Jun-Ting Xu: 0000-0002-7788-9026 Donald J. Darensbourg: 0000-0001-9285-4895 Author Contributions

H.-L.W. and J.-L.Y. made equal contributions to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the National Science Foundation of the People’s Republic of China (no. 21474083) and the Distinguished Young Investigator Fund of Zhejiang Province (LR16B040001). D.J.D. is thankful to the Robert A. Welch Foundation (A-0923) for the financial support.

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Figure 5. Isothermal TGA results of PTMMTC (entry 4 in Table 1).

PTMMTC was only 0.8% at 170 °C over 30 min and only 1.1% at 190 °C over 10 min. Such thermal stability meets the general requirement for the heat processing of thermoplastics. Additionally, unlike crystalline PLA14,15 or aliphatic polycarbonates,8,11−13 PTMMTC is a tough material. For example, a wafer of the copolymer having a diameter of 1.6 mm and a thickness of 1.0 mm fabricated by a hot press method could be bent and recovers immediately in hand (Figures S1-D and S15). In conclusion, we have uncovered a semicrystalline copolymer derived from the completely alternating copolymerization of the two monomers, COS and oxetane, in the presence of a (salen)CrCl/cocatalyst system. The copolymerization process could be efficiently performed at high reaction temperatures (100−130 °C), which improved catalytic activity without the occurrence of oxygen/sulfur atom exchange and the production of significant quantities of cyclic byproducts. The molecular weights of these monothiocarbonate polymers were found to be up to 55 700 g/mol. Therefore, this process represents an industrial feasible method for synthesizing these materials which meets several of the green criteria. Herein, we report the synthesis of thermally stable polythiocarbonate devoid of chiral carbon centers which can rapidly crystallize in bulk or in solution. These semicrystalline copolymers display Tm values up to 127.5 °C, which is similar to that of HDPE. Because one portion of the TMMTC unit contains a degradable ester bond, these copolymers should widen the scope of degradable polymeric materials. Our ongoing efforts seek to develop more efficient catalysts, explore crystallization mechanisms, and investigate the physicochemical and biodegradable properties of the copolymers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02285. E

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DOI: 10.1021/acs.macromol.6b02285 Macromolecules XXXX, XXX, XXX−XXX