Alternating copolymerization of epoxides with carbon dioxide or cyclic anhydrides using bimetallic nickel and cobalt catalysts: Preparation of hydrophilic nanofibers from functionalized polyesters
Graphical abstract
New bimetallic bis(benzotriazole iminophenolate) or bis(benzothiazole iminophenolate) nickel and cobalt complexes were developed for versatile ROCOP of internal epoxides with CO2 or phthalic anhydride (PA). Particularly, di-Co complex 3 was able to copolymerize 4-vinyl-1,2-cyclohexene oxide with PA to afford the vinyl-functionalized polyester, which could be further utilized for the preparation of hydrophilic nanofiber via functional modification and electrospinning.
Introduction
Recently, increasing attention has been paid to the development of metal-catalyzed ring-opening copolymerization (ROCOP) of epoxides with carbon dioxide (CO2) or cyclic anhydrides because it allows for obtaining biodegradable polycarbonates or polyesters (Scheme 1) [[1], [2], [3], [4]]. High-molecular weight and stereocontrolled polymeric materials with a broad range of polymer backbone architectures could therefore be produced from this synthetic route. This kind of process also gives the advantages of CO2 removal and reuse while CO2 is utilized as the comonomer source. Extensive efforts have demonstrated that a various array of catalyst systems comprising of Al, Co, Cr and Zn complexes containing diverse ancillary ligands such as porphyrin, β-diiminate, anilido-aldiminate, salen-type, amine-phenolate and macrocyclic derivatives have been developed for ROCOPs of such kind [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. Among these studies, metal complexes introduced by salen-type ligands were proven to be effective and versatile catalysts for CO2/epoxides coupling and cyclic anhydride/epoxides copolymerization due to the easily adjustable electronic and steric effects of metal catalysts through ligand modification. As a result, a series of discrete chromium(III) and cobalt(III) complexes bearing ONNO-tetradentate N,N′-bis(salicylidene)cyclohexanediamine (salcy) ligands as the main catalyst components were synthesized for the use of CO2-epoxide or cyclic anhydride-epoxide ROCOP [3,[14], [15], [16]]. Particularly, the [Co(III)(salcy)Cl] complex was found to be active both in the copolymerization of 1,4-cyclohexadiene oxide with CO2 or phthalic anhydride (PA), whereas the bis(triphenylphosphine)iminium chloride was employed as a nucleophilic co-catalyst in the case of the former reaction [17]. Furthermore, Lee and co-workers developed a new class of bifunctional catalysts combining (salen)Co(III) units with quaternary ammonium salts in one molecule for catalyzing the ROCOP of CO2 with propylene oxide (PO) [[18], [19], [20], [21]]; the (salen)Co(III) acetate complex tethering four cationic species was demonstrated to be a highly active catalyst in both copolymerizations of PO/CO2 and PO/PA, generating the corresponding copolymers with excellent catalytic activity and very high molecular weights (Mn up to 300,000 g/mol and 170,000 g/mol for the PO/CO2 and PO/PA copolymerization, respectively) [19,21]. Recently, Williams et al. have shown the synthesis and catalysis of a family of di-nuclear zinc complexes [22], and the di-Zn salen complex with the more flexible backbone exhibits the best activity for the versatile ROCOP of cyclohexene oxide (CHO) with CO2 or PA, which was also utilized to prepare poly(ester-block-carbonate) through the one-pot terpolymerization of PA/CO2/CHO. Most recently, Lu and co-workers reported excellent bimetallic catalyst systems for the ROCOP of epoxides with CO2 or cyclic anhydrides [[23], [24], [25], [26]].
Polyester, one of the mostly used commodity polymers, has been widely used for a variety of applications in our daily life, such as fibers [27], plastics [28], packaging materials [29] and drug delivery carriers [30]. In particular, the usage amount of polyester in textile applications is much higher than that in other applications [31]. Due to its excellent mechanical and thermal properties, polyester has been considered as an alternative to natural polymers such as cotton for producing textiles. Relative to cotton, polyester, however, has a very poor water retain capability, thus significantly limiting its applicability in the textile industry [32,33]. To improve the innate drawback, numerous approaches have been employed through introducing polar groups into the polymer backbone [31]. Of these approaches, the surface modification via plasma treatment [34] and the Denier reduction by aminolysis or hydrolysis [35] are the most two commonly used methodologies for enhancing the hydrophilicity of polyester. Nonetheless, those methods would inevitably cause the chain scission of the ester linkages, subsequently affecting the mechanical strength and thermal stability of the resulting fabrics. To develop a new functional fiber material to improve the hydrophilic property, we are interested in investigating the functionalized polyester, poly(PA-alt-VCHO) (VCHO = 4-vinyl-1,2-cyclohexene oxide) [36]. The vinyl functional moiety on the polymeric backbone thus can be incorporated with polar groups through thiol-ene click functionalization but without the concern on deteriorating the molecular structure.
Because bis(benzotriazole iminophenolate) (BiIBTP) derivatives were able to efficiently chelate and thus stabilize the formation of bimetallic complexes, our group has received increasing interest on their structures and catalysis applications of nitrogen heterocycle-containing metal phenolates [37]. Previous investigations have focused on the catalysis of these BiIBTP supported di-nuclear nickel and cobalt complexes in ROCOPs of carbon dioxide-epoxides or cyclic anhydride-epoxides [[38], [39], [40]]. The former di-nickel(II) di-trifluoroacetate complexes were demonstrated to effectively catalyze CO2/CHO copolymerization with a high turnover frequency in a controllable fashion, producing a narrowly dispersed poly(cyclohexene carbonate) with a large molecular weight and highly carbonate linkages [40]. Despite the relatively lower catalytic performances for isostructural cobalt(II) analogs, BiIBTP-ligated di-Co acetate complex was proven to be a versatile catalyst for coupling of CO2 with epoxides and ROCOP of CHO with PA [39]. To further study structure-activity relationships of metal complexes incorporated by nitrogen heterocycle-containing bis(iminophenolate) derivatives, we are motivated to prepare the new ligand precursor, bis(benzothiazole iminophenol) (BiIBThP-H) with the structurally-related N-heterocycle group. Moreover, induction of the asymmetric backbone on the salen ligand of metal catalyst was shown to give the enhanced stereoselectivity of epoxide copolymerization [41]. This leads us to continue to develop novel metal acetate catalysts coordinated by N-heterocycle containing bis(iminophenolate) derivatives with the asymmetric backbone and to apply these new complexes as versatile catalysts for the coupling of CO2 with epoxides and copolymerization of cyclic anhydrides with epoxides. Herein, we describe the synthesis, structure and versatile catalysis for CO2-epoxide and cyclic anhydride-epoxide copolymerization of BiIBT(h)P-featuring di-nuclear nickel and cobalt derivatives. The resulting polyester bearing the vinyl functionality was modified to give the tertiary amine-modified polyester through thiol-ene click functionalization. Sequentially, the tertiary amine-modified polyester was further converted to nanofibers via electrospinning. The fibrous property and wetting ability of the resulting nanofibers were also studied and discussed.
Section snippets
Syntheses and crystal structure characterization
The synthetic routes of di-nickel and di-cobalt complexes 1–6 supported by bis(benzotriazole iminophenolate) (BiIBTP) or bis(benzothiazole iminophenolate) (BiIBThP) ligands are depicted in Scheme 2. The N-heterocycle containing salen-type pro-ligands, C8EtBiIBTP-H2, C1EtBiIBTP-H2 and C1EtBiIBThP-H2, were prepared in good yield from the varied benzotriazole phenolate or benzothiazole phenolate substituted benzaldehyde [42,43] with 0.5 molar equiv. of 1,3-diaminopentane in Et2O. The bimetallic
Conclusions
Seven new nickel and cobalt complexes based on bis(benzotriazole iminophenolate) or bis(benzothiazole iminophenolate) derivatives with the asymmetric backbone have been synthesized and fully characterized by X-ray single-crystal structure analysis. The molecular structures of these complexes are ascribed to di-nuclear metal(II) species (metal = Ni or Co) containing a hexadentate BiIBT(h)P ligand and two acetate co-ligands. Versatile catalysis towards ring-opening copolymerization of internal
Acknowledgments
We gratefully acknowledge the financial support from the Ministry of Science and Technology, Taiwan (MOST 106-2113-M-005-003 to B.-T. Ko and MOST 104-2221-E-035-078-MY2, MOST 106-2221-E-035-083, MOST 106-2632-E-035-001 to C.-K. Chen).
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