Polyurethane (PU) elastomers presented the excellent abrasion resistance of both rubber and plastics. Conventional PUs were produced by using polyether polyol, which are inferior in terms of tensile strength, abrasion resistance, and heat resistance^[1~3]. Accordingly, it’s necessary to prepare PUs with good physical properties. The copolymerization of cyclic acid anhydride (AA) with cyclic ether was an easy and economic candidate to overcome these drawbacks and was thus tested by Inoue et al in 1969 with diethylzinc as a catalyst^{[4, 5]}. Polyester synthesized by copolymeri-zation of propylene oxide (PO) and phthalic anhydride (PA) has been widely used in the research of polyurethane (PU) elastomers, which improved the physical properties of PUs, such as abrasion resistance, tensile strength, and heat resistance^{[6, 7]}. Phthalic anhydride (PA) attracts more attentions owing to its commercial purpose, and various metallo-organic compounds catalysts have been used for the copolymerization of PA with epoxides as a viable greener route for the synthesis of polyesters^{[8, 9]}. However, all of these catalytic systems showed only moderate activities or required tedious synthesis procedures that might be commercially unacceptable.

Ball milling, known as mechanochemistry, is of great interest in inorganic and organic synthesis, cocrystals and even successful industrialization in pharmaceutical aspects^[10]. We firstly reported the green and efficient technique for DMC synthesis by mechanochemical method for copolymerization of CO₂ and propylene oxide (PO), which reveals a clear shortcut in terms of free or limited solvent employed^[11~15]. Zn-Ni DMC was synthesized successfully previously without any solvent by ball milling for the copolymerization of CO₂ and propylene oxide (PO)^[15]. Herein, from a continuing study, we synthesized Zn-Ni DMCs with different co-complexing agents (co-CAs) under the protocol of ball milling here. Interestingly, we found that the Zn-Ni DMCs were effective for the copolymerization of PO and PA without solvent. For DMCs, organic co-CAs such as PEG-PPG-PEG are incorporated into the catalyst matrix to make them more active during ball milling. For poly (PO-co-PA), polymers exhibited high yields with relative high molecular weight and narrow polydispersity index.

Zinc chloride (ZnCl₂), potassium hexacyano-nickelate (II) [K₂Ni (CN)₄], tert-butyl alcohol (t-BuOH) and phthalic anhydride (PA), poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (PEG-PPG-PEG), polyethylene glycol (PEG), poly (propylene glycol) (PPG), poly (tetra-methylene ether glycol) (PTMEG) were of analytical purity. Propylene oxide (PO) (≥98%) was used without further purification.

The DMC catalysts were characterized by Fourier transform infrared spectroscopy (FT IR) (Model: 6700/Thermo Fisher Scientific), elemental analysis (EA) (Vario EL Cube elementar), and scanning electron microscopy (SEM) (Model: JEM-7100F). Spectroscopic analyses of products were performed using FT IR and a Bruker NMR spectrometer (Model: Bruker AV 400 MHz) with ¹H and¹³C probes, CDCl₃ as the solvent. Number average molecular weight (MWn) and polydispersity index (PDI) of polymer products were estimated using a gel permeation chromatography (GPC) system. Tetrahydrofuran (THF, HPLC grade) was used as an eluent.

Reaction conditions involved a specific mol ratio mixture of ZnCl₂ and K₂Ni (CN)₄ with a small amount of t-BuOH (ZnCl₂:K₂Ni (CN)₄:t-BuOH=10:1:0.1). The mixture was put into a Planetary Ball Mill equipped with four stainless steel vessels. PEG, PPG, PTMEG and PEG-PPG-PEG as co-CAs were added to the four vessels, respectively (Tab. 1). The mixture was ground with two 20mm, ten 10mm and twenty 6mm steel balls in each vessel (100mL) at 50Hz for 30 min. The resulting products were washed thoroughly with deionized water and t-BuOH (1:1) so as to remove excess reactants, then filtered and dried under vacuum at 55℃. The elemental analysis results of the DMC complexes were shown in Tab. 1. It is obvious that the ratio of C/N is over 1:1, which is a good confirmation of containing co-CAs in Zn-Ni DMCs synthesized by ball milling^[13].

Copolymerization of PO and PA was carried out in an autoclave equipped with a magnetic stirrer and a heating device. Dried DMC (0.2g) and PA (3g) were added into the autoclave and then purged several times with CO₂, followed by adding a desired amount of PO (20mL) under vacuum conditions. The reactor was then heated at a desired temperature (110℃). After 6 h, the autoclave was cooled down to room temperature and opened. The products were dissolved in dichloromethane, stirred for 15 min, then poured into hot water containing 5% HCl, isolated, and dried under vacuum to constant weight.

The FT IR spectra of Zn-Ni DMCs synthesized by ball milling revealed characteristic absorption peaks similar to that of prepared in conventional solvent-based method^[16]. As shown in Fig. 1, the ν (CN) of K₂Ni (CN)₄ shifted to 2186 cm^-1 in DMC-1 and DMC-2, 2188 cm^-1 in DMC-3, and 2193 cm^-1 in DMC-4, respectively. In the far-IR region, Ni-CN stretching vibrations appear in 630~580 cm^-1. Because of the existence of complexing agent t-BuOH, the -OH in all DMCs showed a band assigned to stretching vibration absorption at approximate 3500 cm^-1. Taking DMC-1 for example, -OH stretching vibration absorption appeared at 3505 cm^-1, while C-H stretching vibration absor-ption peak appeared at 2916 cm^-1, Ni-CN flexural vibration absorption appeared at 455 cm^-1. Owing to the coordination of Zn²⁺with C≡N, the C≡N bond was weakened and a resembling C=N bond was formed at 1617 cm^-1. The appearance of C-H (from zinc acetate) flexural vibration absorption confirmed that the co-CAs were readily introduced into the structure of DMC via ball milling^[13].

Fig. 2 showed the SEM images of Zn-Ni DMCs prepared by ball milling. All the DMCs presented smaller but highly coagulated and nonuniform particles. Nanosized DMCs indicated that the co-CAs indeed allowed for a more controlled and fine particle size of the DMC materials. During the process of ball milling, reactants are fully reaction and form small particles on nanoscale, which ensured a better dispersion of the monomers and better exposure of active sites to the reactants, thus yielding high catalytic activity^{[17, 18]}.

Copolymerization of PO and PA occurred at 110℃ for 6 h. The FT IR spectra of the polymers catalyzed by Zn-Ni DMCs were shown in Fig. 3. It was obvious that there were characteristic absorptions at 1730 cm^-1 [C=O (ester)], 1250 cm^-1[C-O-C (ester)], and the aromatic C=C bending absorption of poly (PO-co-PA) at 1590cm^-1, which confirmed the products formations. There was no characteristic absorption at around 1810cm^-1 of the copolymers, indicating no existence of PA homopolymer units. A broad absorption at around 3500cm^-1 was assigned to OH demonstrating that it was hydroxylated polymer^{[19, 20]}. Taking DMC-3 for example, characteristic absorptions of C=O (ester), C-O-C (ester), and C=C appeared at 1738cm^-1, 1257cm^-1and 1589 cm^-1, respectively.

¹H NMR and ¹³C NMR analyses in CDCl₃ shown in Fig. 4 and Fig. 5 also confirmed the product forms. The characteristic peaks for poly (PO-co-PA) appeared at δ7.5~7.8 [CH=CH], 5.1~5.5 [OCH (ester)], 4.1~4.4 [OCH₂(ester)], and 1.3~1.5 [-CH₃(ester)]. Taking DMC-4 for example, ¹H NMR: δ a₄=7.71, b₄=7.51, c₄=5.44, d₄=4.40, e₄=1.36. Considering the spectroscopic assignment and the fact that there was no homopolymerization during this process, the polyester backbone was composed of cyclic PA-PO alternating segments and partial PO self-propagation segments^{[19, 20]}. ¹³C NMR of poly (PO-co-PA) appeared at δ 166~169[-COO-], 128~133[-C-C=C-], 61~76[-O-CH₂-CH-O-], 14~19[-CH₃] demonstrated the product formation, while due to the different polymerization index there were similar but not the same chemical shifts. For DMC-2, ¹³C NMR: δ 167.6[-COO-], 129~131[-C-C=C-], 65~71[-O-CH₂-CH-O-], 16~19[-CH₃].From ¹H NMR and ¹³C NMR, it was clear that the polymers were poly (PO-co-PA). The content of polyester was calculated by ¹H NMR (Tab. 2).

Copolymerization of PO and PA occurred by using the same amounts of catalyst, PA and PO to investigate the catalytic activity of different DMCs. All the DMCs obtained by ball milling exhibited high yields ranging from 79.05% to 90.79% (Tab. 2), high content of poly (PO-co-PA) (from 62% to 95.5%), and narrow PDI (from 1.23 to 1.54) of the resulted copolymers. Among the copolymers produced by different DMC catalysts, DMC-2 had the highest yield (90.79 %) and MWn (2542g/mol), while the DMC-4 had the narrowest PDI (1.23). The DMC catalysts with small amount of co-CAs during the ball milling synthesis were competitive with those synthesized in traditional way as reported in literatures, with regard to the catalytic activity, yields, and PDI in the reaction between PO and acid anhydrides (AA)^[6~9]. When comparing the properties of the obtained copolymers it can also be concluded that incorporation of co-CAs played a significant role in DMC formation and subsequent copolymerization.

Zn-Ni DMCs catalysts with little co-CAs were rapidly synthesized by ball milling within minutes. The incorporation of co-CAs increased the catalyst activity of DMCs. The structure of DMC catalysts were characterized by SEM, EA, and FT IR. The DMC complexes successfully catalyzed the copolymerization of PO and PA without solvent under controlled condition. The Zn-Ni DMC with PPG had the highest yield and poly (PO-co-PA) content, while DMC with PEG-PPG-PEG presented narrowest PDI.