**4. Conclusions**

In this study, surface-modified ZnGA samples were prepared by treatment with di fferent metal chloride salts and used as new, highly active catalysts in the copolymerization of CO2 and PO under a relatively low CO2 pressure of 2.0 MPa. Among the various metal-treated ZnGA catalysts, ZnGA-Zn-10−<sup>3</sup> was found to be highly active with a TON of 100.1 g PPC/g catalyst, which is 38.3% higher than that of std-ZnGA (TON = 72.4). The FeCl3− treated catalyst ZnGA-Fe-10−<sup>4</sup> produced the polymer with the highest molecular weight (262 kg/mol) with a TON of 90.9. These results indicate that this simple and industrially viable procedure e ffectively increases the catalytic activity of ZnGA. This economically beneficial method promotes the use of the ZnGA-M catalytic system for the industrial applications in the production of biodegradable thermoplastics from PO and CO2.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4344/9/11/892/s1, Figure S1: PXRD pattern of std-ZnGA and PXRD pattern calculated from the crystal structure of ZnGA via Mercury 3.7, Figure S2: (a) FT-IR spectra of std-ZnGA and ZnGA-M catalysts, (b) PXRD patterns of std-ZnGA and ZnGA-M catalysts, Figure S3: SEM images of std-ZnGA and ZnGA-MCln, Figure S4: STEM image and EDS mapping. (a) STEM image of ZnGA-Al-1; STEM-EDS mapping of (b) Zn (red)and (c) Al (green) elements in ZnGA-Al-1; (Scale bar: = 200 nm), Figure S5: FT-IR spectra of std-ZnGA and ZnGA-Al-1, Figure S6: Expected coordination modes of MCln on the std-ZnGA surface, Figure S7: 1H NMR spectrum of PPC from entry 3 in Table 1, Figure S8: 13C NMR spectrum of PPC from entry 3 in Table 1, Figure S9: GPC elugrams for some selected polymers from Table 1, Figure S10: DSC curves of selected polymers from Table 1.

**Author Contributions:** S.Y. and S.P. have designed the experiments. S.P. conducted the experiments. S.P. and S.Y. wrote the manuscript and S.Y. supervised the project. All authors reviewed the manuscript.

**Funding:** This research was supported by a Korea CCS R&D Center (KCRC) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (Ministry of Science, ICT & Future Planning) (No. 2014M1A8A1049300).

**Acknowledgments:** We acknowledge the financial support provided by the Korea CCS R&D Center (KCRC) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (Ministry of Science, ICT & Future Planning) (no. 2014M1A8A1049300).

**Conflicts of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
