This thesis "SYNTHESIS OF POLYPROPYLENE CARBONATE FROM PROPYLENE OXIDE AND CARBON DIOXIDE" was prepared by ANG RUI REN and submitted in partial fulfillment of the requirements for the degree of Master of Engineering Science at Universiti Tunku Abdul Rahman. The ZnGA catalysts were then used to copolymerize propylene oxide and carbon dioxide to produce PPC.
Background
In the copolymerization reaction of CO2 with oxirane, it was found that organometallic catalysts, which are made of diethylzinc and a compound with two active hydrogens, are capable of efficiently synthesizing alternating polycarbonate (Lubczak and Cisek-Cicirko, 2002). Examples of compounds with two active hydrogens are water, aromatic dicarboxylic acids, primary amines, di- or trihydroxybenzenes, polymers, etc. (Zarzyka-Niemiec, 2007).
Problem Statements
Aims and Objectives
Project Scope
The monomers
With a demand of more than 1.5 million tons worldwide each year, PC is an important class of commercial polymers (Gross et al., 1999). First, the method reported by Inoue et al. 1969) is a chain growth process compared to BA-PC that involves step growth mechanisms.
Zinc catalyst compound
Diethylzinc
Consequently, the structure and activity of the catalyst were difficult to study, which delayed the design process of the catalyst. Finally, the low catalytic activity and high cost of the diethylzinc made the heterogeneous catalyst not industrially feasible.
Zinc glutarate
- Effects of zinc sources on ZnGA
- Effects of glutarate sources on ZnGA
- Effect of structure and morphology on zinc glutarate
- Novel discoveries on zinc glutarates
It has been found that the crystallinity of the catalyst has a significant influence on the copolymerization activity (Meng et al., 2002). The previous section shows that the catalytic activity of zinc glutarate in the copolymerization mainly depends on the surface area of the catalyst. Particle size alone could not be used to determine the surface area of the catalyst (Kim et al., 2005).
The large surface area of ZnGA-PE3 produced only relatively lower molecular weight PPC, which is a defect in the copolymerization process. Perhaps in the near future, new ways to increase the molecular weight of the copolymer may be discovered (Kim et al., 2005).
Double metal cyanide complexes
To remove these cations, montmorillonite, which is a type of natural clay that has cation exchange ability, was treated with acid according to the literature (Rhodes and Brown, 1993; Clark et al., 1994). However, ZnGA-MMT in copolymerization only produced low molecular weight PPC, which is one of the problems encountered by researchers. 2005) performed the copolymerization of CO2 and epoxides using DMC which was synthesized from zinc salt and K3Co(CN)6 using tertiary butyl alcohol and poly(tetramethylene ether glycol) as complexing reagents. From Table 2.6, it can be observed that high activity and the presence of carbonate content would be favored in the copolymerization of CO2 and alicyclic oxide (such as cyclopentene oxide and CHO) compared to CO2/alkylene oxide copolymerizations.
Moreover, the inclusion of DMC in the copolymerization of epoxides and carbon dioxide has several disadvantages. In addition, the copolymerization conditions involving the DMC catalyst were harsh, with temperatures in the range of about 80 to 130 oC and pressures of 50 to 100 atm.
Bis(phenoxy) zinc complexes
It was found that the degree of copolymerization was increased by the intramolecular proximity of the second metal center, further indicating a binuclear mechanism (Xiao et al., 2005). In addition, Darensbourg made several studies on several large bis(phenoxy)cadmium complexes, trying to reveal the mechanism and coordination chemistry during the copolymerization reaction (Darensbourg et al., 1998). Compared to Zn(II), the relative mildness of Cd(II) enables the separation and characterization of several cadmium epoxide carboxylates (Darensbourg et al., 1995).
Based on the observation that with very bulky ligands, it can be concluded that oxirane was initially introduced into the Zn-aryloxide bond before CO2 can be introduced (Darensbourg et al., 1997). On the other hand, it can also be concluded that the copolymerization reaction needs only one coordination site, while two sites are required for sequential oxirane insertion (Darensbourg et al., 2000).
During a study by Allen et al. several BDI complexes were synthesized with different substituents (Allen et al., 2002). By reducing the reaction temperature to 25oC (see the case of BDI-2a), the formation of PC was suppressed and replaced by the formation of poly(propylene carbonate) (PPC) by almost 85% (Allen et al., 2002). For the copolymerization of CHO and CO2, it has been shown that modifications to the N-aryl ligands (which are the R1 and R2 groups) would greatly affect the catalytic activity of BDI (Byrne et al., 2004).
No TOF was observed for the methyl-substituted BDI complexes, while TOF of 729 h-1 was seen for the copolymerization using isopropyl- and ethyl-substituted BDI (Moore et al., 2003). In an unsymmetrical ligand framework where an electron-withdrawing group was substituted on the diimine parent chain (R1 or R2 = CN or CF3), the catalytic activity was considered to be enhanced with TOF of about 424 h-1 (Moore et al., 2002) Kröger et al., 2005).
Other zinc complexes
Aluminium, chromium complexes and cobalt catalysts
Subsequently, Cr(III) salen complexes were reported to be highly active in coupling terminal oxiranes and CO2 below a reaction temperature of 75 °C at 7 bar (Miller and Nguyen, 2004). Chromium was first specifically used as a copolymerization catalyst in the year 2000 when Mang and his colleagues reported some interesting porphyrinato-chromium complexes (Mang et al., 2000.). In 1978, Soga and colleagues reported the first use of cobalt catalysts for the copolymerization of PO and CO2 (Soga et al., 1978).
It has been reported that higher reaction temperatures (> 100ºC) will favor the formation of cyclic carbonates. It was also found that even if the reaction took place at lower temperature, the initiator group and cocatalyst would still be the determining factors in the production of polycarbonates (Lu and Wang, 2004; Paddock and Nguyen, 2004; Lu et al., 2004; Paddock et al., 2004; Paddock et al., 2004).
Cadmium, manganese and rare earth metal coordination catalysts
The copolymerization of PO and CO2 using rare earth metal catalyst was first made possible by Chen, Shen and Zhang (1991). In addition to the discovered rare earth metal catalyst, rare earth metal catalyst system such as Y(CF3COO)3 or Y(RC6H4COO)3 where (R = H, OH, Me or NO2) together with ZnEt2 and glycerin were developed using yttrium carboxylates (Tan and Hsu , 1997). Importantly, this ternary rare earth coordination catalyst was able to generate an alternating polycarbonate.
From the above two studies, it can be seen that rare earth metal catalysts were beneficial in the copolymerization process by shortening the induction periods, increasing the copolymer yield, improving the alternating conditions during copolymerization, and raising the molecular weight of the copolymer (Qin and Wong, 2010). Further studies are needed to investigate and improve the catalytic activity of this type of heterogeneous catalyst to improve the properties of the copolymer product.
Copolymerization conditions
It was therefore concluded that the CO2 pressure has no influence on the molecular weight and distribution of the copolymer product. Apart from the CO2 pressure, PO loading plays an important role in determining the yield of the copolymer and the molecular weight of the final product. From the results shown above, it can be seen that the copolymer yield has the highest value of 64.0 g per grams of catalyst with 100 ml of PO added.
The copolymer yield increases along with PO loading and then decreases above 100 mL PO loading. The high solubility of PO in the product will increase the miscibility of the reaction medium (which consists of PO and CO2), producing a high yield but high molecular weight copolymer.
Future works
Bhd., Malaysia while acetone, dichloromethane, hydrochloric acid and ethanol were purchased from Labchem Sdn Bhd., Malaysia.
Preparation of Catalyst
After that, five more sets of experiments were carried out using different reaction times and temperatures. It is important to note that in order to obtain more accurate and precise results, each set of experiments was repeated three times.
Copolymerization
Testing Methods and Characterization
- Fourier Transform Infrared (FTIR)
- X-Ray Diffraction Spectroscopy (XRD)
- Field Emission Scanning Electron Microscopy (FESEM)
- Gel Permeation Chromatography (GPC)
- Differential Scanning Calorimetry (DSC)
- Mechanical Testing
The morphologies of the surface of the catalyst were observed under Hitachi BS340 TESLA scanning electron microscope. To determine the melting temperature Tm, all copolymer samples were subjected to differential scanning calorimetry (DSC) analysis using a Mettler Toledo DSC821e analyzer. The system was purged with dry nitrogen gas at a constant volumetric flow rate of 20 mL/min throughout the measurement.
Tensile strength, percent elongation and tensile modulus for each sample specimen were tested based on the standard of ASTM D1822 using Instron Universal Testing Machine 5582 Series IX tensile tester. The stress-strain curve and other tensile properties such as tensile stress as maximum load, tensile strain at maximum tensile stress and tensile elongation at maximum tensile stress were projected on the tensile data sheet generated by Bluehill software.
Zinc Glutarate (ZnGA) Catalyst
Fourier Transform Infrared (FTIR)
The processing conditions (i.e., reaction temperature and time) significantly affect the properties of the catalyst. As can be seen from the figures, peak a is absent in the FTIR spectrum for each ZnGA catalyst synthesized at a different temperature with a 3-h reaction time. However, as the reaction time slowly increases from 3 hours to 6 hours and 9 hours, peak a is shown in the FTIR spectrum, except for ZnGA synthesized at 40 oC with a 6 hour reaction time.
Furthermore, it can be observed that with increasing reaction temperature, the intensities of peaks a, b, and c increase qualitatively, while peak d becomes sharper and higher. This indicates the formation of more active sites in the catalysts for the occurrence of copolymerization between PO and CO2.
X-Ray Diffraction Spectroscopy (XRD)
Field Emission Scanning Electron Microscopy (FESEM)
Polypropylene Carbonate (PPC) Copolymer
- Fourier Transform Infrared (FTIR)
- Gel Permeation Chromotography (GPC)
- Differential Scanning Calorimetry (DSC)
- Mechanical Properties and Crystallinity
From Figure 4.14 it was found that as the catalyst synthesis temperature increased, the molecular weight (Da) of the PPC obtained increased while the PDI of the PPC decreased as observed in Figure 4.15. As catalyst synthesis temperature increases, the catalytic activity of ZnGA increases, thus improving the molecular weight and selectivity of the PPC. From the Figures, it showed that as the molecular weight increased, the Tm of the copolymer also increased.
As observed from Figure 4.19 and Figure 4.20, the tensile strength and Young's modulus of the PPC polymers increase when the ZnGA catalysts used are synthesized at higher reaction temperature and longer reaction time. Subsequently, the high degree of crystallinity increased the tensile strength and Young's modulus of the copolymer.
Conclusion
Recommendations
Copolymerization of carbon dioxide and propylene oxide with highly effective zinc(III) hexcyanocobaltate-based coordination catalyst. Copolymerization of carbon dioxide and propylene oxide with new rare earth catalysts – RE(P2O4)3-Al(i-Bu)3 – R(OH)n. Cobalt catalysts for the alternating copolymerization of propylene oxide and carbon dioxide: Combining high activity and selectivity.
Supercritical carbon dioxide as a solvent for the copolymerization of carbon dioxide and propylene oxide using a heterogeneous zinc carboxylate catalyst. Copolymerization of carbon dioxide and propylene oxide using an aluminum porphyrin system and its components.
