Prominent Catalysts

In this section, a selection of prominent catalysts in the field of CO₂/epoxides copoly-merization will be illustrated and some of their catalytic performances and characteris-tics will be pointed out. Not all noteworthy catalysts can be presented here. For more details, the reader should refer to the cited literature of the corresponding catalyst or to comprehensive reviews.47,70,81,88,96,108–112

A selection of important catalysts are presented in Figure 7 and Figure 8. In Figure 7 the first catalyst displayed (1) is a zinc-cobalt-Double Metal Cyanide (DMC) cata-lyst.¹¹⁹ It represents one of the heterogeneous catalysts known to copolymerize CO₂ and epoxides; zinc glutarates are also known for this.¹¹³ Heterogeneous catalysts are not the topic of this work, but potential immobilization of a homogeneous catalyst can unite the benefits of both. Immobilization can even allow to reuse a homogeneous catalyst.¹¹⁵

Catalyst (2) was introduced by Sugimotoet al. and consists of a tetraphenylporphyrin (TPP) ligand coordinated to a cobalt atom. The (TPP)CoCl-DMAP system relies on a co-catalyst, viz. DMAP (4-Dimethylaminopyridine).¹¹⁴ It is a representative of the porphyrin complexes that have been applied with different metals, some of which are even active at 1 bar of CO₂.^120–123

In Figure 7 a β-diiminato (BDI) zinc complex (3) is shown, which was published in 2003 by Coates et al.⁸⁶ They investigated various BDI-type catalysts and many other catalysts over the years. The catalyst depicted showed a TON of 306 and a TOF of 917 h⁻¹ (T = 50 °C, 20.6 bar CO₂ (300 psi) in toluene) forming PCHC with 90 % carbonate linkages. Even more impressive are their works on the stereoselective poly-merization, which will be discussed in context with Figure 8.

Complex 4 in Figure 7 is a member of the salen type complexes from Lee et al.¹¹⁵ Several salen catalysts have been published over the years,^83,124–126 yet this particular complex demonstrated outstanding catalytic activities and properties. A TON of 13000 after 30 min. resulting in a TOF of 26000 h⁻¹ was achieved (T = 80 °C , 17–20 bar

Figure 7: Selection of catalysts from literature for the copolymerization of CO₂ and epoxides.

1: repeat unit of Zn-Co-DMC, representative for the heterogeneous catalysts¹¹³. 2: Sugimotoet al. investigated a (TPP)CoCl-DMAP system¹¹⁴ which is displayed as representative of the porphirin-like complexes. 3: β-diiminato (BDI) zinc complex from Coates et al.⁸⁶ one out of many BDI-catalysts investigated by the group, 4: Salen-type complexes represented by an outstanding catalyst from Lee et al.¹¹⁵. 5: System by Rieger et al. which holds the current record for the most active system^116,117. 6: Catalyst systems by Williamset al. that is highly active at 1 bar CO₂ with great tolerance to water.¹¹⁸

CO₂ in neat propylene oxide (PO)) yielding in a 99 % carbonate-linked poly(propylene carbonate) (PPC). Furthermore, it maintained activity down to 2 bar in PO.

However, the current record for the most active system is held by a catalyst which was published in 2015 by Rieger et al. (5 in Figure 7).^116,117 A TON of 6740 (over 20 min.) was reported, producing 88 % carbonate linkages in the PCHC product (80 °C, 30 bar CO₂, CHO in toluene). During the polymerization an unbeaten initial TOF of 155000 h⁻¹was observed (over the 20 min. duration of the experiment this corresponds to an average TOF of 20220 h⁻¹).

In terms of high activity at 1 bar of CO₂, the current benchmark is a versatile system by Williamset al. (6).^118,127The complex in Figure 7 showed a TON of 527 and a TOF of 25 h⁻¹ at only 1 bar of CO₂ (100 °C, neat CHO) forming 94 % carbonate linkages.

After the first publication in 2009, the complex was developed and tested further. To-day, this ligand system has also been applied as an iron catalyst¹²⁸ (TON of 2570 and TOF of 107 h⁻¹ at 80 °C, 10 bar CO₂ in neat CHO, 99 % carbonate linkages), a cobalt catalyst¹²⁹ (TON of 340 and TOF of 172 h⁻¹ at 80 °C, 1 bar CO₂ in neat CHO, >99 % carbonate linkages), and a magnesium catalyst¹³⁰ (TON of 360 and TOF of 103 h⁻¹ at 100 °C, 1 bar CO₂ in neat CHO, with O₂CF₃ instead of OAc). These systems are outstanding for their high activity at 1 bar CO₂ and their good tolerability to water. In fact, water can be used as a chain-transfer agent to control the molecular weight.^118,130

In Figure 8, a second selection of catalysts is depicted. Here the focus is on the stere-oselective polymerization of CO₂ and epoxides. Catalyst 7 was published in 1999 by Nozaki et al.^106,131 Its TON value is relatively low with 40 and a TOF of 2 h⁻¹ (40 °C, 30 bar CO₂, CHO in toluene). However, it forms PCHC with goodP_r values down to 15 % (calculated from ee = 73 %).¹⁰⁶

In 2005 Dinget al. used the Trost-type ligand^135,136forming a zinc catalyst (8) that is active even under 1 bar of CO₂.¹³² The displayed zinc catalyst provided a TON of 285 and TOF of 142 h⁻¹ (T = 80 °C, p = 20 bar CO₂, neat CHO).¹³² Coordination of a magnesium atom showed a TON of 86 and TOF of 43 h⁻¹ (T = 60 °C, p = 1 bar CO₂, CHO in toluene).⁸⁷ More research has been put into the ligand design, and the zinc system was further developed. By applying an azetidine ring instead of a pyrrolidine one, ee values of >90 % were observed in the obtained PCHC.¹³⁷

The last two displayed systems were developed, again, by the group of Coates et al.

Compound 9 represents a variety of imine-oxazoline zinc-based catalysts. The one de-picted produced anee value of 72 %.¹³³ The axially chiral cobalt system10(which was also published as a binaphtyl system) allowed to form polycarbonates with an optimal ee > 99 %.¹³⁴

Figure 8: Selection of catalysts from literature for the copolymerization of CO₂ and epoxides with focus on influencing the stereoregularity within the polymer. 7: Nozaki et al.¹³¹ formed PCHC with P_r = 15 % (calculated from ee = 73 % ).¹⁰⁶, 8: Trost-type ligand applied by Ding et al.^87,132, 9: Imine-oxazoline zinc-based catalysts from Coates et al. (this one: ee = 72 %).¹³³ 10: Axially chiral systems by Coates et al. achieved ee > 99 %.¹³⁴

Overall, a selection of state-of-the-art catalysts have been described, illustrating the current achievements in the field of CO₂/epoxide copolymerization. This PhD thesis aims at expanding this current knowledge.