Cyanide Catalysts
The resulting solid complex catalyst was separated by filtration (or by centrifugation).
In order to eliminate as much as possible of the resulting potassium chloride, which has an inhibitory effect in PO polymerisation, the solid was reslurried in a mixture of ligand- water and finally in pure ligand and then filtered. The catalyst was dried at moderate temperatures (60–70 °C) and under vacuum (665–1,330 Pa), for several hours. The catalytic activity of the catalyst increases substantially if the water content in the final catalyst is very low, water having an inhibitory effect on PO polymerisation.
The analysis of the best catalysts proved that water is always present, around 0.5-1 mol of water/mol of catalyst (z = 0.5–1 in Structure 5.1). The catalysts with the general formula shown in Structure 5.1 synthesised with 1,2-dimethoxy ethane as ligand, were considered for many years a standard model for DMC catalysts [2, 51, 52]. The flow diagram for synthesis of DMC catalysts is presented in Figure 5.1.
The double-bond content of polyether polyols synthesised with the standard DMC catalyst is very low, around 0.015–0.02 mequiv/g at a MW of 6,000–6,500 daltons. Very high catalytic activity DMC catalysts were obtained using tert-butyl alcohol as ligand or with combinations of ligands such as: tert-butyl alcohol – PPG (MW = 1,000-4,000), tert-butyl alcohol – PEG (preferred MW = 2,000), tert-butyl alcohol – sorbitans, tert-butyl alcohol – polytetramethylene glycols, tert-butyl alcohol – tert-butoxy ethanol [17, 32], tert-butyl alcohol – hydroxyethyl pyrolidone, tert- butyl alcohol - poly(N-vinyl pyrolidone), tert-butyl alcohol – alkyl (polyglucosides) [8–10, 12 –29, 32–39, 48–50].
These catalysts lead to an extremely low unsaturation, of around 0.005 mequiv/g, impossible to obtain with other catalysts.
In Figure 5.2 one observes the unsaturation increase versus the polyether MW for potassium hydroxide (KOH), compared with DMC catalysts.
A high unsaturation proved a high concentration in polyether monols and, as an immediate consequence, the functionality (f) of the resulting polyether triols is much lower than 3 OH groups/mol. Thus, a polyether triol of MW of 6,000 daltons, obtained with KOH, has a functionality of 2.14-2.21 OH groups/mol, much lower than that of polyether triols (2.94 OH groups/mol) obtained with DMC catalysts.
The effect is very important in polyether diols. A polyether diol with a MW of 4,000 daltons, obtained by anionic polymerisation has a functionality of 1.61 OH groups/
mol, but a polyether diol obtained with DMC catalysts has a functionality close to the theoretical functionality (f = 1.98–2.00 OH groups/mol).
The polyurethane (PU) elastomers obtained from polyether diols with DMC catalysts
Synthesis of High-Molecular Weight Polyether Polyols with Double Metal Cyanide Catalysts mechanical properties when compared with PU elastomers made from the polyether diols, obtained by anionic catalysis.
ZnCl2 Water
Water Precipitation of Zn3[Co(CN)6]2
Complexation reaction
Filtrate
Filtrate
Filtrate
Water, ligand Filtration 1
Filtration 2
Filtration 3
Drying Reslurry 1
Reslurry 2 Ligand
Water K3[Co(CN)6]
Solution 2 Solution 1
DMC catalyst
Figure 5.1 Flow diagram for synthesis of DMC catalysts
0.1
1,000 0.2
0.3 0.4 0.5 0.6 0.7
Unsaturation mequiv/g
0.8 0.9 1.0
2,000 3,0004,000
DMC catalysts KOH
MW (daltons)
5,000 6,000 7,000
Figure 5.2 The polyether triol unsaturation obtained with DMC catalysts, compared with that obtained with KOH
Table 5.1 shows the double-bond content and the functionality of polyether triols, with a MW of 6,000 daltons, synthesised with different catalysts.
DMC catalysts are real heterogeneous coordinative catalysts [2, 51, 52]. At the end of polymerisation, the catalyst is dispersed in the liquid polyether polyols in the form of small solid particles of around 200 nm (0.2 μm) diameter. By dilution with n-hexane and filtration, it is possible to achieve a quantitative removal of the DMC catalyst [51, 52].
It is very interesting that pure crystals of Zn2[Co(CN)6]2 are catalytically inactive in PO polymerisation [6, 7]. It is only in the presence of an excess of ZnCl2 and in the presence of ligands that the catalyst becomes very active catalytically.
X-ray diffraction studies proved that DMC catalysts, similar to many non- stoichiometric chemical substances, have many defects and vacancies in the crystalline structure [62]. These defects and vacancies are very strong coordination points. The oxiranic monomer is strongly coordinated by these centres at the oxygen atom and
Synthesis of High-Molecular Weight Polyether Polyols with Double Metal Cyanide Catalysts This activation of the monomer by coordination explains the very high catalytic activity of DMC catalysts.
Table 5.1 The double-bond content of polyether triols synthesised with various alkoxylation catalysts
Catalyst Mechanism Unsaturation (mequiv/g)
Functionality (OH groups/mol)
References
KOH Anionic 0.09–0.1 2.14–2.21 [2]
CsOH Anionic 0.045–0.055 2.46–2.55 [53]
Ba(OH)2 Anionic 0.03–0.04 2.60–2.68 [54–56]
Phosphazene Anionic 0.018–0.02 2.78–2.8 [57–61]
DMC Coordinative 0.005 2.94 [18–20]
The alkylene oxide polymerisation, catalysed by DMC catalysts, is characterised by some specific points:
a) The first characteristic is an induction period that varies from 20–30 min to several hours. In this period of time, the consumption rate of PO is extremely low. After the induction period, the polymerisation rate of PO becomes extremely high and the catalyst is considered activated [1-52]. This behaviour is observed in Figure 5.3, where a typical curve for PO consumption in a PO polymerisation reaction catalysed by DMC catalyst is presented and compared with the reaction obtained with a classical KOH catalyst.
A possible explanation of the induction period is the substitution of the soft ligands from the crystalline structure with PO that are in excess. After the induction period, the PO polymerisation rate is so high that in 1 to 3 h it is possible to add all the PO that is needed for the reaction. As a comparison, the polymerisation time, in the presence of KOH, is around 7–11 h.
A very efficient method to avoid a long induction period is to obtain an ‘activated masterbatch’ of DMC catalyst. Thus, a quantity of DMC catalyst, 10–20 times higher than for normal PO polymerisation, is suspended in a purified polyether polyol used as starter (e.g., a polyether triol with a MW = 650–700 daltons) [51, 52]. To this suspension of DMC catalyst is added a quantity of PO, and the mixture is stirred under pressure (200–400 MPa), at 105–120 °C, until the pressure begins to decrease
rapidly. A concentrated suspension of an ‘activated’ DMC catalyst was obtained. By using a part of this ‘activated masterbatch’ in normal PO polymerisation, the PO consumption starts immediately, without any induction period (Figure 5.4).
100 200 ml PO reacted 300
400 500 600 700 800
0 0 60 120 180 240 Time (min)
KOH DMC
catalyst Induction
period
300 360 420
Figure 5.3 The PO consumption versus time in PO polymerisation with DMC catalysts and classical KOH catalyst. Temperature: 110 °C; pressure: 300 MPa;
catalyst concentration: [KOH] = 0.25%; and DMC catalyst: 200 ppm (0.02%)
With the synthesised ‘activated masterbatch’ of DMC catalyst it is possible to make 10–20 normal PO polymerisation reactions, leading to a considerable economy of time, due to the absence of the induction period, corresponding to each batch.
b) Another important characteristic of PO polymerisation with DMC catalysts is the impossibility of initiating the reaction by direct addition of PO to a starter such as glycerol or 1,2-PG [1–7, 51, 52]. The explanation of this abnormal behaviour is given by the formation of very strong, stable and inactive zinc chelates, with the 1,2-glycol structure of the starters (Structure 5.3). This is explained by the fact that water has an inhibitory effect on PO polymerisation with DMC catalysts.
During PO polymerisation, water reacts with PO and is transformed into 1,2-PG, which blocks the activity of DMC catalyst by the formation of strong, catalytically