Anionic polymerisation alkylene oxides

In Section 4.1.1, the general mechanistic aspects of anionic polymerisation of alkylene oxides (especially PO) were discussed. The anionic polymerisation of PO initiated by hydroxyl groups is considered as a pseudo living polymerisation. This type of polymerisation has some important aspects of living polymerisations the active centre (alcoholate type) is stable and active, and during the polymerisation reaction the number of active alcoholate centres remains constant. This characteristic of living polymerisations is very important for the synthesis of block copolymers. For example if after the addition of PO to the living polymer EO (or BO) are added, then block copolymers are obtained. 

It is well known that during anionic polymerisation of alkylene oxides, initiated by hydroxyl groups, there is a permanent equilibrium of alcohol - alcoholate. The distribution of alkylene oxide sequences/hydroxyl groups depends very much on the value of the equilibrium constant Ke which, in its turn, depends on the acidity of hydroxyl groups ... 

The industrial processes currently used worldwide for polyether polyol synthesis by anionic polymerisation of alkylene oxides are discontinuous processes, a fact that is explained by the great number of polyether polyol types produced in the same reactor and by the relatively low reaction rate of the propoxylation reaction. 

In the history of PU, some continuous processes for polyether polyol synthesis by anionic polymerisation were developed, but only at small scale (i.e., pilot plant). Tubular reactors with static mixing systems or a column with plate reactor types were used, but these technologies were not extended to industrial scale levels. The first continuous process for high MW polyether synthesis was developed by Bayer (IMPACT Technology) and is based on the very rapid coordinative polymerisation of alkylene oxides, especially PO, with dimetallic catalysts (DMC catalysts - see Chapter 5). A principle technological scheme of a polyether polyol fabrication plant is presented in Figure 4.30. 

The anionic polymerisation of alkylene oxides initiated by different polyolic starters is the most important step of polyether polyol manufacture. 

Figure 4.30 Scheme for polyether polyol fabrication by anionic polymerisation of alkylene oxides, initiated by glycerol or diols (variant). 1 - Reactor for potassium glycerolate synthesis 2 - Reactor for prepolyether synthesis 3 - Reactor for polyether synthesis 4 - Reactor for purification 5 - Filter press 6 - Storage tank for final purified poly ether 7 - Heat exchangers for removal of the reaction heat 8 - Condensers 9 - Vacuum pumps 10 - Vessels for distilled water 11 - Recirculation pumps 12 - Gear pump or screw (or double screw) pump... 

The gas-liquid contactor type reactors are extremely safe and may be considered the best reactors for the synthesis of polyether polyols by anionic polymerisation of alkylene oxides, initiated by various polyolic starters (Figure 4.31). 

Anionic Polymerisation of Alkylene Oxides Catalysed by Phosphazenium Compounds... 

An important characteristic of alkylene oxide polymerisation with DMC catalysts is the very low reaction rates obtained in EO coordinative polymerisation. EO, which is much more reactive than PO in anionic polymerisation, is less reactive than PO in the coordinative polymerisation [35, 68]. A possible explanation of this behaviour is the fact that PO is a more basic monomer than EO due to the electron release effect of the methyl substituent in the oxiranic ring (the electron density at the oxygen atom of the PO ring is higher than that in the EO ring). As an immediate consequence, PO, is more basic, and is more strongly co-ordinated (and more strongly activated too) to the active sites of DMC catalysts than EO, the less basic monomer. 

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