Alkylene oxides polymerisation

Polyether Polyols. Polyether polyols are addition products derived from cyclic ethers (Table 4). The alkylene oxide polymerisation is usually initiated by alkah hydroxides, especially potassium hydroxide. In the base-catalysed polymerisation of propylene oxide, some rearrangement occurs to give aHyl alcohol. Further reaction of aHyl alcohol with propylene oxide produces a monofunctional alcohol. Therefore, polyether polyols derived from propylene oxide are not truly diftmctional. By using sine hexacyano cobaltate as catalyst, a more diftmctional polyol is obtained (20). Olin has introduced the diftmctional polyether polyols under the trade name POLY-L. Trichlorobutylene oxide-derived polyether polyols are useful as reactive fire retardants. Poly(tetramethylene glycol) (PTMG) is produced in the acid-catalysed homopolymerisation of tetrahydrofuran. Copolymers derived from tetrahydrofuran and ethylene oxide are also produced. 

The catalysts used for alkylene oxide polymerisation, initiated by hydroxyl groups, are ... 

Unfortunately, DMC catalysts are not efficient for EO polymerisation, and it is practically impossible to obtain PO-EO block copolymers with this catalyst. Acidic catalysts are not used on an industrial scale for alkylene oxide polymerisation due to the formation of substantial amounts of cyclic ethers as side products. Acidic catalysts are used industrially only for the synthesis of polytetrahydrofuran polyols or, to a lesser extent, for tetrahydrofuran - alkylene oxide copolyether polyol fabrication (see Sections 7.1, 7.2 and 7.3) Other catalysts have a minor importance for large scale polyether polyol production. 

If trifunctional initiators such as glycerol or trimethylolpropane are used as starters for the alkylene oxides polymerisation, star-like polyether triols are formed [1-13, 15-17, 54, 60, 69, 75] ... 

To explain the catalytic mechanism of the alkylene oxide polymerisation with phosphazenium compounds, several considerations concerning the peralkylated polyamino-phosphazenes should be made. 

The alkylene oxide polymerisation, catalysed by DMC catalysts, is characterised by some specific points ... 

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. 

The polyether polyols for rigid PU foams based on polyols which are liquid under the conditions of alkylene oxides polymerisation are glycerol and TMP polyether triols, of various molecular weights, sorbitol-based polyols (based on a mixture of sorbitol - glycerol, sorbitol - dipropyleneglycol, sorbitol - dithylene glycol) and xylitol-based polyether pentaols. 

Poly(alkylene oxide)-based (PEO-PPO-PEO) triblock and diblock copolymers are commercially successful, linear non-ionic surfactants which are manufactured by BASF and ICI. Over the last four decades, these block copolymers have been used as stabilisers, emulsifiers and dispersants in a wide range of applications. With the development of ATRP, it is now possible to synthesise semi-branched analogues of these polymeric surfactants. In this approach, the hydro-phobic PPO block remains linear and the terminal hydroxyl group(s) are esteri-fied using an excess of 2-bromoisobutyryl bromide to produce either a monofunctional or a bifunctional macro-initiator. These macro-initiators are then used to polymerise OEGMA, which acts as the branched analogue of the PEO block (see Figures 2 and 3). 

The polyalkylene oxide polyols are obtained by the polymerisation of alkylene oxides, initiated by different polyols called starters or chain initiators. The most important alkylene oxides (oxiranes or epoxides) used in oligo-polyols synthesis are propylene oxide (PO), ethylene oxide (EO) and butylene oxide (BO) (see Figure 4.4). 

The general polymerisation reaction of an alkylene oxide initiated by one hydroxyl group is ... 

The general polymerisation of an alkylene oxide, initiated by a polyfunctional starter, is ... 

Catalysts for the polymerisation of alkylene oxides initiated by hydroxyl groups must have some general and very important qualities ... 

Tetrafunctional starters (such as pentaerythritol and ethylene diamine) are used to a small extent for the synthesis of high MW poly ethers. An interesting tetrafunctional starter is ethylene diamine. In the first step the alkylene oxide reacts with the -N-H groups forming a tetraol. By the polymerisation reaction of alkylene oxides initiated by the tetraol formed in situ, a high MW polyether tetraol is obtained ... 

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... 

Unfortunately, the cationic polymerisation of alkylene oxides leads to unpleasant side reactions formation, together with the required polymer, of cyclic oligomers, of dioxane type and crown ether type (reaction 7.17) [3, 9, 35, 36, 45-53, 55]. 

The formation of relatively high yield (15-25%) cylic oligomers, means that the cationic polymerisation of alkylene oxides cannot be used for high MW polyether polyol synthesis on an industrial scale [38, 56]. The cationic polymerisation process is only used industrially for producing PTHF- and THF-alkylene oxide copolymers [2, 3, 7, 35, 36, 54, 57, 58]. The cyclic oligomers are totally inert in the chemistry of PU formation because they do not have hydroxyl groups (are simple diluents) and confer a very unpleasant odour to the synthesised polyether polyols. 

The big advantage of THF copolymerisation with alkyleneoxides is the fact that the equilibrium polymerisation characteristic of THF homopolymerisation is practically suppressed, at relatively high concentrations of alkylene oxides (30-50%). This behaviour leads to high yields of the resulting copolyether, THF-alkylene oxides, of around 85-90% (Figure 7.3). 

The general synthesis reaction of polyether polyols for rigid PU foams by polymerisation of alkylene oxides (PO, EO) initiated by polyolic starters is presented in reaction 13.1. 

Copolymerisation of propylene oxide as well as other oxiranes with carbon dioxide in the presence of zinc-based coordination catalysts is generally accompanied with the formation of a cyclic five-membered carbonate, propylene carbonate or another alkylene carbonate [147,206,207,210,212,230]. The alky-lene carbonate, however, is not the precursor for poly(alkylene carbonate), since it hardly undergoes a polymerisation under the given conditions [142-146],... 

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