Liquid polyether polyols polymerisation

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 im) diameter. By dilution with w-hexane and filtration, it is possible to achieve a quantitative removal of the DMC catalyst [51, 52]. 

PHD polymer polyols are a special class of filled polyols developed successfully by Bayer, PHD being the abbreviation of the German name polyharnstoff dispersion or polyurea dispersions [67-69]. PHD polyols contain organic urea, oligomeric or polymeric polyurea, finely dispersed in liquid polyether polyols [67-73]. The difference between PHD polyols and graft polyether polyols is the different nature of the solid polymer dispersed (it is a heterocatenary polymer - polyurea - instead of carbocatenary polymer) which is obtained by another synthetic procedure (polyaddition reaction between a diisocyanate and a diamine instead of radical polymerisation). The reaction between the diisocyanate and the diamine, takes place in situ (reaction 6.19), in liquid poly ether. The resultant polyurea being insoluble in polyether, precipitates in the form of very fine particles ... 

The synthesis of polyether polyols by anionic polymerisation of gaseous monomers such as PO (bp 33.6 °C) and EO (bp 10.3 °C), at 100-125 °C, is a strong, diffusion dependent process. Of course the polymerisation reaction takes place in a liquid state, where the anionic catalyst is present. 

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). 

Graft polyether polyols are synthesised by in situ radical polymerisation of vinylic monomers in liquid polyethers, by batch, semi-continuous or continuous processes. The solid fraction varies between 10-50%, more frequently being between 10-40% [1-10]. 

The practical technique to obtain polymer polyols by radical polymerisation is to add an homogeneous mixture of vinylic monomer, initiator, chain transfer agent and a part of polyether polyol, to the rest of polyether polyol containing the NAD (macromer or nonreactive NAD), at 115-125 °C. The mechanism of solid polymer particle formation during radical polymerisation of vinylic monomers in liquid polyethers, in the presence of a nonreactive NAD, in the form of very stable dispersions, is described next. 

Polyether polyols for rigid PU foams are obtained in the same type of polymerisation reactors as those used for high molecular weight polyether polyols, i.e., in stainless steel loop reactors, with an external heat exchanger, preferably with the possibility of generating a large surface of the liquid reaction mass, by a spray technique or by an ejector technique... 

In order to decrease the total reaction time, a small reactor, with a stirrer, is linked to the polymerisation reactor, for the preparation of the initial starters - catalyst mixture. In this reactor, there are 1-3 polyols used as starters, the catalyst (KOH, NaOH or a tertiary amine) and sometimes, for solid polyols, an initial liquid medium (for example a part of an intermediary or final polyether polyol called heel , or an inert solvent). Generally, in the synthesis of polyether polyols for rigid foams it is preferred to avoid the utilisation of inert solvents, which need recycling and a more complicated installation. 

The synthesis of rigid polyether polyols, by polymerisation of PO or EO, initiated by polyols which are liquid under the conditions of the polymerisation temperature, is simple, and similar to the synthesis of the prepolyether by propoxylation of glycerol (see Chapters 4.1.1 and 4.1.5). 

The polymerisation of PO and EO, initiated by polyfunctional starters, to make short chain polyether polyols is a reaction that is strongly dependent on diffusion. The consumption rate of PO or EO is given by two simultaneous factors the rate of the chemical reaction in the liquid phase and the efficiency of the monomer mass transfer from the gaseous phase to liquid phase (see details in section 4.1.5). The PO (or EO) consumption rate, considering the mass transfer, is described by equation 13.27 [45-50] ... 

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. 

Polyether polyols for rigid PU foams or intermediate polyether polyols (with a higher hydroxyl number than the final polyether) proved to be excellent reaction media for the propoxylation of solid polyols, especially to obtain very high functionality polyols. These polyether polyols, used as a liquid reaction medium, are called heel . For example, at the PO polymerisation temperature (110-120 °C) a mixture of 60% sucrose and 40% sucrose-based polyether polyol is a perfectly stirrable mixture. 

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