1,3-Polyols synthesis

Numerous other approaches to 1,3-polyol synthesis exist however, most fall outside the scope of this review. Some of these approaches have been reviewed 1831. Particularly noteworthy advances in the direct preparation of protected 1,3-diols have recently been reported by Smith [84], Leighton [85], and Evans [86]. 

Sun, Y., Mayers, B., Herricks, T. and Xia, Y. (2003) Polyol synthesis of uniform silver nanowires a plausible growth mechanism and the supporting evidence. Nano Letters, 3, 955-960. 

Polyols synthesis A two-directional application of chelation-controlled [1,2]-Wittig rearrangement has been used to prepare the all syn polyol 31 from 30 (equation 16) °. 

Tamoa notes how this is particularly useful for polyol synthesis ... 

A general, asymmetric polyol synthesis has been developed by Schneider et al. [18] which takes advantage of the benefits of the silyloxy-Cope rearrangement of the chiral aldol product... 

Herricks, T., Chen, J. and Xia, Y. (2004). Polyol synthesis of platinum nanoparticles Control of morphology with sodium nitrate. Nano Lett. 4 2367-2371. 

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 presence of water in starters or in monomers (PO, EO or BO) always leads to polyether diols. The control of water content in the raw materials, used for polyether polyol synthesis, has a great practical importance for two reasons ... 

A very interesting group of random copolyethers is obtained by anionic copolymerisation of EO (a highly hydrophilic monomer) with BO (a highly hydrophobic monomer). Because EO does not isomerise to double bond structures and BO has a much lower tendency to isomerise to allyl structures than PO (see Chapter 12.2), the BO-EO copolyethers have a very low unsaturation level compared to PO homopolymers or even PO-EO copolymers [82]. This variant of polyether polyols synthesis in the form of BO-EO copolymers is a very interesting way to obtain low unsaturation polyether polyols directly from synthesis. Another group of low unsaturation polyether polyols, obtained directly from synthesis, are the tetrahydrofuran (THF)-EO and THF-PO copolymers synthesised with cationic catalysts (see Chapter 7.3). 

In Sections 4.1, 4.1.1, 4.1.2, 4.1.3 and 4.1.4, the chemistry of polyether polyol synthesis, the mechanism and kinetics of alkylene oxide polyaddition to hydroxyl groups and the most important structures of polyalkylene oxide polyether polyols for elastic polyurethanes - PO homopolymers, random PO-EO copolymers and PO-EO block copolymers - were discussed. 

The manufacturing process for polyether polyol synthesis using alkaline catalysts consists of the following characteristic steps ... 

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. 

DMC catalysts are considered to be the ones that perform best at this time for PO polymerisation initiated by hydroxyl groups. Bayer developed the first continuous process, with a very high productivity, for the synthesis of polyether polyols with DMC catalysts (IMPACT Catalyst Technology). In a short and simple production cycle, a large variety of polyether diols of very low unsaturation for elastomers, sealants, coatings and low monol content polyether triols destined for flexible polyurethane foams are obtained. This is one of the best developments in the last few years in the field of polyether polyol synthesis [2],... 

The concentration of these kinds of macromers, used in polymer polyol synthesis, is around 2-5% compared to the final polymer polyol. Higher concentrations lead to an undesired, substantial viscosity increase together with a decrease in median diameter of the resulting solid polymer particles. A lower concentration of macromer leads to poor stabilisation of the resulting polymer dispersion. 

A simple and convenient method to obtain macromers useful for polymer polyols synthesis is based on the following succession of reactions (a-c) [52] ... 

Generally, the nonreactive NAD is used in higher concentrations, than the macromers in graft polyether polyols synthesis, of around 10-15% compared to the final polymer polyol. 

The Mechanism of Polymer Particle Formation in Polymer Polyols Synthesis by Radical Polymerisation [19]... 

The most important radical initiators used for polymer polyol synthesis are azoderivatives, such as azoisobutyrodinitrile (AIBN) [1, 3,13, 22]. Other initiators used successfully are peroxides (tert-amyl peroxides are very efficient), hydroperoxides and percarbonates, but the half life has to be lower than two minutes, at the polymerisation temperature (115-125 °C) [23-28]. 

Figure 6.11 Technological scheme for PHD polyols synthesis (variant). 1) Static mixer for polyether with hydrazine 2) Static mixer for polyether with TDI 3) Static mixer for TDI-hydrazine reaction 4) Loop reactor with total recirculation 5) Recirculation...

An ideal reaction for PIPA polyols synthesis is shown in reaction 6.24 ... 

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 third type of reaction for polyester polyols synthesis is the ring opening polymerisation of cyclic esters, such as -caprolactone (reaction 8.4) or cyclic carbonates, such as ethylene glycol carbonate, propylene glycol carbonate, neopentyl glycol carbonate, etc., (reaction 8.5) initiated by diols (or polyols) and catalysed by specific catalysts [7, 16]. 

The intensive research done in the last few years proved that the hydrophobicity of the glycol or of the dicarboxylic acid used for polyester polyol synthesis is one of the most important parameters to improve the hydrolysis resistance of polyester-based PU. 

Figure 8.3 Installation for polyester polyols synthesis. 1) polyesterification reactor 2) separation column 3) induction elements for heating 4) condenser 5) vessel for condensed water 6) vacuum pump 7) filter...

A second method for polycarbonate polyol synthesis is the ring opening polymerisation of cyclic carbonates of 5-6 members, initiated by various polyols as starters (reaction 8.38) [68-76]. 

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