Polyol reaction

The groups of Burczyk, Takeda, and others have made thorough studies of cyclic acetals, such as 1,3-dioxolane (five-membered ring) and 1,3-dioxane (six-membered ring) compounds, illustrated in Fig. 13. They are typically synthesized from a long-chain aldehyde by reaction with a diol or a higher polyol. Reaction with a vicinal diol gives the dioxolane [40-42] and 1,3-diols yield dioxanes [43,44]. 

High-Temperature Organic Polyol Reactions HIM Nanoparticle Synthesis... 

To achieve rapid nucleation in polyol reactions, a general mle of thumb is that the higher the temperature of the glycol, the faster the nucleation and the more uniform the nanoparticles formed. This empirical mle prompted the evaluation of other solvents, such as propylene glycol. In the mid-1990s, researchers at IBM... 

Chemistry MDl-based isocyanate prepolymer, polyether and/or polyester polyols, reacts with water to form polyurea-polyurethane backbone Often MDl-based isocyanate prepolymer (some are ahphatic isocyanate based), polyether and/or polyester polyols (some contain epoxy based materials), reaction forms polyurethane backbone MD1-, TDl- and some aliphatic isocyanate-based prepolymers, polyether and/or polyester polyols, reaction forms polyurethane backbone... 

Figure 11.36 By extending the polyol reaction for a ven time period, various polyhedral shapes capped with 100 and 111 faces can be obtained in high yield, a) A schematic of the nucleation and growth process, in which silver continuously deposits onto the 100 faces to eventually result in a completely 111 -bound octahedron, b to f) SEM images of cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra, respectively (scale bar 100 nm). Reproduced with permission from reference [91]. (2006) Wiley-VCH Verlag GmbH Co. KGaA.

Catalysts are also required in many stepwise polymerizations. For example, reaction of polycarboxylic acids and polyols (Reaction 7) is catalyzed by acids ester interchange, by metal compounds such as titanium alkoxides. On the other hand, polyurea synthesis (Reaction 6) generally does not require a catalyst. Metallic compounds are also useful in oxidative polymerization of phenols to give poly(phenylene oxides), illustrated in Reaction 14. 

The noble metals were the favorite metals for demonstrating the usefulness of the microwave operation in conducting the polyol reaction. Among the noble metals platinum was synthesized most frequently. Polymer-stabilized platinum colloids with nearly uniform spherical shape were prepared by Yu and coworkers by microwave dielectric heating [172]. The average diameters of the as-prepared platinum colloids were 2-4 nm with a narrow size distribution in regard to the preparation conditions. 

Finally, using the polyol reaction and the same reactants as in [172] in a third... 

Komarneni and coworkers [175] conducted a polyol reaction for the preparation of Pt and Ag nanopartides. The synthesis of the metal nanopartides [175] was conducted in a double-walled digestion vessel which has an inner liner and a cover made of Teflon PFA and an outer high-strength shell of Ultem polyetherimide. 

In a MWH process one precursor can be heated at much higher heating rates and reach a higher temperature than its surroundings. In this respect it is similar to sonochemistry, where hot spots are formed in the liquid. In the polyol reactions where the first step was the formation of metallic fine particles, we could see the solution of ethylene glycol hot points reaching 600-700 °C. The difference between sonochemistry and MWH is that in the latter method there is no direct contact between the energy source and the solution, while in the sonochemistry the horn is dipped into the solution. 

For a review on solid catalysts in fatty acid/polyol reactions, sec C. Marquez-Alvarez, F. Sastre and J. Perez-Pariente, Top. Catal. 27 (2004) 105. 

In Figure 3.11, one observes the kinetic curves of the oligo-polyol reaction, having only secondary hydroxyl groups compared to oligo-polyols having primary and secondary hydroxyl groups. 

Figure 3.11 Graphical representation of second order kinetics of the oligo-polyols reaction with different primary hydroxyl contents with phthalic anhydride, a initial concentration of phthalic anhydride b initial concentration of hydroxyl groups t time for total consumption of primary hydroxyl x concentration of a or b reacted at time t. Temperature 30 °C Solvent pyridine...

A very interesting variant of cationic polymerisation of CPL is based on the polymerisation initiated by hydroxyl compounds, at room temperature [42,43,44]. The mechanism called hydroxo-mechanism is very similar to the activated monomer mechanism developed for cyclic ethers. This kind of polymerisation is practically a living cationic polymerisation and in the particular case of CPL, using various polyols as starters, it is possible to obtain hydroxy-telechelic poly CPL) polyols, with various MW, depending on the molar ratio of CPL per polyol (reactions 8.28). 

It is very interesting that acrylic polyols can be used as precursors to synthesise hybrid structures, such as acrylic - polyester polyols, by the polymerisation of some cyclic monomers such as e-caprolactone, initiated by hydroxyl groups of acrylic polyols (reaction 10.2). 

The amino groups (-NH2) generated by the reaction with alkylene oxides (PO, EO) hydroxyalkyl groups, transforming the initial amine into an amino polyol (reaction 13.2). 

By hydrolysing a polyether PU, a polyether polyol is obtained with a similar structure to those of the initial virgin poly ether polyol. For polyester PU the hydrolysis reaction is more complicated because the esteric groups of polyesters are hydrolysed back to monomers, such as diacids and glycols or polyols (reaction 20.3). 

Reaction voiih Polyols. The diisocyanate-polyol reaction is an example of the cpndensation class of polymers, as described originally by Carothers. The reaction proceeds without the evolution of a by-product, unless the pobrol is water. For example, a simple glycol and a diisocyanate react in-tennolecularly as follows ... 

Chem. Descrip. Organotin Uses Catalyst for PU coatings, adhesives, sealants Features Provides delayed action catalysis of isocyanate/polyol reaction Properties Lt. yel. liq. sol. in alcohols, org. soivs., insol. in water sp.gr. 1.11 vise. 33 cps f.p. -23 C decomp. pt. 255 C flash pt. (PMCC) 130 C 17.5% total tin... 

Some strategies for developing a polyol process capable of generating well-dispersed alloyed PtRu nanoparticles deposited on carbon supports have been also investigated. " Recently, Sau et alf reported how, by careful selection of the polyol, reaction pH, temperature, and modality of combining the reactants, it was possible to control not only the size and dispersion of the bimetallic nanoparticles but also the relative spatial distribution of the two elements. 

Catalysts (Figure 3.5). Isocyanate + polyol reactions go quite rapidly at room temperature. Isocyanate + amine reactions go rapidly at room temperature. However, most processors add catalysts to make the polymeiization/cure reactions even faster and to control the foaming process. 

Isocyanate/Isocyanurate. Isocyanates react with polyols to form rigid polyurethane foams, a major type of thermoset plastics. While these are very useful in thermal insulation, they are limited by failure at high temperature and by flammability. One way to solve these problems is to convert part of the isocyanate to isocyanurate by cyclotrimeriza-tion (Fig. 3.64). Whereas the isocyanate-polyol reaction forms polyurethane rapidly at room temperature, the cyclotrimeiization of isocyanate to isocyanurate requires strong alkaline catalysis and heat to compete successfully. The resulting isocyanurate rings build considerable heat resistance (150 to 250°C, short-term <800°C) and flame-retardance into the polyurethane foam. They are useful for insulating pipelines and boilers. 

Amine catalysts are primarily used to catalyze the isocyanate-water reaction ( blowing catalyst ), while tin or other metal catalysts are used to regulate the rate of the isocyanate polyol reaction ( gelling catalyst ). Surfactants are used up to 2 pph (parts per hundred) to regulate the cell size. Higher amounts of the surfactant produce thinner cell walls and smaller cells. An excessive amount would cause collapse of the foam as the walls and ribs of the foam cells could not support the pressure of the gas. 

It will be noted that in the production of rigid foam it is not necessary to use a complex catalyst system. Commonly tertiary amines are used alone to catalyze the isocyanate-polyol reaction metal catalysts are not widely used. 

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