Polyurethanes synthesis methods

Krol, P. (2008), Linear polyurethanes Synthesis methods, chemical structures, properties and applications, Leiden Boston VSP... 

Krol, P Linear Polyurethanes Synthesis Methods, Chemical Structures, Properties and Applications, VSP, Boston, MA, 2008. 

Several main synthesis methods widely applied to produce carbon nanotube-polyurethane nanocomposites were summarized above. In addition, latex technology (27), thermally induced phase separation (28), electrospinning (29,30) and many other methods also show their own advantages and promises, however, these methods will not be discussed here. 

The above analysis takes the synthesis methods, the performance affected by the dispersion of CNTs, enhanced physical properties and the latest applications of carbon nanotube/polyurethane composites described in literature reports as the reference point. In the interest of brevity, this is not a comprehensive review, however, it goes through numerous research reports and applications which have been learned and described in the recent years. Despite that, there are still many opportunities to synthesize new carbon nano-tube/polyurethane systems and to modify carbon nanotubes with new functional groups. The possibility of producing modern biomedical and shape memory materials in that way makes the challenge of the near future. 

In regard to Step 3, the telechelic diolefin II can be quantitatively converted to the diol V by hy-droboration/alkaline oxidation. Specifically, hydro-boration of II by BH 3 THF (or 9-BBN) yields an intermediate III which without isolation can be converted in situ by alkaline oxidation (NaOH/H2C>2) to the diol IV, the starting material for polyurethane synthesis. The number average functionality of IV was found to be 2.0 within experimental error. The analytical methods used were 1H NMR and UV spectroscopy [4]. The latter method involved the reacting of the diol with phenyl isocyanate, PhNCO, removing the excess PhNCO, filtration, Mn determination and differential UV spectroscopy. Table I shows representative data. 

A practical route to reduce the effects of photo-degradation in PUs (as in the case of other polymers) is the addition of suitable light stabilizers by different ways. The most used procedure is the additive stabilization method. A more effective method to increase the maintenance of the stabilizer in the polymer is the chemical bonding of these compounds to the macromolecular chain even in the course of polyurethane synthesis. Some attempts to introduce in such way the stabilizers in polymers are known. Thus, a benzotriazole UV absorber was mixed with a prepolymer containing isocyanate groups and the chain extender directly into moulding machine (cone. 0.5%) [64, 323]. The stabilizer can be also mixed into polyol component before its reaction with the isocyanate component. The stabilizer that usually contains OH groups could form chemical bonds with the macromolecular chain in some extent [324]. 

Kr61, P. Synthesis methods, chemical structures and phase structures of linear polyurethanes. Properties and apphcations of linear polyurethanes in polyurethane elastomers, copolymers and ionomers. Progress in Materials Science, 2007,52, 915-1015. 

Polyurethanes are condensation polymers formed by step-growth polymerization in which the chain length of the polymer increases steadily as the reaction progresses. Two major routes of polymerization of polyurethanes are one-step and two-step synthesis methods. In one-step synthesis, the diisocyanate, polyol, and a chain extender are mixed together and allowed to react. In the two-step route, an oligomer or prepolymer synthesized from diisocyanate and polyol and the chain extender are allowed to react. The two-step route offers some advantages over the one-step route, such as a reduced polydispersity index and a higher extent of phase separation. 

Marega, C., and Turturro, A. (2001) Synthesis methods of fluorinated polyurethanes. 2. Effects on morphology and microstructure. Polymer, 42, 9705-9711. 

It should be mentioned here that the practical use of the solvolytic methods for the recycling of TPE block copolymers i.e., composed of polyester and/or polyamide and/or polyurethane blocks) is quite limited. A reason for this is the rather complex composition of the reaction products, hmiting the potential field of their application. On the basis of Figures 2 to 4, it may be concluded that only glycolysis of these copolymers seems to offer products with a clear destination — for polyurethane synthesis. 

Example 2. Two-Shot Cast Elastomer (Prepolymer Method). Synthesis of polyurethane elastomers via the prepolymer method is challenging and requires especial attention to details and techniques. This example is a relatively easy one, using a low-viscosity, 9.6% NCO MDI-based poly(tetramethylene ether) glycol prepolymer. It is adapted from the Bayer Product Information Bulletin for Baytec ME-090 prepolymer. 

DMAP has been found to improve the coupling reaction in solid phase peptide synthesis (4). Polypeptides synthesized via the DMAP-DCC method were found to be of higher purity compared to other methods of synthesis. In polymer chemistry, DMAP has been used for hardening of resins and in the synthesis of polyurethanes, (5) polycarbonates, (6) and polyphenyleneoxides (7) ... 

Synthesis of Polyurethanes. In the "prepolymer method" employed in this study, MDI (2 equivalents) and PPG (1 equivalent) were reacted at 60°C in the presence of 1% dibutyltin dilaureate as catalyst, in the melt. The course of the polymerizations was followed spectroscopically by observing the intensity of the — NCO peak in the IR (Scheme III). 

This section describes the synthesis of oxazolidine esters used as polymer hardeners that cannot be synthesized using chemical catalysis, the synthesis of polyurethane polymers with methods that avoid the use of isocyanates and the enzymatic synthesis of polyesters with low molecular weight dispersity. 

Another approach was attempted by Seppala and Kylma who reported the synthesis of poly(ester-urethane)s by condensation of hydroxyl terminated tel-echelic poly(CL-co-LA) oligomers with 1,6-hexamethylene diisocyanate (Scheme 33) [94]. The diisocyanate acts as chain extender producing an increase in molecular weight of the preformed oligomers. The authors claim that some of the copolymers present elastomeric properties. Using a similar method. Storey described the synthesis of polyurethane networks based on D,L-LA, GA, eCL,... 

The block lengths and the final polymer molecular weight are again determined by the details of the prepolymer synthesis and its subsequent polymerization. An often-used variation of the one-prepolymer method is to react the macrodiol with excess diisocyanate to form an isocyanate-terminated prepolymer. The latter is then chain-extended (i.e., increased in molecular weight) by reaction with a diol. The one- and two-prepolymer methods can in principle yield exactly the same final block copolymer. However, the dispersity of the polyurethane block length (m is an average value as are n and p) is usually narrower when the two-prepolymer method is used. 

The synthesis of optically active polymers is an important area in macromolecular science, as they have a wide variety of potential applications, including the preparation of CSPs [31-37]. Many of the optically active polymers with or without binding to silica gel were used as CSPs and commercialized [38]. These synthetic polymers are classified into three groups according to the methods of polymerization (1) addition polymers, including vinyl, aldehyde, isocyanide, and acetylene polymers, (2) condensation polymers consisting of polyamides and polyurethanes, and (3) cross-linked gels (template polymerization). The art of the chiral resolution on these polymer-based CSPs is described herein. 

Modern technological practice, particularly the various types of chemical processing, use a great variety of formulations for the synthesis of polyurethanes. Therefore it is nearly impossible to create a general kinetic model which would be valid for various polymerizing systems. However, the same general approach to creating such models can be used for different cases. Therefore, it is useful to demonstrate the method used to construct a model and its characteristic kinetic equations for some typical cases. 

James DF and Walters K, "A Critical Appraisal of Available Methods for the Measurement of Extensional Properties of Mobile Systems" in Collyer AA (Ed), "Techniques in Rheological Measurement", Chapman Hall, London, 1993. Jin S, "Synthesis and Characterization of Side-Chain Liquid Crystalline Polyesters and Polyurethanes", PhD Thesis, Delft, 1995. 

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