Reaction linear polyurethane

The addition polymerization reaction of dihydroxy compounds with diisocyanates sets in on mixing the two components and gentle warming. Under proper conditions, linear polyurethanes with molecular weights up to about 15,000 can be obtained. As in the case of polyamides and polyesters, the softening point of the aliphatic polyurethanes depends on the number of carbon atoms between the urethane groups. 

Some investigators who studied polyurethane synthesis proposed that both the forward and the reverse reactions were important. For example this approach was assumed for the formation of linear polyurethane,49 and the following kinetic equation was derived ... 

Another example of the step-growth polymerization is the synthesis of polyurethanes. Here, linear polyurethanes are produced by the reaction of bifunctional alcohols, HO — R — OH, with bifunctional isocyanates, OCN — R — NCO, to produce a polyurethane (see Figure 3.21). 

When a diisocyanate reacts with a diol, a linear polyurethane is generated, and when it reacts with a polyol, it generates a cross-linked polymer. The isocyanates are commonly prepared by the reaction of phosgene and primary amines as follows ... 

Thus, from high MW diols (MW = 1000-4000 daltons) polyethers (polyalkyleneoxides, polytetrahydrofuran (PTHF)), polyesters, polycarbonates (PC), polybutadienes, etc., by the reaction with diisocyanates [toluene diisocyanate (TDI), or pure diphenyl methane diisocyanate (MDI)], high MW linear polyurethanes are obtained (no crosslinking), with high elasticity (polyurethane elastomers, spandex fibres, some adhesives and sealants, etc). 

There are a number of syntheses leading to the formation of urethane polymers. However, the most important commercial route is the isocyanate addition polymerization, the reaction between di- and polyfunctional hydroxyl compounds such as hydroxy-terminated polyethers or polyesters and di- or polyisocyanates. When difunctional reactants are being used, linear polyurethanes are produced and the reaction can be schematically represented as follows ... 

Previous studies( ) have shown how the number fraction of ring structures formed during irreversible linear random polymerisations leading to polyurethanes may be measured. The work has been extended(7,8) to non-linear polyurethane formation using hexa-methylene diisocyanate(HDI) and POP triols. For non-linear polymerisations, it is found that the number of ring structures per molecule(Nr) is always significant, even in bulk reactions. 

Figure 1. Number of ring structure per molecule (Np) as a function of extent of reaction(p) for linear and non-linear polyurethane forming reactions in bulk with approximately equimolar concentrations of reactive groups, r =[NC0no/[pHlo = 1) (6.7).

Although it is expected that difunctional monomers will give a linear polyurethane, the polymerization reaction is subject to possible side reactions. The formation of allophanate groups can occur, particularly if reaction temperatnres exceed 400 K. Here, an isocyanate group adds onto the secondary amine in the methane link, and a branched or cross-linked structure is formed. 

Undoubtedly, the influence of filler on the polymerization reaction kinetics [336-338] is very important for the polymer/substrate adhesive bond strength and formation of polymer layer structure at the phase boundary with the filling. To eliminate this influence, all subsequent investigations were performed with filled polymers derived from solutions of linear polyurethanes, synthesized in the absence of filler. 

A drawback to the cast elastomers is limited shelf-life and a need to store them in the absence of moisture. As a result, millable elastomers were developed. These are produced by first forming hydroxy-terminated linear polyurethanes through reactions of linear aliphatic polyesters or polyethers with diisocyanates. The prepolymers are rubber-like gums that can be compounded on rubber mills with other ingredients and crosslinked. Crosslinking is accomplished by adding either more diisocyanates, or sulfur, or peroxides. Diisocyanates dimers that dissociate at about 150 °C are often used ... 

Step addition and step condensation polymerization processes give rise to polymers containing distinctive functional groups at chain ends. The nature of the end groups will depend on the precise chemistry of the polymerization process. For example, linear polyurethanes are produced by reaction between diisocyanates and diols. If a perfect 1 1 stoichiometry of the reactants is used in the synthesis, on average each polymer chain must contain one isocyanate functional group and one alcohol group. If a 2 1 molar ratio of reactants is used, when the isocyanate is in excess, all the chain ends will have isocyanate functionality, and all will have alcohol functionality at the chain ends if a two-fold excess of diol is used. 

Urethanes are a reaction product of a diisocyanate and long- and short-chain polyether, polyester, or caprolactone glycols [2]. The polyols and the short-chain diols react with the diisocyanates to form linear polyurethane molecules. This combination of diisocyanate and short-chain diol produces the rigid or hard segment. The polyols form the flexible or soft segment of the final molecule. Figure 8.1 shows the molecular structure in schematic form. 

As indicated earlier in this chapter, although diisocyanates are the intermediates responsible for chain extension and the formation of urethane links or a variety of crosslinks by further reaction, much of the ultimate polymer structure is dependent upon the nature of the components carrying the groups with which the isocyanates react initially. Such components can be simple acu-diols, such as were employed in early work on linear polyurethanes, giving polymers linked only by —NHCOO—. Examples of such polymers are listed in Table 1.2. Linear polyurethanes of this type are crystalline, fibre-forming polymers but are lower melting than the corresponding polyamides, and none has become of real importance either as a synthetic fibre or as a thermoplastic material. 

Polyurethanes are generally synthesized by addition polymerization between a polyalcohol and a poly-isocyanate. This is an exothermic reaction caused by the release of a proton from the alcohol group followed by a general molecular rearrangement by the formation of the urethane bond [23], If both reagents are bi-functional linear polyurethanes are obtained, while if functionalities are increased some cross-linked chains are formed, with the formation of reticulated structures. In summary, one of the most common s mthesis routes for TPUs consists basically of the reaction of three main components. 

Liquid MDI (Isonate 143-L) is produced by converting some of the isocyanate groups in MDI to carbodiimide groups, which react with the excess isocyanate present to form a small amount of the trifunctional four-memhered ring cycloadduct (16). The presence of the cycloadduct lowers the melting point of MDI to give a liquid product. In most applications the trifunctional cycloadduct will dissociate into difimctional monomers therefore, this type of hquid MDI can be used in the manufacture of linear polyurethanes. Liquid MDI products are also made by reaction of the diisocyanate with small amounts of glycols. These products are called prepolymers. MDI products enriched in 2,4-MDI are also available. The latter are used in the manufacture of flexible MDI foams. 

L. BUhet, O. Gok, A.P. Dove, A. Sanyal, L.T.T. Nguyen, P.E. Du Piez, Metal-free functionalization of linear polyurethanes by thiol-maleimide coupling reactions, Macromole-cnles 44 (October 25, 2011) 7874-7878. 

It will be apparent that this reaction leads to polyurethanes when multifunctional reactants are used. When a diisocyanate and a diol react together a linear polyurethane is obtained whilst a diisocyanate and a polyhydric compound (polyol) lead to a cross-linked polymer. A cross-linked polyurethane could also be derived from a compound containing three or more isocyanate groups and a diol but this approach is of limited commercial importance. 

Linear polyurethanes (PUR) are formed in a polyaddition readion of diisocyanates [e.g. toluene diisocyanate (TDI) or methylene diphenyl diisocyanate (MDI)] and diols (e.g., 1,4-butanediol or 1,6-hexanediol). Crosslinldng is introduced by either the reaction of triisocyanates or the reaction of triols instead of the corresponding difimctional monomer. 

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