Isocyanate and hydroxyl groups

Many of the common condensation polymers are listed in Table 1-1. In all instances the polymerization reactions shown are those proceeding by the step polymerization mechanism. This chapter will consider the characteristics of step polymerization in detail. The synthesis of condensation polymers by ring-opening polymerization will be subsequently treated in Chap. 7. A number of different chemical reactions may be used to synthesize polymeric materials by step polymerization. These include esterification, amidation, the formation of urethanes, aromatic substitution, and others. Polymerization usually proceeds by the reactions between two different functional groups, for example, hydroxyl and carboxyl groups, or isocyanate and hydroxyl groups. 

The chemistry is based on the reaction of isocyanate and hydroxyl groups ... 

In the case of urethane-foam formation, tin catalysts (Table 8) mainly promote the reaction between isocyanate and hydroxyl groups, i.e., the formation of the urethane linkage. 

Urethane-forming reaction between isocyanate and hydroxyl group was utilized by Osakada et al. to prepare polyurethane-cyclodextrin pseudoro-taxanes (Scheme 15) [86]. Polyaddition of a diol and MDI in the presence of permethylated a-cyclodextrin or permethylated (5-cyclo dextrin was carried out in DMF for 20 h at 120 °C to yield the pseudopolyrotaxane. The molar... 

As Equation 1 indicates, urethane formation is an equilibrium reaction. The degree of dissociation of urethane product into reactant isocyanate and hydroxyl groups is dependent upon the ambient temperature and is a constant at given temperature (Equation 2). 

Polyurethanes (single-component system) Block polyisocyanates and hydroxyl polymers Phenol-blocked isocyanates and hydroxyl groups Unblocking or removal of phenol to form a free isocynate followed by addition of hydroxyl groups to form urethane linkages 8 ... 

First, attention will concentrate on the polyurethane curing reaction itself that is, the polyaddition of isocyanate and hydroxyl groups. Degree of cure and isocyanate conversion rates (as deduced from FTIR) will be discussed for the bulk and compared with thin films on Au, Al, and Cu (Section 6.3.1). Secondly, the morphology of cured films (OM, SFM) will be treated in Section 6.3.2. 

Still Open for debate [20, 21], The contents of water and hydroxyl groups are very high in EPI adhesive gluelines, hence the probability of reaction between isocyanate and hydroxyl groups in the wood is most likely very low. The adhesion between the glue and the wood will, however, he improved by ionic interactions and mechanical and chemical bonds, e.g., hydrogen bonds [4, 6]. 

Organometalhc compounds based on lead, tin, bismuth and zinc are also used to catalyse a urethane reaction. Bismuth and zinc carboxylates are used because of the toxicity and disposal problems of lead and tin. Nowadays, alkyl tin carboxylates, oxides and mercaptide oxides such as dibutyltin dUaurate (DBTDL),dioctyltin mercaptide, stannous octoate and dibutyltin oxide are used successfully in all types of polyurethane applications (Table 6.4), among which DBTDL was found to be the most widely used catalyst. The catalytic effect of organometaUic compounds is due to their capacity to form a complex with the isocyanates and polyols. The catalysis mechanism involves interaction of the metal cation with isocyanate and hydroxyl groups, followed by rearrangement of the resulting complex to yield the final urethane product. 

Polyurethanes are a broad class of polymers produced by the polyaddition reaction of a diisocyanate or a polymeric isocyanate with a polyol, in the presence of suitable catalysts and additives. Under the name of polyurethanes, a practically unlimited number of structures can be involved. The only necessary condition is in general reduced to the presence of the urethane group, (-NHC0-0-) on the macromolec-ular chain with a more or less frequency. The urethane group is usually formed by reaction between isocyanate and hydroxyl groups, although alternative routes such as from bischloroformates and amines are used in special cases. 

Figure 4.1 Representative urethane/urea chemistry reactions (a) between isocyanate and hydroxyl groups (to form a urethane bond) (b) between a cyclic carbonate activated carbonate, carbamate, or chloroformate derivative group and amino group (c) between isocyanate and amino groups (to form a urea bond) (d).

In addition to the most intensively researched urethane reactions between isocyanate and hydroxyl groups (Figure 4.1(a)) that form urethane bonds, there are also nonisocyanate urethane reactions, such as the reactions between cyclic carbonate or activated carbonate/carbamate/chloroformate derivative groups and amine groups (Figure 4.1(b) and (c)) [4-6]. 

Table 3.2 Values of t and tmix computed at 80°C and values of tmax obtained from torque versus time curves presented in Figure 3.10. The initial concentration of isocyanate group (Cao) was 0.75 mol 1 and the ratio of the initial concentrations of isocyanate and hydroxyl groups (r) was 1.05. The timescale of mixing (tmix) was obtained using a typical droplet diameter of 40nm as...

Polyurethane (PU) is a polymer composed of a chain of organic units joined by carbamate (urethane) links. PU polymers are formed by combining two bi-fimctional (or higher) monomers, one of which contains two or more isocyanate functional groups with the formula — N=C=0, while the other contains two or more hydroxyl groups. The isocyanate and hydroxyl groups combine to form a urethane linkage ... 

The interest in blending of thermoplastics with thermoplastic polyurethanes (TPUs) is equivocal some properties of TPUs, such as erosion and abrasion resistance are excellent [85,86], which already justify the blending efforts, but the urethane linkage (-NH-C0-0-) in TPUs is unstable at high temperatures. Isocyanate and hydroxyl groups form via reversible dissociation of the urethane bond, and they are very reactive with the usual functional groups (amine, epoxy. 

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