Macrodiisocyanate prepolymer

To avoid the appearance of terminal blocks of the type -[I-MD]-, the prepolymer technique is usually used in most of the cases. In this type of synthesis, the MD is first reacted with an excess of I so that the whole amount of MD is transformed in a macrodiisocyanate prepolymer ... 

The prepolymers described above are one type of telechelic polymer. A telechelic polymer is one containing one or more functional end groups that have the capacity for selective reaction to form bonds with another molecule. The functionality of a telechelic polymer or prepolymer is equal to the number of such end groups. The macrodiol and macrodiisocyanate telechelic prepolymers have functionalities of 2. Many other telechelic prepolymers were discussed in Sec. 2-12. (The term functional polymer has also been used to describe a polymer with one or more functional end groups.)... 

Polyurethanes are used in four principal types of products foams, elastomers, fibers, and coatings. The majority of polyurethane is used as rigid or flexible foams. However, about 15% is used for elastomer applications. Production of polyurethane elastomers involves a number of steps. As indicated above, an intermediate hydroxyl-terminated low-molecular-weight polyester or polyether is prepared. This intermediate is reacted with an isocyanate to form a prepolymer (macrodiisocyanate). The prepolymers are coupled or vulcanized by adding a diol or diamine ... 

Let us consider the properties of an adhesive based on the following mixture of oligomers an unsaturated polyester resin and a prepolymer with end isocyanate groups (or macrodiisocyanate) based on polydiethylene glycol adipate of molecular weight 800 and on TDI (a mixture of the 2,4- and 2,6-isomers in ratio of 65 35), with the... 

Fig. 7. Reaction scheme for the prepolymer formation and conversion of the macrodiisocyanate into non-reactive methylurethane.

The principles of the prepolymer formation and of the analysis of the composition of the prepolymer have already been described in the Experimental Part. The prepolymer formation in the melt was investigated with monodisperse polyols of different chain lengths and for molar ratios of MDI/polyol varying between 2 and 35. In order to facilitate the chromatogrhic analysis and to avoid side reactions of the macrodiisocyanate samples during the analytic procedure the isocyanate groups were converted into inactive methylurethanes (Figure 7) prior to the HPCL analysis. 

A typical HPLC trace of the prepolymer resulting from the system POTM-12 and MDI (molar ratio 1 3) is depicted in Figure 8. It is evident that the composition of the prepolymer with regard to the mole fraction of residual MDI (p=1) and of the macrodiisocyanate without (p=2) and with pre-extension (p>3) can be determined with high accuracy from the chromatogram resolved into the individual sequences containing two terminal diisocyanate residues ("ideal macrodiisocyanate", p=2) and the pre-ex-tended oligomers with one (p=3) or more (p>4) internal diisocyanate units. 

The variation of the molar ratio POTM-n/MDI in the prepolymer formation in bulk has shown that the pre-extension detectable within the experimental error becomes neglectable only at more than fifty-fold excess of MDI however, a ten-fold excess of MDI already reduces the extension of POTM-n to such an extent that over 90% of the prepolymer consists of "ideal macrodiisocyanate molecules, i.e., POTM-n endcapped with MDI, and only less than 10% of the prepolymer molecules are pre-extended. This is considered to be an acceptable compromise between the objective to prepare model PU elastomers with a soft segment distribution as narrow as possible and a reasonable experimental effort. 

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