Prepolymer molecules

Note A prepolymer molecule capable of entering into further polymerization through reactive end-groups, often deliberately introduced, is known as a telechelic molecule. 

Low-molecular-weight glycols or diamines are used for further propagation of prepolymer molecules and for crosslinking the chains ... 

If the polyurethane molecule is allowed to form a three-dimensional structure, it becomes insoluble and neither wet nor dry spinning processes can be used. A chemical spinning process must be used in this case. The trick is to spin the isocyanate-terminated prepolymer at a stage where it forms a viscous melt, and with the jets produced in an environment that contains a chain extender. This chain extender diffuses into the fibers and reacts to couple the prepolymer molecules (Couper, 1985). 

The DAP prepolymer was prepared by bulk polymerization of DAP, in whieh conversion was about 10%, and characterized as Rus = 0.275, P = 65.8, and Pw/Pn = 3.71. That is, the polymer carries, on average, 36 pendant allyl groups and one crosslinkage per prepolymer molecule. It should be noted that under the same polymerization conditions for the preparation of DAP prepolymer the theoretical gel point was estimated to be 3.4%, being quite low eompared with 10% conversion. 

Flory has also extended his approach to cover cross-linked linear polymers and concluded that, for large prepolymer molecules, a ratio of one cross link per two prepolymer molecules is sufficient for gelation. An extensive review of this subject has been published by Lilley (15). 

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. 

Consider n prepolymer molecules containing an arbitrary number of active groups along the chains and having a lumber average molecular weight of This prepolymer has a glass point of... 

Fig. 2. Random coil model of a prepolymer molecule with active groups A.

For that purpose every prepolymer molecule must have two reactive groups and these must, as far as possible, be situated at the chain ends (telechelic polymers). Every molecule must be exactly difunctional and the reactivity of the ensuing crosslinking step must agree with the preceding chain extension. Liquid NBR is produced with carboxyl, hydroxyl, mercaptan, amine, vinyl, or halogen terminal groups. 

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