Prepolymers reaction sequences

Fig. 2.2. Prepolymer reaction sequence for elastomer synthesis (from AHport Janes, 1973).

The above data prove that the polystyrene-h-polybutadiene prepolymer is quantitatively transformed into block copolymers with perfluorinated side chains. The narrow molecular-weight distribution (I) = MW/MN) of the prepolymers is maintained by the described reaction sequence. 

Polyurethanes offer a convenient method by which iimnobilization of enzymes can be affected. Prepolymers are polymers with active end groups. While the primary purpose of the isocyanate end groups is serving as chain-extending agents, they also react with the amines that characterize an enzyme backbone. Thus, as many of the studies cited will show, the reaction sequence is (1) preparation of an aqueous solution of the enzyme and (2) emulsification of the solution with the prepolymer. The reaction time is on the order of 0.5 hours compared to the 24 hours required by some methods. 

Crosslinked poly(ortho esters) are prepared by a reaction sequence in which an excess of the diketene acetal 3,9-bis(ethylidene 2,4,8,10-tetraoxaspiro [5,5] undecane) is reacted with a diol, and the ketene acetal terminated prepolymer is then crosslinked with a triol. Because the prepolymer is a viscous liquid at room temperature, the therapeutic agent and any excipients used are incorporated into the prepolymer by mixing at room temperature and then cured at temperatures that can be as low as 40°C. 

A typical reaction sequence for the addition of the glycidyl methacrylate to the polyester prepolymer was as follows ... 

The reaction sequences used in this preparation followed the conventional polyurethane prepolymer sequence with the following provision that steps were made to obtain in the final PU elastomer controlled quantities of covalent-type crosslinks by leaving calculated excesses of isocyanate groups in the initially cast and cured PU. 

As prepolymers used in crosslinking reactions of acrylic enamels are usually copolymers of acrylic (x = H) and methacrylic (x = CH ) type monomers, the macroradical I can undergo a number of rearrangements depending on the local sequence distribution. For X = Y = CH3 (a methacrylate sequence) one obtains ... 

The reactions by which urethane foam are produced can be carried out in a single stage (one-shot process) or in a sequence of several stages (prepolymer process and quasi-prepolymer process.) These variations led to 27 basic types of products or processes, all of which have been used commercially. 

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