Segmented Polyurethanes Can Be Represented By Three Basic...

Segmented Polyurethanes Can Be Represented By Three Basic...

Segmented polyurethanes can be represented by three basic components in the following general form: P–(D(CD)n–P)n



Where P is the polyol, D is the diisocyanate and C is the chain extender. Polyol, or the so–called soft segment, is an oligomeric macromonomer comprising a "soft" flexible chain terminated by hydroxyl (–OH) groups. The chain extender is usually a small molecule with either hydroxyl, or amine end groups. The diisocyanate is a low molecular weight compound that can react with either the polyol or chain extender, leading to the interesting segmented structure. In linear polyurethanes, the three components have a functionality of two. If a branched or crosslinked material is desired, multifunctional polyols, isocyanates, and sometimes chain extenders can be incorporated into the formulation. Due to the statistical nature of the copolymerization, polyurethanes have both a distribution in total molecular weight and adistribution in the hard segment sequence length, those copolymer sequences denoted as D(CD)n, that follow essentially a most probable distribution.

The principle chemical reaction involved in the synthesis of polyurethanes is the urethane–forming reaction, i.e., the reaction between isocyanate and hydroxyl groups.

Because this is a nucleophilic addition reaction, it is catalyzed by basic compounds such as tertiary amines and by metal compounds such as organotin. Urethane formation is actually an equilibrium reaction; the presence of catalyst therefore

... Get more on HelpWriting.net ...