1.2- Diisocyanates

The first essential component of a PU is the isocyanate. Molecules that contain two isocyanate groups are called diisocyanates. They are compounds containing the isocyanate group (-NCO). They react with compounds containing alcohol (hydroxyl) 

The PUs hard segments can be either aromatic or aliphatic. The aromatic isocyanates are more reactive than the aUphatic diisocyanates, which can only be utilized if their reactivities match the specific polymer reaction and special properties desired in the final product. Eor example, PUs made from aliphatic isocyanates are light stable [30-33], while materials made from aromatic isocyanates undergo photo degradation [34—36]. Furthermore, the reactivity of an isocyanate group may vary dramatically even within the same class of a diisocyanate. 

The structure, substituents, and steric effect can influence reactivity. For example in 2,4-toluene diisocyanate (TDf), the isocyanate group para to the methyl group is 25 time more reactive than the other NCO group at the ortho position [28]. Moreover, the reactivity of the second NCO group can change as a result of the initial reaction. 

Two of the most important aromatic isocyanates are 2,4-tolylene diisocyanate (TDI) and 4,4-diphenylmethane diisocyanate (MDI). TDI consists of a mixture of the 2,4-and 2,6-toluene diisocyanate isomers [37-39]. 

The isocyanates form the major part of the hard or rigid phase of the polyurethane. The three main isocyanates used in industry for castable materials are toluene diisocyanate (TDI), 4,4 diphenylmethane diisocyanate (MDI), and 1,5-naphthalene diisocyanate(NDI). Aliphatic diisocyanates form a small segment of the diisocyanate market. 

The reactivity of the various isocyanates is important in the processing of any system  

The aromatic diisocyanates are generally more reactive than the aliphatic diisocyanates. The position of the isocyanate group relative to surrounding groups controls the reactivity. 

TDI is used either as an 80 20 mixture of the 2,4 and 2,6 isomers or as 100% 2,4-toluene diisocyanate. The current tendency is to use the 80 20 mixed isomers for normal work and the 100% 2,4 isomer TDI for high-performance material. 

Because of their much higher costs, aliphatic diisocyanates find use mainly in specialized areas where their special properties such as nonyellowing in light are of great importance. The nonyellowing is a result of the aliphatic structure of the isocyanate. There are no series of double bonds that cause the yellowing. 

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The use of hydroxyethyl (also hydroxypropyl) methacrylate as a monomer permits the introduction of reactive hydroxyl groups into the copolymers. This offers the possibility for subsequent cross-linking with an HO-reactive difunctional agent (diisocyanate, diepoxide, or melamine-formaldehyde resin). Hydroxyl groups promote adhesion to polar substrates. 

Elastomeric Fiber. Elastomeric fibers are prepared by the polymerization of polymeric polyols with diisocyanates. 

Haward et al.t have reported some research in which a copolymer of styrene and hydroxyethylmethacrylate was cross-linked by hexamethylene diisocyanate. Draw the structural formula for a portion of this cross-linked polymer and indicate what part of the molecule is the result of a condensation reaction and what part results from addition polymerization. These authors indicate that the crosslinking reaction is carried out in sufficiently dilute solutions of copolymer that the crosslinking is primarily intramolecular rather than intermolecular. Explain the distinction between these two terms and why concentration affects the relative amounts of each. 

CbH)I [AMINES - CYCLOALIPHATIC AMINES] (Vol2) trans-1,4-Cyclohexane diisocyanate [7517-76-2]... 

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