Prepolymers, NCO-terminated

The polycyclotrimerization of difunctional isocyanates (or NCO-terminated prepolymers) produces polymer networks containing heterocyclic, thermostable per-hydro-1,3,5-triazine-2,4,6-trione (isocyanurate) rings as crosslinks ... 

The reaction between TDI and polyols has been simulated in a computer study [154], The objective was to prepare NCO-terminated prepolymers containing a minimum of monomeric TDI. Twelve simultaneous and/or consecutive reactions were treated mathematically. The calculated predictions were in good agreement with experimental results. 

The STPs are prepared by a reaction of NCO-terminated prepolymers and aminosilanes. In particular, STPs prepared from the anilinosilane 1 exhibit superior properties. The STP preparation can be previously performed inside the spray can. As a very small excess of the aminosilane is employed, an NCO-free product is obtained. 

Scheme 3. STP-Preparation from a NCO-terminated prepolymer and anilinosilane 1.

The NCO-terminated prepolymer can be synthesized from a diisocyanate (e.g., TDI, MDI) and a polyol. The anilinosilane 1 can be prepared from the corresponding chloromethylsilane and aniline, i.e. from low cost components, too. 

NCO-terminated prepolymers have been used for producing carbodiimide foams (91). Bayer AG also developed carbodiimide foams (82). 

The formation of step copolymers that are forming blocks is best illustrated through considering polyurethanes that are formed by prepolymers. This was discussed earlier in outline for the polyurethane co-polyureas. In the following example, a polyester-urethane is formed by reacting an —OH-terminated polyester with an -NCO-terminated polyurethane. Each of these is formed by the appropriate stepwise polymerization and then reacted to give a larger -NCO-terminated prepolymer ... 

All chemicals used in this study were reagent grade. Butyl isocyanate (BuNCO, 99% from the Upjohn Chemical Co.), hexamethy-lene diisocyanate (HDI, 99% from the Mobay Chemical Co.), phenyl isocyanate (PhNCO, 99%, from the Upjohn Chemical Co.), p-tolyl isocyanate (MePhNCO, 99% from the Aldrich Chmical Co.), p-chloro-phenyl isocyanate (CIPhNCO, 99%, from the Aldrich Chemical Co.) and cyclohexyl isocyanate (CHI, 98%, from the Aldrich Chemical Co.) were purified by vacuum distillation. Methylene diphenyl diisocyanate (MDI, 99%+, from the Mobay Chemical Co.) was used without further purification. N,N-Dimethylformamide (DMF, reagent grade, from the Mallinckrodt) was dried by molecular sieves 4a. The NCO-terminated prepolymers were prepared from poly(oxy-tetramethylenediol) (POTMD, mol. wt. 650, 1000, 2000, Quaker Oats Chem. Co.) and MDI. 

Prepolymer-MDI Polycocyclotrimers. The NCO terminated prepolymers were prepared by reacting MDI with poly(oxytetramethy-lenediol) (MW=1000, 2000) in the reaction kettle, equipped with the nitrogen inlet and outlet, mechanical stirrer, heating mantle and a thermometer. The reaction was carried out at 70°C without any catalyst and was terminated when the NCO content reached the theoretical values. (11)... 

Better results were obtained by utilizing the second method for the preparation of polycocyclotrimers. This polycocyclo-trimers were prepared from the NCO-terminated prepolymers (MDI-POTMD, MW = 1000, 2000) and difunctional isocyanate (MDI) using trimerlzation catalyst TDAPH. 

Sodium bisulfite-blocked isocyanate-terminated prepolymers are extensively used as shrink-resistant finishes for wool. Polycarbamoylsulfonates (PCS) are produced from NCO-terminated prepolymers with bisulfite salts in aqueous alcohol as the solvent, shown schematically as follows (78-80) ... 

The hydroxyl-terminated homo- and copolymers of butadiene have been utilized in the preparation or modification of urethane elastomers and sealants (102). Owing to the presence of unsaturation in these polymers, reinforcement with certain fillers such as carbon black can be achieved. Hydroxyl-terminated polyisobutylenes have also been used for the preparation of NCO-terminated prepolymers (103). Very recently (104). a,u-di-(hydroxy)polyisobutylenes have been prepared by a different route. Polyurethane films prepared from these polyisobutylene glycols have exhibited excellent hydrolytic stability and very low water absorption (105). However, these materials are not yet commercially available. 

The role chitin as a material of highly ordered crystalline structure has been reported in the study [96]. X-ray diffraction analysis was carried out in order to find the changes of the crystalline structure upon the substitution reaction with NCO terminated prepolymer. The X-ray diffraction studies showed that crystallinity mainly depends on the concentration of chitin in the polyurethane backbone, crystallinity increased as the concentration of chitin into the final PU increased (Fig. 3.22). The crystallinity of some polymers was clearly observed by optical microscopic studies [114]. The results of X-ray diffraction experiments correlate with optical microscopy findings. A crystalline polymer is distinguished from an amorphous polymer by the presence of sharp X-ray Unes superimposed on an amorphous halo. Under an optical microscope, the presence of polycrystalline aggregates appear as spherulites [114]. The spheruhtes are made of small crystallites and grow Irom a nucleus at their centre. They consist of narrow chain folded lamellae growing radially. Since the fibrous crystals are radial, the chains folded with the lamellae are circumferentially oriented. From the evaluation of the X-ray and optical microscopic studies, it has been observed that the involvement of chitin in the PU formulation and have improved crystallinity of the final polyurethane. 

NCO - terminated prepolymer-ionomer diluted with NMP or acetone... 

The prepolymer can be synthesized by a simple one-pot synthesis . During a first step an NCO-terminated prepolymer is prepared by a reaction of the polyol with an excess of TDI. The second step comprises the reaction of all remaining NCO groups with the silane 1. Phenyltrimethoxysilane 2 serves to reduce the viscosity of the prepolymer mixture. It has already been added during the prepolymer synthesis. 

In the melt-dispersion process (95), an NCO-terminated prepolymer is prepared next from a polyester or polyether diol, a diisocyanate, and a glycol containing a potential ionic group. The NCO/OH is between 1.2 and 1.8. Then, the prepolymer is terminated with urea to form biurets, which are further alkylated to give an ionomer as shown in Fig. 17. The bis-biuret can be considered to be somewhat hydrophilic to be methylolated with formaldehyde. Upon dilution with water, a stable dispersion is formed. 

Liquid-NCO terminated prepolymers, which are primarily based on low-viscosity polyoxypropylene glycols and TDI require only a small amount of solvent (5-10%). However, the solvent remains in the finished dispersion. In addition the reaction of the primary emulsion with water or diamines, which results in the formation of poly urea, is dependent upon diffusion and difficult to control. 

One component of a two-part urethane adhesive typically contains an NCO-terminated prepolymer and possibly fillers. The second component consists of polyfunctional, NCO-reactive materials, such as polyols and/or polyamines, and may also contain fillers and catalysts. The work life and rate of cure can be adjusted by the selection of catalysts, the catalyst level, and the starting materials used to prepare the resins. Table III lists the cure times for three adhesives which have identical bond strengths when cured, but different work lives. Two-part urethane systems usually cure at room temperature, but the cure can be accelerated by the use of heat. Although... 

Scheme 3 gives the reaction route to the preparation of the shape memory TPUs crosslinked by trifunctional polyols. PTMG (M =250) and a molar excess of MDI were first reacted to obtain NCO-terminated prepolymers (NTP), which were... 

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