Isocyanates, polymerization catalysts

I would like to acknowledge the contributions of my former co-workers at the Donald S. Gilmore Research Laboratories of the Upjohn Company, especially Dr R.H. Richter and B. Tucker who were involved in the synthesis and cycloaddition reactions of carbodiimides Dr L.M. Alberino who participated in the synthesis of polycarbodiimides Dr K. Onder and Dr WJ. Farrissey, Jr, who played a major role in the development of thermoplastic polyamides based on carbodiimide chemistry Dr H.W. Temme and Dr C.P. Smith, who developed novel polymeric catalysts for the conversion of isocyanates into carbodiimides and A. Odinak, who developed the liquid MDI process. 1 would especially like to acknowledge the encouragement of the late Dr A.A.R. Sayigh. 

Polymeric catalysts are also developed. For example, phospholene oxide modified divinylbenzene/styrene copolymers, as well as a polystyrene anchored triph-enylarsine oxide catalyst were prepared. The solid phase catalysts can be removed by filtration after partial conversion of an isocyanate to the carbodiimide. Such a catalyst is useful for the preparation of carbodiimide modified liquid MDl (4,4 -diisocyanatodiphenylmethane) products, which are of considerable commercial interest. 

Development of molecularly imprinted enantioselective hydrogenation catalysts based on immobilised rhodium complexes was reported by Gamez et al. [29]. The imprinted catalysts were prepared by polymerising Rh(I)-(A,A -dimethyl-l,2-diphe-nylethanediamine) with di- and tri-isocyanates, using a chiral alkoxide as the template (9). The imprinted polymer, after removal of the template, was tested for the reduction of ketones to alcohols. An enhanced enantioselectivity was observed in the presence of the imprinted polymeric catalyst, in comparison to the control polymer. 

Which polymer is formed depends upon the relative rates of subsequent reactions. If chain termination then occurs with loss of X, a cyclic dimer is produced if a third isocyanate molecule is added, followed by loss of X, a cyclic trimer occurs if chain termination is relatively slow, addition of further monomers takes place with formation of a linear polymer. Conditions such as temperature, catalyst concentration, and character contribute to the reaction pattern. The tendency to cyclize no doubt plays a specially large part in isocyanate polymerization. 

Free radical catalysts, such as potassium persulfate and azo compounds, are ineffective in initiating this type of polymerization of isocyanates. Also, catalysts, such as triethylphosphine and triethylamine, which are known to catalyze dimer and triraer formation at higher temperatures, are not effective in initiating the polymerization to linear 1-nylons. 

The polyisocyanates used for preparing carbodiimide foams include TDl, TDl-based prepolymers, liquid-MDl oligomers and polymeric isocyanates. Many catalysts for producing carbodiimide foams have been disclosed in the patent literature. Some of these are shown in Table 13. 

Phosphino)amines and their complexes have been shown to be efficient catalysts for the palladium-catalyzed Suzuki coupling reaction of chloroarenes,449 rhodium-catalyzed hydroformy-lations458 and asymmetric hydrogenations,463,466 allylic substitution reactions,47, 472 conversion of isocyanates to isocyanurates,478 and as ethylene polymerization catalysts.479... 

Isocyanates polymerize through the carbon-to-nitrogen double bonds by anionic mechanism. Reactions can be catalyzed by sodium or potassium cyanide at-58 °C. N,N -dimethylformamide is a good solvent for this reaction. Other anionic catalysts, ranging from alkali salts of various carboxylic acids to sodium-naphthalene, are also effective. In addition, polymerizations can be carried out by cationic, thermal, and radiation-induced methods. 

Tri-n-butyl phosphine n. (C4H9)3P. A curing agent for epoxy resins, and a catalyst for vinyl and isocyanate polymerizations. 

Polymerization of isocyanate to polyisocyanates (polyamide 2) proceeds in presence of anionic polymerization catalysts, such as NaCN, triethylphosphine, butyllithium and strong bases, according to the following scheme ... 

The DMAP analogues bound to cross-linked PS are active in non-polar solvents for esterification of sensitive tertiary alcohols as in equations (21) and (22), dimerization of phenyl isocyanate as in equation (23), nucleophilic acyl rearrangements as in equation (24), and synthesis of dipalmitoylphosphatidylcholine from palmitic anhydride as in equation (25). The polymeric catalysts are slightly less active than DMAP, but they have been recovered and recycled three times with no loss of activity. " The spacer chain catalyst (58 n = 3, DF = 0.16-0.48, 2% DVB) was more active than catalyst (58 n= ) for acetylation of 1-methylcyclohexanol. Spacer chains (n = 4,7) and DF 0.15-0.20 gave highest activity for acyl rearrangements. A mixture of catalyst (58) and cross-linked poly(V,V-diethylaminomethylstyrene), as a proton acceptor, was more active for acetylation of linalool (equation 21), than catalyst (58) alone. 

Oligomerization and Polymerization Reactions. One special feature of isocyanates is their propensity to dimerize and trimerize. Aromatic isocyanates, especially, are known to undergo these reactions in the absence of a catalyst. The dimerization product bears a strong dependency on both the reactivity and stmcture of the starting isocyanate. For example, aryl isocyanates dimerize, in the presence of phosphoms-based catalysts, by a crosswise addition to the C=N bond of the NCO group to yield a symmetrical dimer (15). 

Commercially, polymeric MDI is trimerized duting the manufacture of rigid foam to provide improved thermal stabiUty and flammabiUty performance. Numerous catalysts are known to promote the reaction. Tertiary amines and alkaU salts of carboxyUc acids are among the most effective. The common step ia all catalyzed trimerizations is the activatioa of the C=N double boad of the isocyanate group. The example (18) highlights the alkoxide assisted formation of the cycHc dimer and the importance of the subsequent iatermediates. Similar oligomerization steps have beea described previously for other catalysts (61). 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

A novel chemoenzymatic route to polyester polyurethanes was developed without employing highly toxic isocyanate intermediates. First, diurethane diols were prepared from cyclic carbonates and primary diamines, which were subsequently polymerized with dicarboxylic acids and glycols by using lipase CA as catalyst, yielding the polyurethanes under mild reaction conditions. 

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

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