Polymerization of Isocyanates

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... 

The anionic polymerization of isocyanates using NaCN as an initiator was first reported in I960.998 The living coordinative polymerization of n-hexylisocyanate has been described using the titanium(IV) complexes (345) (348).999-1001 A trifunctional initiator has also been used to prepare star polyisocyanates.1002... 

Reactions with Isocyanates, TYZOR TPT catalyzes the trimerization of isocyanates and polyisocyanates to isocyanurates and polyisocyanurates (38). Titanium alkoxides of the type Cl3TiOR initiate the living polymerization of isocyanates. Polyisocyanates possessing controlled molecular weights and narrow polydispersities can be synthesized using these catalysts (39) ... 

A stereospecific nature of the cathodically initiated anionic polymerization of isocyanates in DMF containing BU4N has been mentioned [16]. 

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 polymerization of isocyanates to 1-nylons is thought to be a new example of anionically polymerizable systems requiring basic catalysis and low temperatures. Thus, the initiation step can be considered as the attack of the anion (X ) at the isocyanate group as shown below. 

An i.r. study of the reactions between C2H5NCO and Si02 surfaces has been undertaken.299 The products of the adsorption process include a surface urethane, 1,3-diethylurea, a biuret, dissociatively adsorbed isocyanate and ethoxy-groups, and compounds formed by the polymerization of isocyanic acid and HCN. 

Miscellaneous Routes. Polyamides have been prepared by other reactions, including addition of amines to activated double bonds, polymerization of isocyanates, reaction of formaldehyde with dinitriles, reaction of dicarboxylic acids with dllsocyanates, reaction of carbon suboxide with diamines, and reaction of diazlactones with diamines. These reactions are reviewed in Reference 4. 

The rate of self-polymerization of isocyanates to dimers (uretidine diones) depends upon the electronic or steric influences of ring substituents. Ortho substitution greatly retards dimerization of the NCO groups, with the ortho NCO slower to dimerize. This dimerization is catalyzed strongly by trialkyl phosphines (38, 39) and more mildly by tertiary amines, such as pyridine (40. 41). MDI dimerizes slowly on standing at room temperature even without catalysts but is stable at low or at slightly elevated temperatures (40-50 °C). 

Polyurethanes are formed by polymerization of isocyanates with polyether polyols or polyester polyols. The products are quite versatile and can be manufactured as either foamed or elastomerics. In addition, a small amount of isocyanates are used for adhesives and coatings and a minor quantity of monoisocyanates are used as precursors for carbamate pesticides. Phosgene is a key reactant in the commercial processes for all of the isocyanates. 

The latter type of catalysts (Cp-TiCl3) are very tolerant toward the polymerization of isocyanates with other functional groups such as unsaturation. For example, 2-isocyanatoethylmethacrylate give an unsaturated polymer. 

At present, we only know how to prepare Nylon 1 by anionic polymerization of isocyanates. This reaction is discussed in Chapter 3 ... 

Unbranched chains are formed when the functionality of the molecule is equal to 2. The bifunctionality of the molecule can result from a single bifunctional group, as occurs with the base-initiated polymerization of isocyanate groups ... 

Polyimides contain the group —CO—NR—CO—, in the main chain. The basic member of this series is formed by the spontaneous polymerization of isocyanic acid at 15 C ... 

Substituted nylon 1 types (—NR—CO—) are obtained by polymerization of isocyanates, RNCO, with, for example, KCN as initiator in solvents such as dimethyl formamide. The products have no commercial application. 

Reaction polymerization reactions of isocyanates with suitable monomers can he performed in an extruder or in a RIM machine. In the latter reaction thermosets (cross-hnked polymers) are produced. In an extruder usually linear polymers are manufactured. For example from methylene di-p-phenylene isocyanate (MDI), with some macroglycols and 1,4-hutanediol as extenders, segmented polyurethane elastomers are produced in an extruder (6). However, linear condensation polymers are also produced in a vented extruder. For example from MDI, with macrodicarboxylic acids and dicarboxyhc acids as extenders thermoplastic block copolyamide elastomers are produced. The by-product of the condensation reaction, carbon dioxide, is removed in the vented extruder. The polycondensation process can also be performed in solution. For example, MDI can be added to a solution of dicarboxyhc acids in tetramethylene sulfone, with simultaneous removal of the carbon dioxide. Tetramethylene sulfone is the solvent of choice for solution polymerization of isocyanates (7). In addition to dicarboxyhc acids trimellitic acid anhydride and benzophenonetetracarboxylic acid dianhydride (BTDA) are utilized as monomers for condensation polymers. With these monomers poly(amide imides) and poly(imides) are produced. The diisocyanate-derived commercial polycondensation products are listed in Table 1. 

Miscellaneous. Patten and Novak have described (236) the polymerization of isocyanates using CpTiCl2(OCH2CF3) as a single-component catalyst. Unlike non-metallocene titanium catalysts, this species is active for the... 

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 ... 

Dienes or olefins are polymerized in the presence of a 7r-allylnickel complex and a phosphate as catalysts to afford the polymers having a Tt-allyl group at the end position as shown in eq. (19.50). Then they are able to easily copolymerize with styrene or isocyanates [61,72,92-96]. Further, the polymerization of isocyanate and cyclohexadiene has been tried in the presence of nickel catalysts [94,95]. 

It may be noted that the anionic polymerization of isocyanates to high molecular weight polymers is mentioned in Section 1.4.2.3 but this process is not of commercial importance. 

T. E. Patten, B. M. Novak, Oiganotitanium(IV) compounds as catalysts for the polymerization of isocyanates the polymerization of isocyanates with functionalized side chains. Macromolecules 26,1993 436. 

Patten, T.E. and Novak, B.M. (1991) Living titanium(IV) catalyzed coordination polymerizations of isocyanates. 

Curing reactions in the case of polyurethanes can be divided into two main classes (a) the reaction of isocyanates with compounds containing reactive hydrogen atoms to give addition products and (b) the polymerization of isocyanates, le, self-addition. 

Lastly, mention will be made of two further amide block copolymer syntheses not mentioned above. The first employs living polymer anions from styrene, isoprene or methyl methacrylate to initiate the polymerization of isocyanates to diblock polymers containing nylon 1 sequences. In this approach, selective polymerization can be achieved at the unhindered isocyanate group of diisocyanates such as tolylene 2,4-diisocyanate to give products with pendant NCO groups (32) which are crosslinkable with diols to give tough resins. 

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