Polymerisation of Isocyanates

Liquid Ghromatography/Mass Spectrometry. Increased use of Hquid chromatography/mass spectrometry (Ic/ms) for stmctural identification and trace analysis has become apparent. Thermospray Ic/ms has been used to identify by-products in phenyl isocyanate precolumn derivatization reactions (74). Five compounds resulting from the reaction of phenyUsocyanate and the reaction medium were identified two from a reaction between phenyl isocyanate and methanol, two from the reaction between phenyl isocyanate and water, and one from the polymerisation of phenyl isocyanate. There were also two reports of derivatisation to enhance either the response or stmctural information from thermospray Ic/ms for linoleic acid hpoxygenase metabohtes (75) and for cortisol (76). 

This system was slightly modified by R J. Flory, who placed the emphasis on the mechanisms of the polymerisation reactions. He reclassified polymerisations as step reactions or chain reactions corresponding approximately to condensation or addition in Carother s scheme, but not completely. A notable exception occurs with the synthesis of polyurethanes, which are formed by reaction of isocyanates with hydroxy compounds and follow step kinetics, but without the elimination of a small molecule from the respective units (Reaction 1.3). 

Contact with the basic solvent causes violent polymerisation of the isocyanate. 

Some of the reaction products of polymerisation and cure can be toxic, for example, aromatic amines from hydrolysis of isocyanates and bisphenol A from... 

The mechanism of high temperature bulk polymerisation of NCA s is still obscure. It was suggested that the reaction involves the isomerisation of the anhydride into isocyanate, viz. 

The coordination polymerisation of heterounsaturated monomers, such as aldehydes [101-103] and ketones [104], isocyanates [105] and ketenes [106,107], in homopolymerisation systems has not been widely described in the literature. However, the coordination copolymerisation of heterounsaturated monomers not susceptible to homopropagation, such as carbon dioxide [71,108-113], with heterounsaturated monomers such as cyclic ethers has been successfully carried out and is of increasing interest. 

Coordination polymerisations of alkyl isocyanates have not been widely studied, since these monomers could be polymerised via their C=N bond by using anionic initiators. However, such anionic polymerisations require low-temperature conditions [264]. It has been found recently [265] that alkyl isocyanates are capable of polymerisation in the presence of coordination catalysts at ambient temperature. By contrast, phenyl isocyanate appeared capable of coordination copolymerisation with oxirane [266]. 

Polymerisation of w-butyl isocyanate was carried out with the Pruitt-Baggett adduct Re(OR)2Cl [267] at ambient temperature, yielding polyamide, a polymer of the nylon-1 type (Table 9.3) [268]. For more detailed mechanistic studies, -butyl isocyanate was also subjected to polymerisation with a catalyst derived from the reaction of the Pruitt-Baggett adduct with /i-naphthyl-/V-( -butyl) urethane (for the sake of clarity, the catalyst association and internal complexation of the Fe atom by etheral O atoms of the OR substituent are omitted in formulae) ... 

Polymerisation of n-butyl isocyanate was accompanied with cyclic trimer formation, which has been explained in terms of the backbiting reaction [268],... 

A typical polyester could be poly (diethyleneglycol adipate) with a molecular weight of between 2000 and 3000. A typical polyether is obtained by polymerisation of propylene oxide, usually in the presence of a small proportion of glycerol or sorbitol to provide branched structures. Two of the most commonly used isocyanates are 2,4- and 2,6- tolylene di-isocyanate (TDI). These are chosen because of their reactivity, cheapness and relative low toxicity. The resulting prepolymer has -OH terminal groups when a deficiency of isocyanate is used, but -NCO terminal groups if an excess of isocyanate is employed. 

Scher HB, Rodson M and Lee K Microencapsulation of pesticides by interfacial polymerisation utilising isocyanate or aminoplast chemistry, Pesticide Science, 54 394-400... 

Wood furniture coatings usually contain urethane/isocyanates in addition to UF, volatile plasticisers, residual solvents and free monomers from incomplete polymerisation of the coating. Nitrocellulose lacquer, acrylic, cellulose acetate butyrate and polynrethane (with plasticisers like epoxy, di-bntyl phthalate, butyl benzyl phthalate and isopropyl myristate) are the coatings commonly nsed on clear finished wood furniture. In these, the amonnts and... 

The applications of this technique have been reviewed [68]. In addition to chemical reactions associated with thermal and oxidative decomposition, DSC can also be used to monitor the curing of thermosets (see Section 10.3). Examples are the polymerisation of unsaturated polyester resins, poly-addition of epoxy resins with curing agents, and isocyanates with polyols. 

DSC can be used to monitor the curing of thermosets. Examples are the polymerisation of unsatured polyester resins, poly-addition of epoxy resins with curing agents and isocyanates with polyols. 

Figure 4a represents interfacial polymerisation encapsulation processes in which shell formation occurs at the core material—continuous phase interface due to reactants in each phase diffusing and rapidly reacting there to produce a capsule shell (10,11). The continuous phase normally contains a dispersing agent in order to faciUtate formation of the dispersion. The dispersed core phase encapsulated can be water, or a water-immiscible solvent. The reactant(s) and coreactant(s) in such processes generally are various multihmctional acid chlorides, isocyanates, amines, and alcohols. For water-immiscible core materials, a multihmctional acid chloride, isocyanate or a combination of these reactants, is dissolved in the core and a multihmctional amine(s) or alcohol(s) is dissolved in the aqueous phase used to disperse the core material. For water or water-miscible core materials, the multihmctional amine(s) or alcohol(s) is dissolved in the core and a multihmctional acid chloride(s) or isocyanate(s) is dissolved in the continuous phase. Both cases have been used to produce capsules. 

Figure 5 illustrates the type of encapsulation process shown in Figure 4a when the core material is a water-immiscible Hquid. Reactant X, a multihmctional acid chloride, isocyanate, or combination of these reactants, is dissolved in the core material. The resulting mixture is emulsified in an aqueous phase that contains an emulsifier such as partially hydroly2ed poly(vinyl alcohol) or a lignosulfonate. Reactant Y, a multihmctional amine or combination of amines such as ethylenediamine, hexamethylenediamine, or triethylenetetramine, is added to the aqueous phase thereby initiating interfacial polymerisation and formation of a capsule shell. If reactant X is an acid chloride, base is added to the aqueous phase in order to act as an acid scavenger. 

Step-Growth Gopolymerization. A sample of a block copolymer prepared by condensation polymerisation is shown in equation 30 (37). In this process, a prepolymer diol (HO—Z—OH) is capped with isocyanate end groups and chain extended with a low molecular-weight diol (HO—E—OH) to give a so-called segmented block copolymer, containing polyurethane hard blocks and O—Z—O soft blocks. 

Type AD-G is used in an entirely different sort of formulation. The polymer is designed for graft polymerisation with methyl methacrylate. Typically, equal amounts of AD-G and methyl methacrylate are dissolved together in toluene, and the reaction driven to completion with a free-radical catalyst, such as bensoyl peroxide. The graft polymer is usually mixed with an isocyanate just prior to use. It is not normally compounded with resin. The resulting adhesive has very good adhesion to plasticised vinyl, EVA sponge, thermoplastic mbber, and other difficult to bond substrates, and is of particular importance to the shoe industry (42,43). 

The third approaeh to synthetic polymers is of somewhat less commereial importance. There is in fact no universally accepted deseription for the route but the terms rearrangement polymerisation and polyaddition are commonly used. In many respects this process is intermediate between addition and condensation polymerisations. As with the former teehnique there is no moleeule split out but the kinetics are akin to the latter. A typical example is the preparation of polyurethanes by interaction of diols (di-alcohols, glycols) with di-isocyanates Figure 2.7). 

Amongst the catalysts used or the polymerisation-trimerisation reactions are alkali metal phenolates, alcoholates and carboxylates and compounds containing o-(dimethylaminomethyl)phenol subgroups. Fluorocarbons such as trichloro-fluoromethanes are used as the sole blowing agents in the absence of any isocyanate-water reaction. 

The basic RIM process is illustrated in Fig. 4.47. A range of plastics lend themselves to the type of fast polymerisation reaction which is required in this process - polyesters, epoxies, nylons and vinyl monomers. However, by far the most commonly used material is polyurethane. The components A and B are an isocyanate and a polyol and these are kept circulating in their separate systems until an injection shot is required. At this point the two reactants are brought together in the mixing head and injected into the mould. 

These high energy species are extremely reactive, with themselves and with nucleophiles, and can generate runaway exotherms. With water, rapid evolution of carbon dioxide results. Some instances are reported [1], A compound of this class was resposible for the worst chemical industry accident to date. Di-isocyanates are extensively employed, with polyols, to generate polyurethane polymers. The polymerisation temperature should be held below 180°C or decomposition may occur which, in the case of foams, may induce later autoignition. 

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