Isocyanate oligomers

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 volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

The above processes are only selected examples of a vast number of process options. In the case of carbonylation, the formation of by-products, primarily isocyanate oligomers, allophanates, and carbodiimides, is difficult to control and is found to greatly reduce the yield of the desired isocyanate. Thus a number of nonphosgene processes have been extensively evaluated in pilot-plant operations, but none have been scaled up to commercial production of diisocyanates primarily due to process economics with respect to the existing amine—phosgene route. Key factors preventing large-scale commercialization include the overall reaction rates and the problems associated with catalyst recovery and recycle. 

Many polymers have been analysed by SFC, namely styrene and vinyl oligomers, polysiloxanes, polysaccharides, polyethers, polyesters, polyolefins and waxes, and low-molecular mass epoxy, acrylate and isocyanate oligomers. Each type of polymer will be dealt with in a separate subsection. A summary of polymer applications by SFC is listed in Table 9.3. 

Oligomers of phosgene, such as diphosgene [503-38-8] (COCl2)2, have found use in the laboratory preparation of isocyanates. Carbamoyl chlorides, A[,A/-disubstituted ureas, dimethyl- and diphenylcarbonates, and arylsulfonyl isocyanates have also been used to convert amines into urea intermediates, which are subsequendy pyroly2ed to yield isocyanates. These methods have found appHcations for preparation of low boiling point aUphatic isocyanates (2,9,17). 

An excess of phosgene is used during the initial reaction of amine and phosgene to retard the formation of substituted ureas. Ureas are undesirable because they serve as a source for secondary product formation which adversely affects isocyanate stabiUty and performance. By-products, such as biurets (23) and triurets (24), are formed via the reaction of the labile hydrogens of the urea with excess isocyanate. Isocyanurates (25, R = phenyl, toluyl) may subsequendy be formed from the urea oligomers via ring closure. 

Liquid-Injection Molding. In Hquid-injection mol ding (LIM), monomers and oligomers are injected into a mold cavity where a rapid polymerization takes place to produce a thermoset article. Advantages of these processes are low cost, low pressure requirement, and flexibiHty in mold configuration. Conventional systems, such as isocyanate with polyol, release Htfle or no volatiles. The generation of substantial volatiles in the mold is obviously undesirable and has represented a significant obstacle to the development of a phenoHc-based LIM system. A phenoHc LIM system based on an... 

Polymeric isocyanates or PMDI ate cmde products that vary in exact composition. The main constituents are 40—60% 4,4 -MDI the remainder is the other isomers of MDI, trimeric species, and higher molecular weight oligomers. Important product variables are functionaHty and acidity. Rigid polyurethane foams are mainly manufactured from PMDI. The so-called pure MDI is a low melting soHd that is used for high performance polyurethane elastomers and spandex fibers. Liquid MDI products are used in RIM polyurethane elastomers. 

Solvent-home adhesives are of two different types reactive and non-reactive. The reactive solvent-home adhesives are usually high molecular weight oligomers with isocyanate functionality. When applied, these adhesives can react further, increasing physical properties. The non-reactive solvent-home adhesives will not react further after application. 

An alternative route starts with a carboxy-terminated oligomer [36], This is reacted with glycidyl methacrylate to provide the methacrylate-terminated polymer. The resulting linkage is susceptible to hydrolysis, so the hydroxy group may be reacted with an isocyanate to improve environmental resistance (Scheme 3). 

ABA type poly(hydroxyethyl methacrylate) (HEMA) and PDMS copolymers were synthesized by the coupling reactions of preformed a,co-isocyanate terminated PDMS oligomers and amine-terminated HEMA macromonomers312). Polymerization reactions were conducted in DMF solution at 0 °C. Products were purified by precipitation in diethyl ether to remove unreacted PDMS oligomers. After dissolving in DMF/toluene mixture, copolymers were reprecipitated in methanol/water mixture to remove unreacted HEMA oligomers. Microphase separated structures were observed under transmission electron microscope, using osmium tetroxide stained thin copolymer films. 

Polyurethanes are thermoset polymers formed from di-isocyanates and poly functional compounds containing numerous hydroxy-groups. Typically the starting materials are themselves polymeric, but comprise relatively few monomer units in the molecule. Low relative molar mass species of this kind are known generally as oligomers. Typical oligomers for the preparation of polyurethanes are polyesters and poly ethers. These are usually prepared to include a small proportion of monomeric trifunctional hydroxy compounds, such as trimethylolpropane, in the backbone, so that they contain pendant hydroxyls which act as the sites of crosslinking. A number of different diisocyanates are used commercially typical examples are shown in Table 1.2. 

Waste PETP was depolymerised by glycolysis to give hydroxyl-terminated oligomers(DPET), which were used in the synthesis of urethane oils. The effect of depolymerisation temps., the type of glycol and the amount of catalyst on the yield and composition of the depolymerisation products was studied. The physical properties of the urethane oils were compared with those of a commercially-available product. The reaction of DPET with isocyanates produced random linkage between different molecules with or without terephthaloyl groups. 15 refs. 

The anionic method of polymerization is most useful for the synthesis of low molecular weight hydroxy-terminated oligomers and polymers that are to be further processed. For example, the treatment of hydroxy-terminated oligomers with isocyanates has been used to obtain polyester-urethanes (9,20), while triblock copolymers (PCL-PEG-PCL) are prepared by initiating the polymerization of e-caprolactone with the disodium alcoholate from polyethylene glycol (26). 

We can make polyurethanes via one- or two-step operations. In the single-stage process, diols and isocyanates react directly to form polymers. If we wish to make thermoplastic linear polymers, we use only diisocyanates. When thermosets are required, we use a mixture of diisocyanates and tri- or polyisocyanates residues of the latter becoming crosslinks between chains. In the first step of the two-stage process, we make oligomers known as prepolymers, which are terminated either by isocyanate or hydroxyl groups. Polymers are formed in the second step, when the isocyanate terminated prepolymers react with diol chain extenders, or the hydroxyl terminated prepolymers react with di- or polyisocyanates. 

Epoxies - The term "reactive oligomer" is relatively new but the concept is fairly old. This concept has been used for many years with systems such as epoxies, phenolics, unsaturated esters, cyanates, isocyanates and many other crosslinked systems. An example of a 177°C curing epo y system (Narmco s 5208) which was introduced into the marketplace about 1971 is shown in Eq. 1. 

Reactive oligomers such as epoxy resins, isocyanate-terminated compounds, and urethane-acrylates are extremely useful in the adhesive, coating, reaction injection molding (RIM), sealant and... 

The synthesis of the oligomers involved the known reaction of isocyanates and cyanamide (Nl CN). For example, N-cyano-N -phenyl urea has been synthesized from phenyl isocyanate and an aqueous alkaline solution of cyanamide in high yield.(3) Recently, similar reactions were used to prepare various di-N-cyanourea compounds from diisocyanates. ( 1) These monomers were also synthesized directly by reacting diisocyanates with cyanamide at melt temperatures. 

Chain Extendable Urethane Modified Epoxy Oligomer. The chain extendable urethane modified oligomers were prepared by combining equimolar amounts of epoxy-diol adduct and half-blocked diisocyanates, and heating the resulting mixture at 80°C for 4-6 hours until the isocyanate Infrared band disappeared. 

Figures la and lb show the OH and NH infrared bands of the oligomer as a function of temperature in uncatalyzed and catalyzed formulations. The uncatalyzed urethane modified epoxy oligomer shows only small changes in the OH/NH absorbance ratio at temperatures below 165°C only about a 60% conversion of the blocked isocyanate was observed. In contrast, sample of the oligomer catalyzed with 0.5% dlbutyl tin dilaurate shows nearly complete chain extension at temperatures as low as 130°C.

Isocyanates are capable of co-reacting to form dimers, oligomers and polymers. For example, aromatic isocyanates will readily dimerize when heated, although the presence of a substituent ortho to the -NCO group reduces this tendency. For example, toluene diisocyanate (TDI) is less susceptible to dimer formation than diphenylmethane diisocyanate (MDI). The dimerization reaction is reversible, with dissociation being complete above 200 °C. It is unusual for aliphatic isocyanates to form dimers, but they will readily form trimers, as do aromatic isocyanates. The polymerization of aromatic isocyanates is known, but requires the use of metallic sodium in DMF. 

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