Blocked, isocyanate monomers

Spindler, R., and Frechet, J. M. J. 1993. Synthesis and characterization of hyper-branched polyurethanes prepared from blocked isocyanate monomers by step-growth polymerization. Macromolecules, 26, 4809 1813. 

Spindler and Frechet1391 prepared hyperbranched polyurethanes by step-growth polymerization (Scheme 6.8) of protected, or blocked , isocyanate AB2 monomers. The method is dependent on the thermal dissociation of a carbamate unit into the corresponding isocyanate and alcohol moieties.140,411 Decomposition temperatures range from ca. 250°C for alkyl carbamates to ca. 120°C for aryl carbamates.1401... 

Various systems are available depending on the end use. Typical systems are based on saturated polyesters, a.tw-acid-terminated cured with an epoxy (triglycidyl isocyanurate TGIC), or a.ty-OH-terminated cured with an isocyanate (or a blocked isocyanate). Recently, bio-based formulations with aliphatic nonyellowing monomers have been proposed [15]. 

Polyester resins for powder coating are brittle solids with a relatively high Tg (50-60°C) and so the powder coating does not sinter during storage. Terephthalic acid (TPA) and NPG are used as the principal monomers with smaller amoimts of other monomers to increase fn and to reduce Tg to the desired level. Both hydroxy-and carboxy-terminated polyesters are used. The former are most commonly cross-linked with blocked isocyanates and the latter with epoxy resins. Other crosslinkers include 2-hydroxyalkylamides and tetramethoxymethylglycolmil. 

If the grafted monomer retains reactivity it can take part in chemical bonding. Such reactivity could include isocyanates, silanes, epoxides, or even heat reactive adducts, such as blocked isocyanates. 

Thermosetting (crosslinking) acrylics normally require a crossUnker to be present. Their actual cure behaviour is dependent upon the pendant (side chain) functionality of the copolymerised co-monomers. Acrylamide will crosslink with itself, epoxy or amino resins, whilst amino or melamine crosslinkers will react with hydroxyl or acid groups. Isocyanates can react with hydroxyl groups at room temperature. To reduce the hazards of isocyanates and the inconvenience of two pack systems, blocked isocyanates can be used. At elevated temperatures used for stoving they can unblock generating an alcohol and an isocyanate which will react with any hydroxyl or acid groups on the acryUc resin. 

Isocyanates can be added to solvent-borne CR adhesive solutions as a two-part adhesive system. This two-part adhesive system is less effective with rubber substrates containing high styrene resin and for butadiene-styrene block (thermoplastic rubber) copolymers. To improve the specific adhesion to those materials, addition of a poly-alpha-methylstyrene resin to solvent-borne CR adhesives is quite effective [76]. An alternative technique is to graft a methacrylate monomer into the polychloroprene [2]. 

Polyaddition reactions based on isocyanate-terminated poly(ethylene glycol)s and subsequent block copolymerization with styrene monomer were utilized for the impregnation of wood [54]. Hazer [55] prepared block copolymers containing poly(ethylene adipate) and po-ly(peroxy carbamate) by an addition of the respective isocyanate-terminated prepolymers to polyazoesters. By both bulk and solution polymerization and subsequent thermal polymerization in the presence of a vinyl monomer, multiblock copolymers could be formed. 

In order to investigate dendrimers of a different nature [230], Bradley described the synthesis and transfection efficiency of polyamidourea dendrimers synthesised from isocyanate-containing AB3 monomers [231-234]. The use of this kind of tris-branched building block was addressed to enhance dendrimer synthesis by replacement of the 1,4-addition step typical of PAMAM synthesis and to a rapid increase in terminal functionality. The dendritic structures were synthesised using a divergent, microwave-assisted, solid-phase approach with the dendrimers assembled on polystyrene resin via an acid labile linker (see Fig. 26). In particular, a G3.0 polyamidourea bis-dendron with the peripheral amino groups conjugated to L-lysine residues demonstrated remarkable transfection abilities [234],... 

Aluminum isopropoxide has been used for the preparation of block copolyesters [147, 148]. Tri-block poly(e-CL-b-DXO-e-CL) was prepared by the sequential addition of different monomers to a living polymerization system initiated with aluminum isopropoxide in THF or toluene solution [95]. An alternative route for the preparation of the tri-block copolymer was to react the diblock poly(e-CL-b-DXO) containing an -OH functionality at the chain end using a difunctional coupling agent such as isocyanate or acid chloride (Scheme 18). However, the molecular weights were low and full conversion of monomers was not achieved. 

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