Catalysis polyurethane formation

A few polymerizations can be reasonably employed either in a catalyzed or an uncatalyzed process. Polyurethane formation is an example of this type of behavior. The reaction between diols and diisocyanates is subject to base catalysis. However, the polymerization is often carried out as an uncatalyzed reaction to avoid various undesirable side reactions. 

The influence of reaction temperature increase on the catalytic effects of DBU (83MI3), and the specific catalysis of DBU for polyurethane formation (83MI2), have been studied. 

An extensive review of the reactions of isocyanates with hydroxy compounds, including a thorough survey of the catalysis and kinetics of the reactions, was published in 1962 [116]. Other relatively recent surveys of the subject of special interest include those of Farkas and Mills [117] and Lyman [118]. Other reviews which are of special supplemental value with respect to control of the reactions in polyurethane formation include those of Johnson [119] and Saunders and Frisch [120]. 

A New Concept for the Catalysis of Polyurethane Formation via Tertiary Amines and Organometallic Compounds... 

The catalysis of the polyurethane formation by organometallic compounds has especially lead to a number of very different interpretations. 

Thus, several mechanistic pathways based on polarization effects have been proposed to explain the catalysis of the alcohol-isocyanate reaction. These propositions appear to be often unsatisfactory and cannot explain even the majority of the experimental results reported in the literature. For an example, why is the polyurethane formation catalyzed by potassium acetate (JJ and not at all by MgC03 nor CsCl (14) The role played by the nature of the metal remains also unexplained. Robins reports the incremental temperature rise noted 1 minute after the mixing of reagents and catalyst (7, 16). This parameter is related to the catalytic activity and is an effective way to show the role played by the organometallic compound in its interaction with the alcohol. A similar conclusion can be drawn from Table I (15) where the Sn+ derivative is much more active than the Sn+2 oxidation state or Pb+. ... 

The main experimental results relative to the catalysis of the polyurethane formation by organometallic compounds can be explained by taking into account the structure of the catalysts and of coordination vacancies. Several cases are to be considered. 

In general, the quantitative results of the polyurethane cure substantiate increased isocyanate conversion rates in the vicinity of any of the three metals, being most pronounced on Cu and to some extent on A1 but also detected even in ultrathin films on Au. Nevertheless, considerable catalysis, by some orders of magnitude, of polyurethane formation is only found for films on Cu. Once more, the effectiveness of copper even in films of some microns thickness is easily explained by the dissolution of Cu ions. 

The spontaneous reaction of polyurethane formation in solutions follows a second-order rate equation in the early stages. Toward the end of the reaction, the polymer formation rate constant increases. The rate constant of polyurethane formation from DPMDI and PTMG calculated by the second-order equation for 60°C up to 76% conversion is 9.0 x 10 l/(mol.s), and from 76% conversion to the end of the reaction is 15.5 x 10 l/(mol.s). Some increase of the reaction rate constant at 76% conversion seems to be related to catalysis of the reaction by the urethane groups formed or by secondary reactions in the system. 

The formation of isocyanurates in the presence of polyols occurs via intermediate allophanate formation, ie, the urethane group acts as a cocatalyst in the trimerization reaction. By combining cyclotrimerization with polyurethane formation, processibility is improved, and the friability of the derived foams is reduced. The trimerization reaction proceeds best at 90-100°C. These temperatures can be achieved using a heated conveyor or a RIM machine. The key to the formation of PUIR foams is catalysis. Strong bases, such as potassium acetate, potassium 2-ethylhexoate, and tertiary amine combinations, are the most useful trimerization catalyst. A review on the trimerization of isocyanates is available (104). 

Huynh-Ba, G., and Jerome, R. (1981). Catalysis of isocyanate reactions with protonic substrates a new concept for the catalysis of polyurethane formation via tertiary amines and organometaUic compounds. In Urethane Chemistry and Applications (Edwards, D. N., ed.), ACS, Washington D.C. 

In parallel with these developments, organotin compounds have found a variety of applications in industry, agriculture, and medicine, though in recent years these have been circumscribed by environmental considerations. In industry they are used for the stabilization of poly(vinyl chloride), the catalysis of the formation of the polyurethanes, and the cold vulcanisation of silicone polymers, and also as transesterification catalysts. 

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