Anionic Polymerization Of Isocyanates Research Articles

Palladium(0)-Catalyzed Trimerization of Arylisocyanates into 1,3,5-Triarylisocyanurates in the Presence of Diimines: A Nonintuitive Mechanism

We show here that palladium(0) (dibenzylideneacetone) complexes bearing 1,10-phenanthroline constitute efficient catalysts for the cyclotrimerization of aromatic isocyanates. For the first time, the mechanism of this reaction has been investigated experimentally and theoretically with group 10 catalysts. This investigation provides a very consistent picture of the catalytic cycle. Notably, we establish that the reaction does not proceed by stepwise cycloadditions or ring insertions involving metallacyclic intermediates, as might have been anticipated. Rather, in our proposal, the initial steps of the mechanism resemble the chain-growth process operative during the anionic polymerization of isocyanates and feature charge-separated intermediates. These steps are then followed by ring closure on the metal center of the last intermediate formed to yield a seven-membered metallacycle that reductively eliminates the cyclotrimer and re-forms the active species. In addition, we conclusively show that the (known) palladacycles that could be isolated during the experimental investigations are not catalytic intermediates but result from catalyst deactivation. Thus, with Pd(0) diimine catalysts, the actual trimerization mechanism appears to be a blend between the two types of mechanisms proposed thus far for the oligomerization of heterocumulenes with very different catalysts. In conclusion, this work contributes to a better understanding of the reactivity of arylisocyanates in the vicinity of late group 10 metal centers in low oxidation state and sheds some light on the detrimental self-poisoning processes observed during the reductive carbonylation of nitroaromatic substrates catalyzed by related catalysts in non-nucleophilic media.

The Effect of Alkyl Side Chain and Additives on the Anionic Polymerization of Isocyanates with Carbamate Group

AbstractThe synthesis of new isocyanate monomers and their polymerization by anionic route is reported. Reaction of 1,6‐diisocyanatohexane with aliphatic alcohols such as methanol, n‐propanol and n‐pentanol in 1:0.5 molar ratios was carried out in the presence of pyridine such that one NCO group remained unreacted. The anionic polymerization of n‐alkoxycarbonylaminohexyl isocyanates was carried out using sodium napthalenide (Na‐Naph) initiator in the presence of 15‐crown‐5 (15C5) and sodium tetraphenylborate as the additives. While polymerization of n‐propyloxycarbonylaminohexyl isocyanate (PAHI) and n‐pentanoxycarbonylaminohexyl isocyanate (PEAHI) was feasible that of methoxycarbonylaminohexyl isocyanate (MAHI) led to an insoluble material. The polymers were isolated in high yields with NaBPh4 as the additive.

Anionic Polymerization of Isocyanates with Optically Active Properties

Random copolymers, an optically functionalized isocyanate containing disperse red 1 (DR1NCO) with a chiral isocyanate containing (s)-(-)-2-methylbutanol (MBI), were prepared by anionic polymerization in THF at -98°C with Na-Naph as a two-directional initiator. As the contents of MBI in the copolymer increased, the optical rotation of the polymer was also increased. To study about the optical activity change with initiator systems, homo polymer of MBI was synthesized with one-directional and two-directional initiator. The difference of optical activity in both systems was investigated using Circular Dichroism (CD) spectroscopy and optical rotation.

Anionic polymerization of isocyanates with optical functionalities

Polyisocyanates of 2-[4-(4-nitrophenylazo)- N-ethylphenyl amino]ethoxy carbonyl amino hexyl isocyanate, DR1NCO, and ( s)-(−)-2-methyl-1-butyloxy carbonyl amino hexyl isocyanate, MBI, were synthesized by anionic polymerization using sodium naphthalenide as an initiator in THF at −98°C. In the polymerization reaction, crown ether (15C5) and octyl isocyanate (Oct-NCO) were used to suppress side reactions such as backbiting or chain transfer reaction. In the homopolymerization of DR1NCO, the polymers with high yield (∼94%) were obtained due to suppression of backbiting by 15C5. In the random copolymerization of DR1NCO with MBI, the chain transfer reaction was suppressed due to the low reactivity of anionic chain end induced by octyl isocyanate. The random copolymers showed the optical activity in CD and ORD measurements, and specific rotations increased with increasing the component of MBI.

A quantum chemical study of mechanisms underlying chain growth and cyclization in anionic polymerization of isocyanates

The semiempirical methods CNDO/BW and MINDO/3 were used to study anionic polymerization of isocyanates leading to the formation of isocyanurate rings. The cyclic trimer of isocyanate can form a complex with the anionic active centres; structure of these hypothetical complexes has been determined. Evidence on the electronic structure of reactants has been used to explore the possible role of urethane anions as initiators. A qualitative agreement exists between the calculated results and experimental data on the relative rates of competitive reactions.

Initiation of the anionic polymerization of isocyanates with sodium naphthalane

Initiation of the anionic polymerization of isocyanates with sodium naphthalane

イソシアナートのアニオン重合

イソシアナートのアニオン重合