Abstract: The present invention provides methods and compositions for inhibiting polymerisation of ethylenically unsaturated monomers, which involve the use of nitroxide compound of formula (I): wherein R1 is C4-20 hydrocarbyl; and R2, R3, R4 and R5 are independently each C1-6 alkyl.

Abstract: A monomer composition, stabilized against premature polymerization, comprising: a) an ethylenically unsaturated monomer or mixture of monomers polymerizable by free radical initiation, and b) an effective amount, sufficient to inhibit premature polymerization of component (a) of a mixture of: i) 1 to 99% by weight, based on the total weight of components (i) and (ii), of a mixture of at least one aromatic amine and at least one organic acid in a molar ratio of 10:1 to 1:10, and ii) 99 to 1% by weight, based on the total weight of components (i) and (ii), of at least one stable radical compound.