The present invention relates to a liquid composition comprising a monomer, a (meth)acrylic polymer and at least two initiators. In particular the present invention relates to a liquid composition comprising a monomer, a (meth)acrylic polymer and at least two initiators that have a different half life time. That liquid composition can be used as a syrup and especially as a syrup for impregnation of fibres or fibrous material. Also concerned is a thermoplastic material obtained after polymerization of the liquid composition. The invention also relates to a process for manufacturing such a liquid composition. The invention also relates to a process for impregnating a fibrous substrate of long fibres with said liquid composition. The invention also relates to a fibrous substrate impregnated with said liquid composition which is useful for manufacturing composite parts. The present invention also relates to a process for manufacturing mechanical parts or structural elements made of composite material and to mechanical parts or structural elements made of composite material obtained via a process using such a liquid composition.