A radiation-sensitive composition includes a free-radically polymerizable component, an initiator composition capable of generating free radicals sufficient to initiate polymerization upon exposure to imaging radiation, an infrared radiation absorbing compound, and a polymeric binder. The initiator composition includes a unique polyonium borate comprising a polyvalent onium cation with multiple onium moieties and sufficient organic borate counterions to provide a net neutral charge. The radiation-sensitive composition can be used to prepare a negative-working imageable element that is sensitive to suitable imaging infrared radiation, can be imaged at relatively low energy, and can be developed either on-press or off-press.