Abstract: A silver halide photographic emulsion chemically sensitized by a sulfur compound of the formula: ##STR1## wherein R.sub.1 represents a 5-membered or 6-membered heterocyclic ring containing a nitrogen atom; R.sub.2 represents a hydrogen atom, a lower alkyl group, an allyl group, an aryl group, or an aralkyl group; and R.sub.3 has the same definition as R.sub.1 or R.sub.2 ; or R.sub.2 and R.sub.3 may join to form a heterocyclic ring; is disclosed.

Abstract: Unsymmetrical azonitriles of the formula, ##STR1## wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently straight- or branched-chain alkyl having 1 to 7 carbon atoms or alkoxy substituted straight- or branched-chain alkyl having 2 to 11 carbon atoms but at least one of R.sup.1 and R.sup.2 and at least one of R.sup.3 and R.sup.4 must be different, have good efficiency as initiator for polymerizations or copolymerizations of vinyl monomers. These unsymmetrical azonitriles can be prepared by oxidizing a corresponding unsymmetrical hydrazo compound.