Photopolymerization initiator and producing method thereof, polymerizable composition, ink jet recording method, and acylphosphine oxide compound

A photopolymerization initiator which is an acylphosphine oxide compound having a structure in which one or more carbonyl groups that link to a carbon atom are further directly bonded to an aromatic ring of an aromatic acyl group that bonds to a phosphorus atom in an acylphosphine oxide structure,

• wherein the carbon atom that links to the carbonyl group is an aliphatic carbon atom,
• wherein the photopolymerization initiator has a constitutional unit represented by Formula 1A, Formula 2A, or Formula 2B:
• wherein, in Formula 1A, Formula 2A, or Formula 2B, L's each independently represent a single bond, RP's each independently represent a hydrogen atom or an alkyl group, R1A's each independently represent a monovalent hydrocarbon group in which a part of a hydrocarbon chain may be substituted with a carbonyl group, an ester bond, an amide bond, an oxygen atom, NR, and/or a sulfur atom, and R1A is a group that links at an aliphatic carbon atom to the carbonyl group in Formula 2A, R1B represents a divalent hydrocarbon group in which a part of a hydrocarbon chain may be substituted with a carbonyl group, an ester bond, an amide bond, an oxygen atom, NR, and/or a sulfur atom, and R1B is a group that links at an aliphatic carbon atom to the carbonyl group in Formula 2B, R represents a hydrogen atom, an alkyl group, or an aryl group, R2 to R4 each independently represent an alkyl group, an aryl group, or an alkoxy group, m1 represents 3, m2 represents an integer of 0 to 5, and p represents an integer of 0 to 3.