Polyphenylene ether, production method of the same, thermosetting composition, prepreg, and laminate

A polyphenylene ether comprising a repeating unit derived from a phenol of the following formula (1), a repeating unit derived from a phenol of the following formula (2), and a structural unit derived from a phenol of the following formula (3):

• (in the formula (1), each R11 is independently an optionally substituted saturated hydrocarbon group having a carbon number of 1 to 6, an optionally substituted aryl group having a carbon number of 6 to 12, or a halogen atom, and each R12 is independently a hydrogen atom, an optionally substituted hydrocarbon group having a carbon number of 1 to 6, an optionally substituted aryl group having a carbon number of 6 to 12, or a halogen atom):
• (in the formula (2), each R22 is independently a hydrogen atom, an optionally substituted saturated or unsaturated hydrocarbon group having a carbon number of 1 to 20, an optionally substituted aryl group having a carbon number of 6 to 12, or a halogen atom, two R22 cannot be both hydrogen atoms, and R21 is a partial structure represented by the following formula (4):
• (in the formula (3), X is any a-valent linking group, a is an integer from 2 to 6, R3 is one of a linear alkyl group having a carbon number of 1 to 8 and a partial structure represented by the following formula (4), and is bonded to the carbon atom of at least one of Position 2 or Position 6 when the position of the carbon atom of the benzene ring to which —O— is bonded is designated as Position 1, and k is each independently an integer from 1 to 4):
• (in the formula (4), R41 is each independently an optionally substituted linear alkyl group having a carbon number of 1 to 8, or is a cyclic alkyl structure having a carbon number of 1 to 8 to which two R41 are bonded, each R42 is independently an optionally substituted alkylene group having a carbon number of 1 to 8, b is each independently 0 or 1, R43 is one of a hydrogen atom, an optionally substituted alkyl group having a carbon number of 1 to 8, or an optionally substituted phenyl group), and
• the partial structure represented by the formula (4), which is R21 in the formula (2), and the partial structure represented by the formula (4) in R3 in the formula (3) may be the same or different.