Polymers comprising at least one unit of formula wherein n is 0 or 1, m and p are independently from each other 0, 1, 2, 3, 4, 5 or 6, provided that the sum of n, m and p is at least 2, and n and p are not 0 at the same time, Ar 1 and Ar 2 are independently from each other C 6-14 -arylene or C 6-14 -aryl, which may be substituted with 1 to 4 substituents independently selected from the group consisting of C 1-30 -alkyl, C 2-30 -alkenyl, C 2-30 -alkynyl, C 5-8 -cycloalkyl, C 6-14 -aryl and 5 to 14 membered heteroaryl, X 1, X 2 and X 3 are independently from each other and at each occurrence O or S, R 1 and R 2 are independently from each other and at each occurrence selected from the group consisting of C 1-30 -alkyl, C 2-30 -alkenyl, C 2-30 -alkynyl, C 5-8 -cycloalkyl, C 6-14 -aryl and 5 to 14 membered heteroaryl, wherein C 1-30 -alkyl, C 2-30 -alkenyl and C 2-30 -alkynyl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, CN, C 5-6 -cycloalkyl, C 6-10 -aryl and 5 to 10 membered heteroaryl, C 5-8 -cycloalkyl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, C 1-10 -alkyl, CN, C 2-10 -alkenyl, C 2-10 -alkynyl, C 6-10 -aryl and 5 to 10 membered heteroaryl, and C 6-14 -aryl and 5 to 14 membered heteroaryl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, CN, C 1-10 -alkyl, C 2-10 -alkenyl, C 2-10 -alkynyl and C 5-6 -cycloalkyl. n is 0 or 1, m and p are independently from each other 0, 1, 2, 3, 4, 5 or 6, provided that the sum of n, m and p is at least 2, and n and p are not 0 at the same time, Ar 1 and Ar 2 are independently from each other C 6-14 -arylene or C 6-14 -aryl, which may be substituted with 1 to 4 substituents independently selected from the group consisting of C 1-30 -alkyl, C 2-30 -alkenyl, C 2-30 -alkynyl, C 5-8 -cycloalkyl, C 6-14 -aryl and 5 to 14 membered heteroaryl, X 1, X 2 and X 3 are independently from each other and at each occurrence O or S, R 1 and R 2 are independently from each other and at each occurrence selected from the group consisting of C 1-30 -alkyl, C 2-30 -alkenyl, C 2-30 -alkynyl, C 5-8 -cycloalkyl, C 6-14 -aryl and 5 to 14 membered heteroaryl, wherein C 1-30 -alkyl, C 2-30 -alkenyl and C 2-30 -alkynyl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, CN, C 5-6 -cycloalkyl, C 6-10 -aryl and 5 to 10 membered heteroaryl, C 5-8 -cycloalkyl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, C 1-10 -alkyl, CN, C 2-10 -alkenyl, C 2-10 -alkynyl, C 6-10 -aryl and 5 to 10 membered heteroaryl, and C 6-14 -aryl and 5 to 14 membered heteroaryl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, CN, C 1-10 -alkyl, C 2-10 -alkenyl, C 2-10 -alkynyl and C 5-6 -cycloalkyl. C 1-30 -alkyl, C 2-30 -alkenyl and C 2-30 -alkynyl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, CN, C 5-6 -cycloalkyl, C 6-10 -aryl and 5 to 10 membered heteroaryl, C 5-8 -cycloalkyl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, C 1-10 -alkyl, CN, C 2-10 -alkenyl, C 2-10 -alkynyl, C 6-10 -aryl and 5 to 10 membered heteroaryl, and C 6-14 -aryl and 5 to 14 membered heteroaryl can be substituted with 1 to 5 substituents independently selected from the group consisting of halogen, CN, C 1-10 -alkyl, C 2-10 -alkenyl, C 2-10 -alkynyl and C 5-6 -cycloalkyl.