Process for making an oxymethylene polymer

A process for making an oxymethylene polymer comprising polymerizing at least one compound capable of forming —CH2O— repeat units (monomer) and at least one comonomer which is a cyclic ether or cyclic formal in the presence of at least one ester of the formula I (ester)

• R1—CO—O—R2 (I)
• wherein
• R1 can be hydrogen, with the proviso that R2 is C2 alkyl or linear C3 to C4 alkyl, wherein R1 and R2 are independently of each other
• linear or branched C1 to C10 alkyl
• and in the presence of at least one Lewis acid, wherein the Lewis acid is an halogenide of boron, tin, titanium, phosphorous, antimony or arsenic.

An oxymethylene polymer consisting essentially of repeat units derived from at least one compound capable of forming —CH2O— repeat units (monomer) and at least one comonomer which is a cyclic ether or cyclic formal and further comprising that one of its endgroups is an endgroup of general formula (II)

• R1—CO— (II)
• wherein R1 is
• hydrogen, with the proviso that R2 is C2 alkyl or linear C3 to C4 alkyl,
• linear or branched C1 to C10 alkyl,
• and comprising at least one endgroup of general formula (III)
• —O—R2 (III)
• wherein R2 is
• linear or branched C1 to C10 alkyl
• C5 to C7 cycloalkyl
• —[R3—O—]nR4
• wherein
• R3 is a linear or branched C2 to C5 alkylene and
• R4 is a linear of branched C1 to C5 alkyl and
• n is an integer of from 1 to 5.