Abstract: Described herein are methods for the controlled de novo synthesis of RNA oligonucleotides using enzymatic catalysis. For example, provided herein are methods for preparing RNA oligonucleotides via controlled, template-independent addition of nucleotides to an initiator oligonucleotide 3?-terminus via enzymatic catalysis (also known as terminal transferase activity). Single nucleotides can be iteratively added by a compatible polymerase (e.g., a poly(N) polymerase such as a poly(U) polymerase) until a desired RNA oligonucleotide sequence is synthesized. Also provided are nucleotides and polymerases useful in the methods described herein.