The invention relates to a method for the chemical synthesis of RNA, comprising the following steps: a) bonding to a solid support of a monomer having formula (II) in which—X
1
is a dimethoxytrityl group,—X
6
is H or an OAc group or OX
3
, in which X
3
is a group having formula (A), in which X is O or S, R′ is H or CH
3
and R is selected from a linear or branched alkyl group at C
1
to C
4
and a R
1
—O—R
2
group in which R
1
is an alkyl group at C
1
to C
2
and R
2
is a CH
3
group or CH
2
CH
2
—O—CH
3
or aryl; b) assembly with the monomer having formula (II) bound to the support thereof obtained in step (a) of at least one monomer having formula (III) in which X
1
, Bp, X
3
are as defined for formula (II) and X
5
is a hydrogen phosphonate monoester or phosphoramidite group, preferably a 2-cyanoethyl-N,N-diisopropylphosphoramidite group, which is used to obtain a protected single-strand RNA bound to a support.