This application relates to monomers of the general formula (I) for the preparation of PNA (peptide nucleic acid) oligomers and provides method for the synthesis of both predefined sequence PNA oligomers and random sequence PNA oligomers: wherein E is sulfur or oxygen; J is nitrogen or C—R′; R1, R′ is independently H, halogen, alkyl, nitro, nitrile, alkoxy, halogenated alkyl, halogenated alkoxy or a functional group having one of following general formulae: wherein A1, A2, A3, A4, A5, is independently selected from H, halogen such as F, Cl, Br or I, CF3, alkyl, preferably C1-C4 alkyl, nitro, nitrile, alkoxy, preferably C1-C4 alkoxy, halogenated (such as F, Cl, Br and I) alkyl, preferably halogenated C1-C4 alkyl, halogenated (such as F, Cl, Br and I) alkoxy, preferably halogenated C1-C4 alkoxy, phenyl, and halogenated (such as F, Cl, Br and I phenyl; R2 is H or protected or unprotected side chain of natural or unnatural α-amino acid; and B is a natural or unnatural nucleobase, wherein when said nucleobase has an exocyclic amino function, said function is protected by protecting group which is labile to acids but stable to weak to medium bases in the presence of thiol.