High-capacity digital poly(phosphodiester)s were synthesized by stepwise automated phosphoramidite chemistry. Coding libraries containing either 4 or 8 phosphoramidite monomers of different mass were used to encode the polymers, thus enabling storage densities of 2 or 3 bits/monomer, respectively. In addition, a monomer containing a cleavable alkoxyamine and nucleotide mass tags enabling fragment identification were included in the chains to allow their decryption by electrospray pseudo-MS3 sequencing. As a proof of concept, black and white images with sizes ranging from 80 to 144 pixels were encoded in single polymer chains and decoded by mass spectrometry. Six different polymers were prepared in this work; three with the 4-monomer alphabet and three with the 8-monomer alphabet. In all cases, uniform macromolecules were obtained and deciphered. Yet, the bulkiest monomers of the 8-symbol alphabet required optimized protocols for uniform polymer synthesis. A macromolecular storage capacity of 144 bits per chain was obtained in this work, which is the highest capacity ever attained for a synthetic informational polymer.