The invention features linker molecules that have at one terminus a amino-protecting group and at the other terminus a phosphorous activating group, typically phosphoramidite. The linker molecules can be used, for example, to produce amino-modified linkers that space an oligonucleotide from a solid support. The invention also features an amino-protected nucleotide that includes an activated phosphorous group such as a phosphoramidite for the production of a 5' amino-modified oligonucleotide. The invention further provides a phthalimido-modified support that can be used to synthesize a polynucleotide that includes an amino group at the 3' terminus.