Abstract: Aspects of the present disclosure include compositions that make use of phosphorus and/or nucleobase protecting groups which find use in the synthesis of long polynucleotides. Phosphorus protecting groups are provided that help increase the stepwise coupling yield and/or phosphorous protecting groups that can be removed during the oxidation step. Amidine nucleobase protecting groups are provided that find use in the subject compositions and methods which provides for e.g., increased resistance to depurination during polynucleotide synthesis. In some instances, the methods and compositions disclosed herein utilize a combination of the phosphorus and amidine nucleobase protecting groups in the synthesis of polynucleotides having a sequence of 200 or more monomeric units in length. Also provided are methods for synthesizing a polynucleotide (e.g., a DNA) using one or more compounds disclosed herein.