DNA and RNA oligonucleotides are molecules of great interest due to their potential application as antisense and RNAi drugs. Currently, solid phase synthesis (SPS) is the method of choice for their synthesis due to its simplicity, automation, and simple intermediate purification. However, large scale synthesis is challenging due to its 2- phase reaction medium. This research investigates a novel method – Membrane Enhanced Oligonucleotide Synthesis (MEOS) – for the synthesis of DNA oligonucleotides. This novel process combines synthesis on a soluble polymeric support with the emergent separation technology Organic Solvent Nanofiltration (OSN). The synthesis of support-bound oligonucleotide dimers was first attempted using linear MeOPEG112 as the soluble support, and ceramic and cross-linked polyimide OSN membranes. The performance of OSN membranes was characterised prior to synthesis in order to selected membranes with appropriate performance properties. Poor process yields lead to the development of novel branched PEG supports. PEG silyl ethers, phosphate and carboxylic esters were prepared in a novel methodology using an OSN membrane for the separation of the branched PEG from excess PEG diol. The use of branched PEGs resulted in an increase of the MEOS process yields in the range of ~3 to 6 fold. Best results for the MEOS process were obtained in the synthesis of a fully unprotected dimer, dApdA, using a 3-arm PEG carboxylic ester with MW of ~ 9,000 g mol-1 [benzene-1,3,5-(CO2-PEG67)3]. The synthesis of DNA monomers using OSN was also investigated. This investigation also describes a novel synthetic method for the synthesis of heterobifunctional PEGs. PEGs of MW of ~ 2,000 g mol-1 were prepared and characterised with monophthalimide, monoamine, and monoazide functionalities. Overall, this research investigates the use of OSN in two different applications for organic synthesis, namely the synthesis of DNA oligonucleotides and derivatised (branched and linear) PEGs.