One of the most cost-effective and clean separation technologies is membrane separation. However, the performance of membranes is often limited by two bottleneck issues: (i) a universal trade-off between membrane permeability and selectivity, and (ii) reduced permeability and selectivity over time due to membrane fouling. This thesis focuses on the design and synthesis of ultrathin and functionalized polymer coatings to overcome those bottleneck issues by leveraging a surface modification technique termed initiated Chemical Vapor Deposition (iCVD). iCVD is an all-dry technique that enables polymerization and coating application in a single step with outstanding conformality, thus giving rise to uniform film thickness over micro/nano-structured substrates, including the nanopores in ultra/nano-filtration membranes. The large variety of chemical functionalities that are attainable via the iCVD technology, including hydrophobic, hydrophilic (e.g., zwitterionic), and amphiphilic polymers, enabled the design of unprecedented antifouling and antimicrobial polymer coatings that are uniquely effective at the air-liquid-solid three-phase interface. Towards advanced membrane design, the thesis established a framework to understand iCVD polymerization under nanoconfinement. It first reveals the transition between a linear and a quadratic polymerization kinetics regime under common iCVD conditions, which serves as the foundation for understanding the deposition kinetics under nano-confinement. The thesis reveals the unprecedented effects of radical amplification under nano-confinement to (i) dominate the dependence of polymerization kinetics on the initiator concentration, (ii) enable pore-sculpturing, i.e., manipulation of the coating thickness profile in the longitudinal direction of nanopores, and (iii) program pore size distribution by tuning the fractional saturation pressure of a monomer. The thesis systematically unravels the effects of nanoconfinement on vacuum-based polymerization kinetics and the resulting pore sizes in three dimensions, pointing to the enormous potential of iCVD polymerization to enable novel separation technologies.