Polymer chemical vapor deposition (CVD) offers superior conformality and substrate compatibility compared to solution-based thin film fabrication. Unique among polymer CVD processes, initiated CVD (iCVD) decouples radical generating reactions from polymerization reactions, enabling use of mild conditions, and diverse polymer chemistries. Most iCVD research falls into one of three categories: (i) implementation, (ii) process innovations, and (iii) post-deposition modifications. This dissertation targets critical knowledge gaps in these areas by investigating the fundamental processes relevant to them. Beginning with implementation, Chapter 2 addresses the gap in understanding of defects which might pose a risk to use of iCVD in industries requiring smooth, uniform films, such as optical coatings and soft electronics. Films fabricated using iCVD at sample temperatures below ~30oC are shown to be susceptible to defects originating from build-up of oligomeric species (n < 10) and prevention methods are developed. Chapters 3 and 4 address the understudied iCVD vapor phase. Even though innovations like batch iCVD and condensed droplet polymerization were built on understanding of the vapor phase, no models of the iCVD vapor phase have been validated experimentally. Chapter 3 first investigates heat transfer and mixing in the iCVD reactor, introducing the Peclet and Knudsen numbers to quantify these effects. Chapter 4 then thoroughly characterizes the reactive vapor phase, developing a robust model of initiator (di-tert-butyl peroxide) decomposition at a range of flowrates, pressures, temperatures and monomer (cyclohexyl methacrylate) compositions. Moving to post-deposition modification, Chapter 5 investigates post-deposition base catalyzed hydrolysis of two fluoroacrylic polymers. The high pH conditions required to hydrolyze the passivating fluoroacrylate moiety pushes the limits of post-deposition modification. Quantification of hydrolysis kinetics shows that both crosslinking and covalently grafting the film to the substrate are necessary. The eventual degradation of the grafting agent indicates a need for improved grafting procedures.