Conformal coating of ultrahigh-aspect-ratio nanostructures with functional polymer thin films is highly desirable in many applications, ranging from biosensing to energy storage. Nevertheless, achieving uniform surface coverage on nanostructures is challenging due to the difficulty of controlling molecular transport and reaction kinetics under nanoconfinement. Here we demonstrated the conformal coverage of ultrahigh-aspect-ratio nanopores by polymer nanolayers deposited using initiated chemical vapor deposition (iCVD) and unraveled the fundamental mechanisms governing the coating growth kinetics under nanoconfinement. A molecular-collision model was developed by using statistical methods and validated by systematic kinetic experiments. The results indicated that nanoconfinement amplified radical-surface collisions, resulting in higher effective radical concentration. The approach for validating the molecular-collision model can be broadly adopted to study vapor-based reaction systems without needing extensive nanofabrication or characterization instruments. Together, the results reported here could improve the control over nanocoating growth during nanostructured material/device fabrications across industries of manufacturing, healthcare, and sustainability.