There is a recognized need to create well-defined polymer probes for in vivo and clinical positron emission tomography (PET) imaging to guide the development of new generation polymer therapeutics. Using the RAFT polymerization technique in combination with the reactive ester approach, here we have synthesized well-defined and narrowly distributed N-(2-hydroxypropyl)methacrylamide homopolymers (pHPMA) (P1* and P2*) and random HPMA copolymers consisting of hydrophilic HPMA and hydrophobic lauryl methacrylate comonomers (P3* and P4*). The polymers had molecular weights below (P1* and P3*) and above the renal threshold (P2* and P4*). Whereas the homopolymers dissolve in isotonic solution as individual coils;. the random copolymers form larger aggregates above their critical micelle concentration (similar to 40 nm), as determined by fluorescence correlation spectroscopy. Structure-property relationships of the pharmacokinetics and biodistribution of the different polymer architectures were monitored in the living organism following radiolabeling with the positron emitter F-18 via fluoroethylation within a few hours. Ex vivo organ biodistribution and in vivo mu PET imaging studies in male Copenhagen rats revealed that both size and the nature of the aggregate formation (hydrophobically modified copolymers) played a major role in blood clearance and biodistribution, especially concerning liver and kidney accumulation. The high-molecular-weight random copolymer P4* (hydrophobically modified), in particular, combines low liver uptake with enhanced blood circulation properties, showing the potential of hydrophobic interactions, as seen for the represented model system, that are valuable for future drug carrier design.