PEGylation is a widely used approach for prolonging plasma circulation and improving the in vivo activity of therapeutic proteins. It is also being used to generate antibody-drug conjugates for cancer therapy. However, PEGylation is limited by the formation of anti-PEG antibodies that can accelerate plasma clearance of the therapeutic protein long term, potentially leading to under-dosing and failed therapy. Alternatives to PEGylation are therefore being explored, including a range of polymers synthesised using the reversible addition-fragmentation chain-transfer polymerisation (RAFT) approach. The aim of this study was therefore to evaluate the utility of using poly(N-(2-hydroxypropyl)methacrylamide)-poly(N-isopropyl acrylamide) (HPMA-NIPAM) as an alternative to PEG. This was achieved by comparing the pharmacokinetic behaviour of ~20kDa p(HPMA-NIPAM) to ~20 kDa PEG, and ~40 kDa p(HPMA-NIPAM) conjugated trastuzumab to ~40 kDa PEG conjugated trastuzumab in rats after IV and SC dosing. The formation of anti-PEG and anti-HPMA-NIPAM antibodies after IV and SC dosing in rats was also evaluated. HPMA-NIPAM alone exhibited more prolonged systemic exposure after IV administration to rats compared to PEG, and was less efficiently eliminated via the urine. When conjugated to trastuzumab, the HPMA-NIPAM and PEG conjugated proteins generally exhibited similar IV pharmacokinetics. However, the HPMA-NIPAM conjugated antibody exhibited more prolonged plasma exposure beyond 7 days compared to the PEG conjugated antibody. This difference in plasma exposure was due to the formation of anti-PEG IgM in all rats administered PEGylated trastuzumab SC, compared to only one rat out of four exhibiting anti HPMA-NIPAM IgM after 7 days. The HPMA-NIPAM co-polymer therefore exhibits pharmacokinetic properties that suggest it may be a suitable replacement for PEG. Shorter chain lengths of HPMA-NIPAM may also be used to exert the same pharmacokinetic effects as PEGylated proteins. Further, HPMA-NIPAM appears to be less immunogenic that PEG, meaning that plasma concentrations of HPMA-NIPAM conjugated therapeutic proteins may be more stable over long term therapy than PEGylated protein constructs.