Herein, we investigate the influence of spacer length on the homoassociation and heteroassociation of end-functionalized hydrogen-bonding polymers based on poly(n-butyl acrylate). Two monofunctional ureido-pyrimidinone (UPy) end-functionalized polymers were prepared by atom transfer radical polymerization using self-complementary UPy-functional initiators that differ in the spacer length between the multiple-hydrogen-bonding group and the chain initiation site. The self-complementary binding strength (K-dim) of these end-functionalized polymers was shown to depend critically on the spacer length as evident from H-1 NMR and diffusion-ordered spectroscopy. In addition, the heteroassociation strength of the end-functionalized UPy polymers with end-functionalized polymers containing the complementary 2,7-diamido-1,8-naphthyridine (NaPy) hydrogen-bond motif is also affected when the aliphatic spacer length is too short. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4253-4260, 2011