Addition of styrene (S)/4-hydroxystyrene (HS) block, blocky gradient, or blocky random copolymer to 80/20 wt% polystyrene (PS)/polycaprolactone (PCL) blends is examined as a compatibilization strategy. Four copolymers are synthesized by controlled radical polymerization, each with an S block and the other block being a HS block or S/HS random or gradient copolymer. Compatibilization depends on copolymer level and HS sequence distribution and content. Using a two-step solution-mixing/melt-mixing process, addition of 2 wt% and 5 wt% nearly symmetric S/HS diblock copolymer leads to compatibilization with average PCL domain diameters of 390-490 nm and 90-110 nm, respectively. In contrast, adding 0.25-0.75 wt% copolymer leads to microscale dispersed-phase domains and only reduced melt-state coarsening. Results with 2-5 wt% added copolymer indicate that a major reduction in interfacial tension is facilitated by hydrogen bonding of HS units and PCL carbonyl groups. Nanoscale confinement of normally semi-crystalline PCL within blends with 100 nm dispersed phases impedes PCL crystallizability, yielding liquid-state PCL domains at room temperature and demonstrating that properties of nanostructured blends and microstructured blends can differ greatly. Polystyrene/PCL blends are also made by one-step melt mixing with low mol% HS copolymers. Adding 5 wt% blocky gradient S/HS copolymer (86/14 mol% S/HS) leads to compatibilization with an average dispersed-phase diameter of 360-420 rim. In contrast, adding 5 wt% blocky random (87/13 mol% S/HS) or 5 wt% diblock (81/19 mol% S/HS) copolymer yields microscale dispersed-phase domains and only reduced coarsening. Crystallization in these blends is less hindered than in blends containing 2-5 wt% nearly symmetric S/HS diblock copolymer, indicating that both hydrogen bonding and confinement suppress PCL crystallization. (c) 2008 Elsevier Ltd. All rights reserved.