Ureidopyrimidone (UPy) end‐functionalized linear and star‐shaped poly(ethylene‐co‐propylene)s (hydrogenated polyisoprene) with molecular weights between 12K and 90K and narrow molecular weight distributions (PDI = < 1.10) were studied with SAXS and AFM. These hydrogen bond end‐functionalized polymers (0.45–1.14 mol.‐% UPy end‐groups) unexpectedly exhibited microphase separated domains with interdomain spacings of approximately 10–15 nm suggesting a solid‐state clustering of the hydrogen bonding end‐groups beyond simple dimerization. The interdomain spacings that were obtained from SAXS measurements systematically increased with molecular weight and decreased for monofunctional oligomers relative to telechelic analogs of the identical molecular weight. Variable temperature AFM measurements confirmed the presence of microphase separation at the surface for the star‐shaped UPy end‐functional poly(ethylene‐co‐propylene) and revealed a decrease in phase contrast upon heating to 130 °C with retention of the microphase separated texture.
< 1.10) were studied with SAXS and AFM. These hydrogen bond end‐functionalized polymers (0.45–1.14 mol.‐% UPy end‐groups) unexpectedly exhibited microphase separated domains with interdomain spacings of approximately 10–15 nm suggesting a solid‐state clustering of the hydrogen bonding end‐groups beyond simple dimerization. The interdomain spacings that were obtained from SAXS measurements systematically increased with molecular weight and decreased for monofunctional oligomers relative to telechelic analogs of the identical molecular weight. Variable temperature AFM measurements confirmed the presence of microphase separation at the surface for the star‐shaped UPy end‐functional poly(ethylene‐co‐propylene) and revealed a decrease in phase contrast upon heating to 130 °C with retention of the microphase separated texture. 