Here we report dynamic and thermodynamic aspects of the self-assembly in a series of branched-chain soybean-oil-derived styrenic block copolymers produced via reversible addition-fragmentation chain transfer (RAFT) polymerization. These biobased materials display unusual domain expansion which has not been seen in conventional petroleum-based block copolymers. The chain architecture of poly(styrene-b-acrylated epoxidized soybean oil) (PS-PAESO) block copolymers varies from star-like to star-brush-like owing to the bulk and multifunctionality of the PAESO repeat unit. The self-assembly behavior is evaluated through a combination of (ultra) small-angle X-ray scattering (SAXS), transmission electron microscopy, and dynamic shear rheology. Microphase separation in PS-PAESO is kinetically limited due to sluggish chain dynamics that cannot be thermally accelerated due to interference from cross-linking reactions. Selective hydrogenation of the residual PAESO vinyl groups retards but does not eliminate cross-linking reactions, indicating that transesterification is also at play. SAXS experiments show that the PS-PAESO domain size scales nearly linearly with the number of statistical segments, with structures as large as d = 155 nm observed. We believe that domain dilation is related to bulky aliphatic side chains in PAESO domains that extend the primary chain backbone to produce a brush-like conformation. The understanding of the structure-property relationships of these new biobased block copolymers gives insights on the design of novel polymer architectures benefiting new applications.