The chain conformation of polymers plays an important role in controlling their phase behaviour and associated material properties, In the case of flexible polymers, conformation is controlled by the degree of polymerization (DP), with low-DP polymers having extended polymer chains and high-DP polymers adopting random-coil conformations in solution and the bulk amorphous state(1), and folded conformations in the crystalline phase(2). Exceptions to this general rule are polymers that contain structurally rigid building blocks, or that are subjected to directional shear forces during solidification, The backbones of semi-flexible and rigid rod-like polymers, for example, are always extended in liquid crystalline and crystalline phases(3-5), and gel-spun flexible polymers form extended-chain crystals(2). Here we report a general strategy for the rational control of polymer conformation through the self-assembly of quasi-equivalent monodendritic (branched) side-groups attached to flexible backbones, At low DPs, the conical monodendrons assemble to produce a spherical polymer with random-coil backbone conformation, On increasing the DP, the self-assembly pattern of the monodendritic units changes to give cylindrical polymers with extended backbones, This correlation between polymer conformation and DP is opposite to that seen in most synthetic and natural macromolecules, We anticipate that our strategy will provide new approaches for the rational design of organized supramolecular materials(6-9) in areas such as nanotechnology, functional films and fibres, molecular devices, and membranes, expanding the synthetic and technological uses of dendritic building blocks(7,10-15).