The transient uniaxial extensional response of a series of ultrahigh molecular weight poly(alpha -olefins) (P alpha O) with densely grafted bottlebrush architectures was measured as a function of side chain length, N sc. The P alpha O bottlebrushes studied here have N sc values ranging from 6 carbons [poly(1-octene)] to 16 carbons [poly(1-octadecene)] (POD). To understand the effect of bottlebrush architecture on the nonlinear response, a linear polyolefin (polypropylene), with N sc = 1, was included in this study. All the P alpha O bottlebrushes show strong extensional strain hardening (SH) at Weissenberg numbers, W i R, greater than 1. Strain-hardening ratios, S H R = eta E + ( t ) / eta E , LVE +, show increasing dependence on the rate of extension, described by power laws, with exponents dependent on the N s c values. Moreover, the maximum SHR values, S H R max, are increasing function of the side chain length, reaching values more than one order of magnitude larger for the POD, compared to atactic polypropylene (aPP, e.g., at W i R similar to 200, S H R max = 3.5 and 75 for PP and POD, respectively). Another remarkable feature of these P alpha O bottlebrushes is their large extensibility, which is also an increasing function of both the extension rate and the side chain length. For instance, the maximum macroscopic stretch ratio values (before breakup), lambda max, reach values of similar to 400 (for POD), which is more than an order of magnitude larger than that for aPP ( lambda max similar to 25 ). A mechanism to explain the strong SH and stretchability in the P alpha O bottlebrushes, based on flow-induced side chain reorientation and interdigitation, is discussed.