High trans-1,4-poly(butadiene-co-isoprene) (TBIR) copolymers from the stereo-selective and chemo-selective copolymerization of butadiene (Bd) and isoprene (Ip) are reported with MgCl₂ supported Ziegler-Natta catalyst (TiCl₄/MgCl₂Al(i-Bu)₃). Three major components in the copolymers, separated by stepwise isothermal crystallization fractionation, consist of crystalline trans-1,4-polyisoprene (TPI) blocks and trans-1,4-polybutadiene (TPB) blocks but in varied sequence length. Unique phase morphologies with different crystalline lamellar structures are observed in these TBIR copolymers for the first time. The phase morphologies of TBIR reflecting as the discrete lamellar stacks of TPB blocks and dendrimer-shaped lamellae of TPI blocks become diverse in different components with the varied statistical monomer and comonomer unit sequence distribution. The copolymers with TPB blocks are produced in the initial polymerization stage from the active species with relatively low stereospecificity. With the stereospecificity transformation of the active species from low to high, TBIR copolymers with crystalline TPB and TPI blocks and copolymers with TPI blocks are successively formed.