Synthesis of Statistical Multiblock Copolymers through Chain Shuttling Copolymerization with a Thermomechanical Switch Behavior

A high-throughput experimentation (HTE) approach is adopted for the synthesis of a special class of elastomeric polyolefins with a tailored thermomechanical response. Ethylene/hexadec-1-ene (E/C16) statistical multiblock copolymers (MBCs) are synthesized through chain shuttling copolymerization (CSC). They are characterized by alternation of crystalline (hard) and amorphous (soft) blocks consisting of E/C16 random copolymers with low (0.5 mol %) and high (22 mol %) C16 content, respectively. The MBCs have statistical block length; mass average molecular mass greater than 100 kDa; hard block content wh of 20, 31, and 50 wt %; and elastomeric properties. The regulation of the mechanical properties with the temperature is triggered by the independent melting and crystallization of the side chains at low temperatures facilitated by the high C16 content in the soft blocks. DSC thermograms show two endothermic peaks at ≈10 and 122 °C and two exothermic peaks at ≈2 and 100 °C due to the melting/crystallization of the crystals formed by the side chain pendant from the soft blocks and the crystals formed by the main chain long ethylene sequences belonging to the hard blocks, respectively. Transmission electron microscopy (TEM) reveals that all of the samples show heterogeneous morphologies in the solid state due to the tendency of the hard blocks to crystallize in separated domains. For the samples with wh ≈ 20 and 31 wt %, the partial miscibility of the hard blocks with short length in the soft block rich regions prevents good phase separation, resulting in pass-through morphologies characterized by lamellar crystals crossing the soft block rich region in all directions, low contrast, and diffuse boundary at domain interfaces. The pass-through morphology occurs also for the sample with wh ≈ 50 wt %. However, as it includes a large fraction of long hard and soft blocks with nearly symmetric length, disordered bicontinuous morphologies develop characterized by a better microphase separation, good contrast, and sharp boundaries at domain interfaces. The presence of heterogeneities in the melt is indicated by rheology measurements in the linear viscoelastic regime by the time-temperature superposition principle failure at low frequencies. The tensile properties and X-ray fiber diffraction analysis show that the designed MBCs present elastomeric properties at 25 °C and a remarkable increase of the stiffness at low temperatures. The low-temperature reinforcement effect is caused by side chain crystallization. Upon heating to room temperature, the melting of the side chain crystals induces an almost complete recovery of the pristine dimensions of the sample. It is suggested that the regulation of thermomechanical response, that is, the melting and crystallization temperatures of the side chain crystals along with the strength of reinforcing effect, may be easily achieved for elastomeric MBCs by varying the length of the alk-1-ene counit and/or its content in the soft block.