Synthesis, morphology, and mechanical properties of poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly(methyl methacrylate) triblocks. Ligated anionic polymerization vs atom transfer radical polymerization

Poly(methyl methacrylate)-b-poly(n-butyl acrylate)-b-poly(methyl methacrylate) triblock copolymers have been prepared by ligated anionic polymerization (LAP; 8K-50K-8K) and atom transfer radical polymerization (ATRP; 9K-51K-9K). Size exclusion chromatography, nuclear magnetic resonance, and differential scanning calorimetry have confirmed that the molecular structure of the two triblock copolymers is essentially identical. However, important differences are found in dynamic mechanical properties, viscoelastic properties, and stress-strain behavior. Indeed, the ATRP copolymer has low storage modulus, high complex viscosity, high order-disorder transition temperature, and poor ultimate tensile strength and elongation at break, compared to those of the LAP analogue. Marked differences also observed by tapping mode atomic force microscopy in the microscopic morphology of thin films of these copolymers. All these observations can be explained by the slow initiation of MMA by the poly(n-butyl acrylate) macroinitiator used in ATRP in contrast to what happens when MMA is added to living poly(tert-butyl acrylate) anions. As a result, the polydispersity of the short poly(methyl methacrylate) (PMMA) outer blocks is much broader in the ATRP copolymer, although the polydispersity index of the triblock is only 1.15. This heterogeneous structure of the ATRP triblock is also supported by the comparison of homo-PMMAs prepared by LAP and ATRP.