The low-temperature polymerization of methyl methacrylate initiated with monofunctional lithium carbanionic species has been studied in THF and in a 9:1 toluene-THF mixture. Various types of "ligand" lithium salts have been purposely added to modify the active growth species. Quite interestingly, it has been found that lithium chloride, although not influencing the stereochemistry of the propagation, has an important beneficial effect on the "living" character of the process. That approach provides for a straightforward high-yield synthesis of PMMA samples displaying a predetermined molecular weight and a very low polydispersity. The LiCbinitiator molar ratio and the polarity of the solvent have been shown to be determinant parameters in the association equilibrium involved in these controls.

The present investigation deals with the tensile mechanical properties of the melt‐blended polyethylene/polystyrene pair as controlled by poly(hydrogenated butadiene‐b‐styrene) copolymers. It is clearly demonstrated that moderate amounts of these copolymers (2–10%) significantly increase both the ultimate strength and elongation at break of blends of polystyrene with various types of polyethylene (low‐density, high‐density, linear low‐density, and hydrogenated polybutadiene) and synergistic effects may result. The mechanical performance is strikingly dependent on the molecular characteristics of the copolymers. Over a broad range of molecular weights (60,000–270,000), diblocks are more effective than graft, triblock, or star‐shaped copolymers. It is also demonstrated that using polymeric emulsifiers under usual processing conditions in the melt state is powerful technique for preparing valuable polymer alloys.

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As investigated by optical and electron microscopy, the observed morphology of the meltblended polyethylene/polystyrene pair unambiguously supports the interfacial activity of poly-(hydrogenated butadiene-b-styrene) copolymers. The phase size is significantly reduced, the interfacial adhesion is dramatically increased and the phase dispersion is firmly stabilized against coalescence during subsequent processing. Diblock copolymers with a balanced composition are the most efficient interfacial agents in such an extent that only small amounts (1 -2 wt.4'0) are required to obtain homogeneous and stable phase dispersions. The emulsification concept applied to melt-blended immiscible polymers appears to be a powerful technique to prepare valuable polymer alloys.

Poly(hydrogenated butadiene‐b‐styrene) copolymers are very effective emulsifiers for blends of polystyrene and low‐density or high‐density polyethylene. It is shown that the extent of improvement in mechanical properties is dependent not only on the molecular weight but also on the structure of the diblock copolymer. A comparative study of the morphology and the mechanical behavior of modified low‐density polyethylene/polystyrene blends demonstrates that a tapered diblock is more efficient than a pure diblock with the same composition and molecular weight. It is assumed that the unique behavior of the tapered sample results from its particular miscibility characteristics at the blend interface. The tapered copolymer could behave essentially as a solu‐bilizing agent for the homopolymers at the interface and provide a “graded” modulus responsible for the improved mechanical response of the material.