Copolymerization of ethylene and 1,5-hexadiene by stereospecific metallocenes in the presence of Al(iBu)3/[Ph3C][B(C6F5)4]

Cyclopolymerizations: Synthetic Tools for the Precision Synthesis of Macromolecular Architectures

Monomers possessing two functionalities suitable for polymerization are often designed and utilized in syntheses directed to the formation of cross-linked macromolecules. In this review, we give an account of recent developments related to the use of such monomers in cyclopolymerization processes, in order to form linear, soluble macromolecules. These processes can be activated by means of radical, ionic, or transition-metal mediated chain-growth polymerization mechanisms, to achieve cyclic moieties of variable ring size which are embedded within the polymer backbone, driving and tuning peculiar physical properties of the resulting macromolecules. The two functionalities are covalently linked by a "tether", which can be appropriately designed in order to "imprint" elements of chemical information into the polymer backbone during the synthesis and, in some cases, be removed by postpolymerization reactions. The two functionalities can possess identical or even very different reactivities toward the polymerization mechanism involved; in the latter case, consequences and outcomes related to the sequence-controlled, precision synthesis of macromolecules have been demonstrated. Recent advances in new initiating systems and polymerization catalysts enabled the precision syntheses of polymers with regulated cyclic structures by highly regio- and/or stereoselective cyclopolymerization. Cyclopolymerizations involving double cyclization, ring-opening, or isomerization have been also developed, generating unique repeating structures, which can hardly be obtained by conventional polymerization methods.

Cyclocopolymerization: a mechanistic probe for dual-site alternating copolymerization of ethylene and alpha-olefins

The copolymerization of ethylene with 1,5-hexadiene or 1-hexene was studied with the series of ansa-metallocenes Me2Si(Cp)(9-Flu)ZrCl2 (1), Me2Si(1-Ind)(9-Flu)ZrCl2 (3), and Me2Si(9-Flu)2ZrCl2 (4). 1,5-Hexadiene, a monomer which requires two insertion events to be cyclopolymerized, when copolymerized with ethylene using 3/MAO, gave a copolymer with a novel architecture. When compared with the copolymerization of 1-hexene with ethylene, the observed striking differences between the two copolymers provided compelling evidence for a dual-site alternating copolymerization mechanism in both cases. The copolymerization results from 1/MAO and 4/MAO further support this.