Four discrete half-sandwich dialkyl rare earth metal (REM) complexes incorporating a disilylated indenyl ligand, (1,3 (SiMe3)(2)C9H5)M(CH2SiMe3)(2)(THF) (M = Sc, Y, Dy, Lu), have been investigated for the coordination addition polymerization of naturally renewable methylene butyrolactones, alpha-methylene-gamma butyrolactone (MBL) and gamma-methyl alpha-methylene gamma butyrolactone (MMBL) Initial screening for the polymerization of methyl methacrylate highlighted several differences in catalytic behavior between these half-sandwich REM catalysts and well studied sandwich REM catalysts in terms of reactivity trend, polymer tacticity and solvent dependence Most significantly all four catalysts herein exhibit exceptional activity for polymerization of MMBL in DMF, achieving quantitative monomer conversion in <1 min with a 020 mol % catalyst loading and giving a high turnover frequency of > 30 000 h(-1) Slower polymerizations occur in CH2Cl2 allowing for establishment of the activity trend within this REM series, which follows Dy (largest ion) >= Y > Lu > Sc (smallest ion) The most active and effective Dy catalyst has been examined in detail, demonstrating its ability to control the polymerization for producing PMMBL with high T-g (221 degrees C) and with molecular weight ranging from a medium M-n of 1 89 x 10(4) Da to a high M-n of 1 63 x 10(5) Da, programmed by the [MMBL]/[Dy] ratio Kinetic experiments have revealed a first order dependence on [monomer] and a second order dependence on [REM] These kinetic results, coupled to catalyst efficiencies, NMR studies, as well as with chain end group analysis by MALDI TOF mass spectrometry, have yielded a chain initiation mechanism that involves both alkyl groups on each metal center and a bimolecular chain propagation that involves two metal centers in the rate limiting C-C bond forming step The Dy catalyst response to enolizable organo acids, externally added as chain transfer agents has also been examined