Coordination-insertion polymerization systems have long been superior to their anionic, cationic, and radical polymerization counterparts with regard to stereochemical control. However, until five years ago, these metal-based insertion methods were inferior to ionic and radical mechanisms in the category of living polymerization, which is simply a polymerization that occurs with rapid initiation and negligible chain termination or transfer. In the last half decade, the living insertion polymerization of unactivated olefins has emerged as a powerful tool for the synthesis of new polymer architectures. Materials available today by this route range from simple homopolymers such as linear and branched polyethylene, to atactic or tactic poly(alpha-olefins), to end-functionalized polymers and block copolymers. This review article summarizes recent developments in this rapidly growing research area at the interface of synthetic and mechanistic organometallic chemistry, polymer chemistry, and materials science. While special emphasis is placed on polymer properties and novel polymeric architectures, most of which were inaccessible just a decade ago, important achievements with respect to ligand and catalyst design are also highlighted.

Reductive elimination of C-H and C-C bonds from octahedral platinum(IV) complexes has been shown to occur from a fivecoordinate intermediate generated by ligand dissociation. 1 Given the importance of such intermediates in bond activation at platinum, we wish to communicate isolation of well-defined fivecoordinate platinum(IV) silyldihydride complexes. 2 The chemistry of TpPt(R)(R′)H [Tp ) hydridotris(pyrazolyl)borate; Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate; R,R′ ) combinations of alkyl, aryl, silyl and hydride ligands] has blossomed since 1995. 3 The stability of these complexes stems from canonical tridentate coordination of the tris(pyrazolyl)borate ligand. 4 Other stable platinum(IV) alkyl hydride complexes also contain chelating ligands in the platinum coordination sphere. 5 Protonation under mild conditions induces reductive elimination of methane from Tp′PtMe 2 H and Tp′PtMe(H) 2 via five-coordinate intermediates, leading to isolable cationic platinum(II) complexes after addition of a trapping ligand. 3g,6 Given the ability of silyl and hydride ligands to stabilize highoxidation state metals, 3f,7 and noting the elongated Pt-N bond (2.30 Å) trans to the triethylsilyl ligand in Tp′Pt(SiEt 3 )(H) 2 (1), 3f it seemed plausible that we could access a stable five-coordinate platinum(IV) complex 2 simply by protonation of 1 (eq 1).

A series of chiral cationic platinum(II) complexes of the type [κ 2 -((Hpz*)BHpz* 2 )Pt(R)(L)]-[BAr′ 4 ] (R ) Me, L ) MeCN (3), t BuNC (4), py (5), CO (6), CH 2 dCH 2 (7), PMe 2 Ph (8); R ) Ph, L ) MeCN (10); pz* ) 3,5-dimethylpyrazolyl, BAr′ 4 ) tetrakis(3,5-trifluoromethylphenyl)borate) has been prepared. Protonation of Tp′PtMe 2 H (1) or Tp′PtPh 2 H (2) (Tp′ ) hydridotris-(3,5-dimethylpyrazolyl)borate) occurs selectively at a pyrazole nitrogen atom and induces reductive elimination of methane or benzene. Subsequent addition of ligand, L, traps the platinum(II) intermediate and yields 3-8 and 10. Addition of 2 equiv of PMe 2 Ph led to the formation of trans-[κ 1 -((Hpz*)BHpz* 2 )Pt(Me)(PMe 2 Ph) 2 ][BAr′ 4 ] ( 9), a square-planar complex containing a rare monodentate protonated tris(pyrazolyl)borate ligand. Deprotonation of the pyrazolium ring (present after protonation of 1 or 2 and trapping with added ligand) led to the synthesis of neutral platinum(II) complexes of the type Tp′PtR(L) (R ) Me, L ) MeCN (11), SMe 2 (12), CO (13), CH 2 dCH 2 (14); R ) Ph, L ) MeCN ( 15)). Complexes 11, 12, and 15 exhibit square-planar coordination with a bidentate Tp′ ligand, while complex 14 is trigonal bipyramidal. Tp′PtMe(CO) ( 13) is present in solution in both isomeric forms. X-ray structure determinations for the cationic complexes 3 and 6-9 and the neutral squareplanar complex 12 are reported.

New titanium catalysts with ancillary, non-fluorinated phenoxyketimine ligands have been synthesized and found to be active for living ethylene polymerization.

is unknown is incorrect. In a series of careful experiments with arenes, including benzene, complete potential energy diagrams for the reaction of C 5 R 5 Rh(PMe 3) [R) Me, H] with arenes were constructed. 1-4 Using laser flash photolysis to generate Cp*Rh(PMe 3) in the presence of C 6 H 6 allowed direct measurement of the rate of conversion of Cp*Rh(PMe 3)(η 2-C 6 H 6) to Cp*Rh(PMe 3)(H)(C 6 H 5). 2 The values of ∆H q and ∆S q were extracted, and at 252 K the calculated ∆G q for the Rh case is 12.4 kcal/mol, quite close to the ∆G q value of 12.7 kcal/mol that we reported for the Pt(II) system. A thorough discussion of the arene rhodium adducts is contained in ref 1 and the reactions of benzene with C 5 R 5 (PMe 3) [R) Me, H] are presented in refs 2 and 3. We deeply regret this oversight. (1) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones, W. D.; Perutz, R. N.