The molecular structure of bis(acetylacetonate)cobalt(II) ([Co(acac)2]) in solution and in the presence of the electron donors (ED) pyridine (py), NEt3, and vinyl acetate (VOAc) was investigated using 1H NMR spectroscopy in C6D6. The extent of formation of ligand adducts, [Co(acac)2(ED)x], varies in the order py>NEt3>VOAc (no interaction). Density functional theory (DFT) calculations on a model system agree with Co--ED bond strengths decreasing in the same order. The effect of electron donors on the [Co(acac)2]-mediated radical polymerization of VOAc was examined at 30 degrees C by the addition of excess py or NEt3 to the complex in the molar ratio [VOAc]0/[Co]0/[V-70]0/[py or NEt3]0=500:1:1:30 (V-70=2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile)). As previously reported by R. Jerome et al., the polymerization showed long induction periods in the absence of ED. However, a controlled polymerization without an induction period took place in the presence of ED, though the level of control was poorer. The effective polymerization rate decreased in the order py>NEt3. A similar behavior was found when these electron donors were added to an ongoing [Co(acac)2]-mediated radical polymerization of VOAc. On the basis of the NMR and DFT studies, it is proposed that the polymerization is controlled by the reversible homolytic cleavage of an organocobalt(III) dormant species in the presence of ED. Conversely, the faster polymerization after the induction period in the absence of ED is due to a degenerative transfer process with the radicals produced by the continuous decomposition of the excess initiator. Complementary experiments provide additional results in agreement with this interpretation.

The electron-withdrawing effect of the ligand in the cobalt complex was studied in cobalt-mediated radical polymerization of vinyl acetate (VOAc) initiated by 2,2‘-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70). The polymerization of VOAc with V-70 and a series of bis(acetylacetonate)cobalt derivatives, Co(RCOCHCOR)2 (R1 = R2 = CH3 (1), R1 = CF3, R2 = CH3 (2), R1 = R2 = CF3 (3)), was conducted under the same conditions. Complexes 1 and 2 successfully mediated the radical polymerization of VOAc, resulting in poly(VOAc) with predetermined M n and low polydispersity. In contrast, complex 3 did not control the radical polymerization of VOAc, resulting in poly(VOAc) with higher molecular weight and higher polydispersity. While complex 1 was not able to control the radical polymerization of n-butyl acrylate (nBA), the copolymerization of nBA with various amounts of VOAc (9%, 23%, and 50%) in the presence of V-70 and 1 showed that M n and M w/M n of copolymers decreased with increasing VOAc ratio. This indicated that the extent of control in the copolymerization improved with increasing molar ratio of VOAc. The VOAc content of poly(nBA-co-VOAc) copolymers was determined by 1H NMR spectroscopy when 50% VOAc was initially added to the copolymerization system. The VOAc content increased with conversion, indicating a gradient sequence distribution in the copolymer. Furthermore, the block copolymer composed of a poly(nBA-grad-VOAc) segment and a poly(VOAc) block was successfully synthesized when the copolymerization was performed under the VOAc-rich condition (77%) in the initial feed.

Block and graft copolymers with polyacrylate and linear polyethylene segments were synthesized through a combination of degenerative transfer (DT) ethylene polymerization and atom transfer radical polymerization (ATRP). DT coordination polymerization was mediated by a bis(imino)pyridineiron/diethylzinc binary catalyst system activated by MAO and was studied at various temperatures. The catalyst system achieved a well-controlled DT coordination polymerization of ethylene, resulting in low-polydispersity polyethylene with high chain end functionality. The Zn-terminated polyethylene prepared by DT coordination polymerization was oxidized using dry air, followed by hydrolysis to provide a monohydroxy-terminated polyethylene (PE-OH). The degree of ethylene polymerization (DP ∼ 20) and the chain end functionality (∼80%) of PE-OH were determined by 1 H NMR. The PE-OH was converted into a polyethylene R-bromoisobutyrate macroinitiator (PE-MI), and ATRP of n-butyl acrylate and tert-butyl acrylate was successfully conducted from the PE-MI to produce well-defined block copolymers. A PE-block-PnBA copolymer (Mn ) 10 100, Mw/Mn ) 1.16 with 7.4 wt % PE segment) and a PE-block-PtBA copolymer (Mn ) 11 000, Mw/Mn ) 1.16 with 6.8 wt % PE segment) were obtained successfully. The R-bromoisobutyrate/PE-MI was dehydrobrominated to form a polyethylene macromonomer with a terminal R-methacrylate group (PE-MM). A grafting-through ATRP was performed with the PE-MM and either n-butyl acrylate or tert-butyl acrylate to form well-defined PnBA-graft-PE and PtBAgraft-PE copolymers. Molecular structures were determined by 1 H NMR and GPC analysis. The PtBAgraft-PE was hydrolyzed yielding PAA-graft-PE with 90% carboxylic acid functionality. DSC analysis of the graft copolymers indicated two separate glass transitions, suggesting phase-separated morphology.

The effect of the electron-withdrawing groups on the ligand in a series of bis(acetylacetonate)-cobalt(II) derivatives, Co(R 1 COCHdCOR 2 ) 2 (R 1 ) R 2 ) CH 3 (1), R 1 ) CF 3 , R 2 ) CH 3 (2), R 1 ) R 2 ) CF 3 (3)), was examined by conducting a controlled radical polymerization of vinyl chloroacetate (VClOAc) and N-vinyl-2-pyrrolidone (NVP) under the same reaction conditions. Complex 2 provided better control over the polymerization of VClOAc than complex 1, resulting in the preparation of a poly(VClOAc) with M n closer to the theoretical M n and lower polydispersity at the same monomer conversion. On the other hand, complex 3 was not able to control the radical polymerization of VClOAc. In the case of NVP, only complex 1 produced poly(NVP) with relatively low polydispersity (1.7-2.0). Better control over M n and polydispersity in the polymerization of VClOAc and NVP was achieved by addition of vinyl acetate (VOAc) to the reaction. The M n /M n,th and polydispersity of the copolymers became lower as the initial proportion of VOAc in both copolymerization systems was increased. The VOAc content of the poly(VClOAc-co-VOAc) and poly(NVP-co-VOAc) copolymers calculated from 1 H NMR spectra were in good agreement with the values calculated from monomer reactivity ratios.

Mononuclear 16-electron arene−ruthenium(II)−thiolate complexes of the general formula (η6-arene)Ru(SAr)2 (2, Ar = 2,6-C6H3Me2 = Xyl; 3, Ar = 2,4,6-C6H2(CHMe2)3; arene = C6H6 (a), arene = p-MeC6H4(CHMe2) = p-cymene (b), arene = C6Me6 (c)) have been prepared by treatment of [(η6-arene)RuCl2]2 (1) with the corresponding sodium arenethiolate in methanol. Reaction of 2b with disodium of 1,2-benzenedithiolate (=S2C6H4) led to a mixture of monomer and dimer complexes (η6-p-cymene)Ru(S2C6H4) (4b) and [(η6-p-cymene)Ru(S2C6H4)]2 (5b) in solution. A complex (η6-C6Me6)Ru(S2C6H4) (4c) was predominantly monomeric, revealed by a crystal structure analysis of 4c. All these monomeric complexes show intense blue color of the LMCT band due to the donation from the filled S(pπ) orbital to the empty Ru(dπ*) orbital, which stabilizes coordinatively unsaturated ruthenium(II). The structures of these complexes are characterized by NMR and mass spectra and elemental analysis in addition to the X-ray crystal structure determinations of 2a,b, 3c, and 4c, indicating the two-legged piano stool geometry. The newly prepared 16-electron complexes react with π-accepting molecules such as isocyanide, carbon monoxide, and trialkylphosphine. Treatment of 2b with an excess of tert-butyl isocyanide resulted in the release of the p-cymene ligand to give trans-Ru(SXyl)2(CN t Bu)4 (8). Similarly, reaction of 5b with an excess of tert-butyl isocyanide afforded cis-Ru(S2C6H4)(CN t Bu)4 (10), while reaction of 5b with 6 equiv of tert-butyl isocyanide gave a binuclear complex [Ru(S2C6H4)(CN t Bu)3]2 (11). In the case of more strongly coordinating C6Me6 derivatives 2c and 4c, one molecule of tert-butyl isocyanide can coordinate to the ruthenium atom, resulting in the formation of (η6-C6Me6)Ru(SXyl)2(CN t Bu) (9c) and (η6-C6Me6)Ru(S2C6H4)(CN t Bu) (12c), respectively. Reactions of 5b and 4c with an excess of triethylphosphine afforded (η6-p-cymene)Ru(S2C6H4)(PEt3) (13b) and (η6-C6Me6)Ru(S2C6H4)(PEt3) (13c), respectively. In the carbonylation, 2b gave a binuclear carbonyl complex (CO)3Ru(μ-SXyl)3Ru(CO)2(SXyl) (14), while the carbonylation of the C6Me6 complex 2c afforded (η6-C6Me6)Ru(SXyl)2(CO) (15). Versatile reactivity of 16-electron ruthenium(II) thiolate complexes is thus demonstrated.