Metal-catalyzed routes to rings, chains and macromolecules based on inorganic elements
In this perspective article, some of our recent work directed at the development of new catalytic routes to rings, chains, and macromolecules based on main group and transition elements will be discussed. The preparation of polymers with backbones of alternating phosphorus and boron atoms attracted significant attention in the 1950s and early 1960s as a consequence of their anticipated high thermal stability and resistance to oxidation and hydrolysis. The main synthetic route explored at that
... explored at that time involved thermally induced dehydrocoupling of phosphine-borane adducts at 180-200°C, to afford predominantly six-membered rings. Only negligible yields of low molecular weight, partially characterized polymers were claimed, mainly in patents . We have recently shown that the dehydrocoupling process can be catalyzed by late transition-metal complexes. This has permitted the formation of six-and eight-membered phosphinoborane rings under more facile conditions, novel linear oligomeric chains, and high-molecular-weight polyphosphinoboranes. Catalytic dehydrocoupling of secondary phosphine-borane adducts The uncatalyzed dehydrocoupling of the secondary phosphine-borane adduct Ph 2 PHؒBH 3 at 170°C gives a mixture of the cyclic trimer [Ph 2 P-BH 2 ] 3 (1) and tetramer [Ph 2 P-BH 2 ] 4 (2) in an 8:1 ratio. However, upon heating Ph 2 PHؒBH 3 in the presence of [Rh(µ-Cl)(1,5-cod)] 2 or [Rh(1,5-cod) 2 ][OTf] (0.5-1 mol % Rh) at 120°C, 1 and 2 are formed in a 2:1 ratio (Scheme 1). Upon lowering the temperature to 90°C, the novel linear compound Ph 2 PH-BH 2 -PPh 2 -BH 3 (3) was formed as the exclusive product (Scheme 1) [16, 17] . In the absence of catalyst at 90°C, no conversion of Ph 2 PHؒBH 3 was ob-A. BARTOLE-SCOTT et al. Metal-catalyzed routes to rings, chains, and macromolecules 1997 Scheme 6 This transition-metal-catalyzed ROP methodology has been extended toward the formation of comb and star copolymers with the appropriate Si-H source  . Block copolymers can be formed via the addition of macromonomers with Si-H end groups  . For example, a PFS diblock copolymer with a poly(ethylene oxide) (PEO) block has been prepared which is soluble in water provided the organic block is sufficiently long  . The use of polysiloxane macromonomers with two Si-H termini yields novel PFS-b-polydimethylsiloxane-b-PFS triblock copolymers  . It should be noted that, unlike anionic polymerization, the transition-metal-catalyzed ROP route also provides block copolymers with appreciable polydispersities (ca. 1.4) and subsequent fractionation is necessary to access narrow PDI samples. In addition, telechelic polyferrocenylsilanes with Si-Cl and Si-H end-functionalities were prepared via transition-metal-catalyzed ROP of 20 (R = R' = Me) using chlorosilanes as the terminating agents. Chlorosilanes were found to be much more effective terminating agents than Et 3 SiH for the "capping" of polyferrocenylsilane chains. Further functionalization was achieved by reaction of the Si-Cl bond with poly(ethyleneglycol)methyl ether, which yielded poly(ethylene oxide-b-ferrocenylsilane) diblock copolymers (Scheme 8)  . The mechanism of the platinum-catalyzed ROP of silaferrocenophanes is of interest in our research group. The ROP is proposed to proceed through initial formation of a platinasilaferrocenophane via oxidative-addition to the platinum center  . Colloidal platinum is proposed to subsequently form, and oxidative-addition and reductive-elimination of the silaferrocenophane at the colloid surface is believed to allow growth of the polymer. End-group analysis experiments, and the discovery that mercury inhibits ROP, support this proposition  . However, even in the presence of mercury, a small amount of polymer is formed very slowly, therefore, the overall mechanism of the transition-metal-cat-A. BARTOLE-SCOTT et al. © 2005 IUPAC, Pure and Applied Chemistry 77, 1991-2002 1998 Scheme 7 Fig. 1 Plot of the M w and M n values of the mole ratio of monomer:silane for the synthesis of polyferrocenylsilane, 22. Reproduced from ref.  with permission.