Detailed investigations of the polymerization of ethylene by (R-diimine)nickel(II) catalysts are reported. Effects of structural variations of the diimine ligand on catalyst activities, polymer molecular weights, and polymer microstructure are described. The precatalysts employed were 6). Active polymerization catalysts were formed in situ by combination of 4-6 with modified methylaluminoxane. In general, as the bulk and number of ortho substituents increase, polymer molecular weights, turnover frequencies and extent of branching in the homopolyethylenes all increase. Effects of varying ethylene pressure and temperature on polymerizations are also reported. The degree of branching in the polymers rapidly decreases with increasing ethylene pressure but molecular weights are not markedly affected. Temperature increases result in more extensive branching and moderate reductions in molecular weights. Catalyst productivity decreases above 60 °C due to catalyst deactivation.

A survey of the state-of-the-art in the development of synthetic methods to incorporate p-block elements into polymers is given. The incorporation of main group elements (groups 13-16) into long chains provides access to materials with fascinating chemical and physical properties imparted by the presence of inorganic groups. Perhaps the greatest impedance to the widespread academic and commercial use of p-block element-containing macromolecules is the synthetic challenge associated with linking inorganic elements into long chains. In recent years, creative methodologies have been developed to incorporate heteroatoms into polymeric structures, with perhaps the greatest advances occurring with hybrid organic-inorganic polymers composed of boron, silicon, phosphorus and sulfur. With these developments, materials are currently being realized that possess exciting chemical, photophysical and thermal properties that are not possible for conventional organic polymers. This review focuses on highlighting the most significant recent advances whilst giving an appropriate background for the general reader. Of particular focus will be advances made over the last two decades, with emphasis on the novel synthetic methodologies employed.

Addition polymerization, the most general method of preparation for organic polymers, has successfully been extended to P=C bonds. The polymerization of a phosphaalkene has been initiated by thermolysis or with alkyllithium reagents. The unprecedented poly(methylenephosphine)s are easily oxidized using oxygen or sulfur to give air stable macromolecules. A molecular weight (M(w)) of 35000 g/mol for the poly(methylenephosphine sulfude) was estimated by light-scattering GPC.

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The first chalcogen-bridged [1]ferrocenophanes Fe(η-C5H3R)2E (6, E = S, R = H; 7, E = Se, R = H; 12, E = S, R = Me) have been synthesized and characterized both structurally and spectroscopically. Synthesis of sulfur- and selenium-bridged species 6 and 7 was achieved by the reaction of dilithioferrocene·TMEDA (TMEDA = tetramethylethylenediamine) with bis(phenylsulfonyl) sulfide S(O2SPh)2 and selenium diethyldithiocarbamate Se(S2CNEt2)2, respectively, in 20−30% yields. Structural characterization of both 6 and 7 revealed highly strained structures with tilt-angles between the cyclopentadienyl ligands of 31.05(10)° and 26.4(2)°, respectively. Compounds 6 and 7 are purple and red-purple, respectively; comparison of the structures of known [1]ferrocenophanes 1 showed that when the second period (from group 14−16) is traversed, there is a substantial increase in cyclopentadienyl ring-tilting in main group element bridged [1]ferrocenophanes, and the lowest energy UV/vis absorption peaks become increasingly red-shifted. Extended Hückel MO calculations were performed and, consistent with this observation, predicted a decrease in the HOMO−LUMO gap as the ring-tilt increases. Thermal ring-opening polymerization (ROP) of both 6 and 7 afforded the insoluble poly(ferrocenyl sulfide) [Fe(η-C5H4)2S] n 8 and poly(ferrocenyl selenide) [Fe(η-C5H4)2Se] n 9, respectively. Differential scanning calorimetry studies of the ROP process provided estimates of the strain energies of 6 and 7 which were ca. 130(±20) and 110(±20) kJ mol-1, respectively. Anionic ROP of 6 also yielded the insoluble poly(ferrocenyl sulfide) 8. However, linear soluble dimeric and trimeric trimethylsilyl-capped oligo(ferrocenyl sulfides) 10b and 11b were synthesized by the reaction of 6 with dilithioferrocene·TMEDA followed by the addition of Me3SiCl and were characterized spectroscopically, electrochemically, and, for 11b, by X-ray diffraction, and provide useful models for the analogous high polymer. The dimethylated sulfur-bridged species 12 was prepared as a mixture of isomers from the reaction between dilithiodimethylferrocene·TMEDA and S(O2SPh)2, and X-ray structural characterization of a single isomer 12a showed the presence of a large tilt-angle of 31.46(8)°. Thermal and anionic ROP of the isomer mixture 12 afforded the first soluble poly(ferrocenyl sulfide) [Fe(η-C5H3Me)2S] n 13 which was characterized by 1H and 13C NMR, elemental analysis, thermogravimetric analysis, and gel permeation chromatography. Cyclic voltammetric studies of 13 showed the presence of two reversible oxidation waves with a redox coupling ΔE = ca. 0.32 V, which is consistent with the presence of significantly stronger M···M interactions compared to those present in other ring-opened poly(ferrocenes) derived from [1]ferrocenophanes.

Labile (P−P)Pd(CH3)(OEt2)+BAr‘4 - complexes (2) have been prepared via protonation of (P−P)PdMe2 (1), where P−P = cis-1,2-bis(diphenylphosphino)ethylene (a, dppee), 1,2-bis(diphenylphosphino)benzene (b, dpbz), 1,2-bis(diphenylphosphino)ethane (c, dppe), 1,2-bis(dimethylphosphino)ethane (d, dmpe), 1,3-bis(diphenylphosphino)propane (e, dppp), 1,3-bis(diisopropylphosphino)propane (f, dippp), 1,4-bis(diphenylphosphino)butane (g, dppb) and Ar‘ = 3,5-(CF3)2C6H3. Unstable complex 2d (P−P = dmpe) was generated in situ. X-ray structures are reported for 1e and 2e−g. Treatment of 2a−g with CO in CH2Cl2 at −90 °C yields the (P−P)Pd(CH3)(CO)+ complexes 3a−g. Barriers to migratory insertion in 3a−g were determined with the ordering to be:  3f (dippp) ≈ 3g (dppb) < 3e (dppp) ≪ 3a (dppee) ≈ 3b (dpbz) ≈ 3c (dppe) < 3d (dmpe). Exposure of 2a−g to ethylene at −80 °C yields the ethylene complexes (P−P)Pd(CH3)(C2H4)+ (5a−g). Barriers to migratory insertion in these complexes were determined by NMR spectroscopy to be:  5b (dpbz) ≈ 5e (dppp) ≈ 5f (dippp) ≈ 5g (dppb) < 5a (dppee) ≈ 5c (dppe) < 5d (dmpe). Complexes (P−P)Pd(CH2CH3)(C2H4)+ (6a−e,g) produced from 5a−e,g under C2H4 are catalyst resting states for the dimerization of C2H4 to butenes. In the case of 5f (P−P = dippp), the catalyst resting state produced is the β-agostic ethyl complex (dippp)Pd(CH2CH2-μ-H)+ (8f), which has been isolated. This complex exhibits two dynamic processes studied by VT-NMR:  interchange of Cα and Cβ (ΔG ⧧ = 10.3(2) kcal/mol) and rotation of the agostic methyl group (ΔG ⧧ ca. 6.4 kcal/mol). The β-agostic propyl complex 7f has been generated and identified as the n-propyl isomer (dippp)Pd(CH2CH2-μ-HCH3)+.

Phosphaalkenes (MesP=CRR': R = R' = Ph (1a); R = R' = 4-FC6H4 (1b); R = Ph, R' = 4-FC6H4 (1c); R = R' = 4-OMeC6H4 (1d); R = Ph, R' = 4-OMeC6H4 (1e); R = Ph, R' = 2-pyridyl (1f)) are prepared from the reaction of MesP(SiMe3)2 and O=CRR' in the presence of a trace of KOH or NaOH. The base-catalyzed phospha-Peterson reaction is quantitated by NMR spectroscopy, and isolated yields of phosphaalkene between 40 and 70% are obtained after vacuum distillation and/or recrystallization. The asymmetrically substituted phosphaalkenes (1c, 1e, 1f) form as 1:1 mixtures of E and Z isomers; however, X-ray crystallography reveals that the E isomers crystallize preferentially. Interestingly, E-1e and E-1f readily isomerize in solution in the dark, although the rate of isomerization is much faster when samples are exposed to light. X-ray crystal structures of 1b, E-1e, and E-1f reveal that the P=C bond lengths (average of 1.70 A) are in the long end of the range typically found in phosphaalkenes (1.61-1.71 A). Attempts to prepare isolable P-adamantyl phosphaalkenes following this route were unsuccessful. Although AdP=CPh2 (2a) is detected by 31P NMR spectroscopy, attempts to isolate this species afforded the 1,2-diphosphetane (AdPCPh2)2 (3a), which was characterized by X-ray crystallography.

The reaction of the phosphaalkenes MesPdCPh 2 (Mes ) 2,4,6-Me 3 C 6 H 2 ) and Mes*PdCH 2 (Mes* ) 2,4,6-t Bu 3 C 6 H 2 ) with Lewis (AlCl 3 , GaCl 3 , InCl 3 ) and protic (HOTf) acids has been examined to evaluate the feasibility of cationic polymerization for PdC bonds. Addition of GaCl 3 to Mes*PdCH 2 generates the adduct Mes*(Cl 3 Ga)PdCH 2 , which can be detected spectroscopically at 193 K. At higher temperatures, GaCl 3 migrates from phosphorus to carbon to afford the fleeting phosphenium zwitterion Mes*PCH 2 GaCl 3 . This undetected transient species immediately oxidatively adds to a C-H bond of an o-t Bu group in the P-Mes* substituent, resulting in a GaCl 3 -coordinated ylide that has been characterized crystallographically. The analogous reaction of GaCl 3 with MesPdCPh 2 gives stable Mes(Cl 3 Ga)Pd CPh 2 , for which a crystal structure determination has been conducted. Significantly, treating a highly concentrated solution of Mes*PdCH 2 with substoichiometric quantities of GaCl 3 leads to linear dimerization following a cationic chain growth mechanism; however, the oligomerization is terminated by intramolecular C-H activation. The novel coordinated linear dimer (C-H activated Mes*)PCH 2 PH(Mes*)CH 2 GaCl 3 has been characterized crystallographically. Interestingly, mechanistic studies reveal that the diphosphiranium ring Mes*PCH 2 P(Mes*)CH 2 GaCl 3 derived from the reaction of Mes*PCH 2 GaCl 3 with Mes*Pd CH 2 is an intermediate in this transformation. The reaction of phosphaalkenes with phosphenium species appears to be a general method to prepare diphosphiranium ions. In one case, NMR spectroscopic data suggests that treating MesPdCPh 2 with HOTf gives both the diphosphiranium species [MesPCPh 2 P(Mes)CPh 2 H]OTf and the adduct Ph 2 Cd(Mes)PfP-(Mes)(CHPh 2 )]OTf. Remarkably, treating concentrated Mes*PdCH 2 solutions with HOTf results in oligomers of up to six repeat units, as determined by ESI mass spectrometry. These results suggest that it may be possible to initiate the polymerization of PdC bonds using cationic initiators and that the propagating species will be a cationic phosphenium moiety.