Polyisobutylenes with Controlled Molecular Weight and Chain-End Structure: Synthesis and Actual Applications

The polymerization of isobutylene allows us to obtain a wide spectrum of polyisobutylenes (PIBs) which differ in their molecular weight characteristics and the chemical structure of chain-end groups. The bulk of the PIBs manufactured worldwide are highly reactive polyisobutylenes (HRPIBs) with -C(Me)=CH2 end-groups and low-molecular weights (Mn < 5 kDa). HRPIBs are feedstocks that are in high demand in the manufacturing of additives for fuels and oils, adhesives, detergents, and other fine chemicals. In addition, HRPIBs and CMe2Cl-terminated PIBs are intensively studied with the aim of finding biomedical applications and for the purpose of developing new materials. Both chain control (molecular weight and dispersity) and chemoselectivity (formation of exo-olefinic or -CMe2Cl groups) should be achieved during polymerization. This review highlights the fundamental issues in the mechanisms of isobutylene polymerization and PIB analysis, examines actual catalytic approaches to PIBs, and describes recent studies on the functionalization and applications of HRPIBs and halogen-terminated PIBs.

Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity

Two series of zinc salts, [EtZn][A] and Zn[A]₂, with weakly coordinating anions [A]^- as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me₃NB₁₂Cl₁₁]^- [1]^-, [Pr₃NB₁₂H₅Cl₆]^- [2]^-, [Bu₃NB₁₂H₄Cl₇]^- [3]^-, and [Hex₃NB₁₂H₅Cl₆]^- [4]^- were chosen as potential and more readily available alternatives to carborate anions such as [CHB₁₁Cl₁₁]^- and [HexCB₁₁Cl₁₁]^-. The basicity of anion [4]^- was determined as being close to that of the triflimide anion [N(SO₂CF₃)₂]^-, and the fluoride ion affinities (FIAs) of compounds [EtZn][2] and Zn[2]₂ are lower than those of the Lewis acids B(C₆F₅)₃ and Zn[HexCB₁₁Cl₁₁]₂. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.

A Bibliometric Survey on Polyisobutylene Manufacture

Polyisobutylenes (PIB) constitute a versatile family of polymer materials that have been used mainly as fuel and lubricant additives. Particularly, the current commercial demand for highly reactive polyisobutylene (HR-PIB) products motivates the development of new processes and procedures to produce PIBs with high polymer yields, narrow molar mass distributions and high vinyl contents. For this reason, a bibliometric survey is presented here to map and discuss important technical aspects and technological trends in the field of solution cationic polymerization of isobutylenes. It is shown that investigations in this field are concentrated mainly on developed countries and that industrial initiatives indicate high commercial interest and significant investments in the field. It is also shown that use of catalyst systems based on AlCl3 and ether cocatalysts can be very beneficial for PIB and HR-PIB manufacture. Finally, it is shown that investigations search for cheaper and environmentally friendly catalysts and solvents that can be employed at moderate temperatures, particularly for the production of HR-PIB.

A Highly Lewis Acidic Strontium ansa-Arene Complex for Lewis Acid Catalysis and Isobutylene Polymerization

The potential of a dicationic strontium ansa‐arene complex for Lewis acid catalysis has been explored. The key to its synthesis was a simple salt metathesis from SrI₂ and 2 Ag[Al(OR^F)₄], giving the base‐free strontium‐perfluoroalkoxyaluminate Sr[Al(OR^F)₄]₂ (OR^F=OC(CF₃)₃). Addition of an ansa‐arene yielded the highly Lewis acidic, dicationic strontium ansa‐arene complex. In preliminary experiments, the complex was successfully applied as a catalyst in CO₂‐reduction to CH₄ and a surprisingly controlled isobutylene polymerization reaction.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Are homoleptic complexes of ethylene with group 12 metals isolable in solution? A DFT study

Ethylene efficiently binds late transition metals of groups 10 and 11. In spite of their reactivity, homoleptic compounds of formula [M-(η²-C₂H₄)₃]ⁿ⁺ (with n = 0,1) have been isolated in solution and solid state and characterized spectroscopically throughout the last 50 years with metals from groups 10 and 11. X-ray diffraction studies proved that such homoleptic adducts adopt planar "wheel" structures where ethylene moieties lies flat in the same plane both in group 10 and 11. These experimental findings were also confirmed by several in-depth computational investigations carried out to understand the bond pattern of such peculiar structures. Homoleptic complexes of group 10 and 11 metals with ethylene are normally obtained in poorly coordinating solvents (like CH₂Cl₂ or light petroleum) saturated with ethylene to increase the stability of such species in solution. In the case of coinage metals, Cu(I), Ag(I) and Au(I), weakly coordinating fluorinated counter-ions (like SbF₆^-) succeeded in stabilize the ethylene adducts, but, curiously, no analogous success has been reported for Zn(II), Cd(II), and Hg(II). Isoelectronic congeners along group 12 are still elusive, however, and, to our knowledge, full experimental and theoretical characterizations are still missing. This manuscript focuses on the theoretical study of the thermodynamic stability and properties of homoleptic complexes of ethylene with metals from group 12 in comparison with those from groups 10 and 11.

A twist on Hellwinkel’s salt, [P(2,2′-biphenyl)2]+[P(2,2′-biphenyl)3]–

Treating PCl5 with C12H8Li2, generated from either C12H10, C12H8Br2, or C12H8I2, affords three products in different ratios depending on the source of the lithiated biphenyl. Hellwinkel’s salt [P(C12H8)2][P(C12H8)3] ([1][2]) and another product [P(C12H8)(C24H16)][P(C12H8)3] ([1′][2]) were obtained by reacting PCl5 with 2,2′-dilithiobiphenyl [Route A: 2.5 equiv.; obtained from biphenyl, n-BuLi, and TMEDA; Route B: 3.0 equiv.; obtained from 2,2′-diiodobiphenyl and n-BuLi; Route C: 4.0 equiv.; obtained from 2,2′-dibromobiphenyl and n-BuLi]. The synthesis, isolation, and characterization of the chiral spiro-compound [1′][2] and the characterization of the pentavalent phosphorane [P(C12H8)2(C12H9)] (3) are reported. The complex [1′][2] was characterized by 31P{1H} NMR spectroscopy, X-ray crystallography, and mass spectrometry. The pentavalent compound (3) was characterized by 31P{1H} NMR spectroscopy and X-ray crystallography.

The Lewis superacid Al[N(C6F5)2]3 and its higher homolog Ga[N(C6F5)2]3 - structural features, theoretical investigation and reactions of a metal amide with higher fluoride ion affinity than SbF5

Herein we present the synthesis of the two Lewis acids Al[N(C6F5)2]3 (ALTA) and Ga[N(C6F5)2]3 (GATA) via salt elimination reactions. The metal complexes were characterized by NMR-spectroscopic methods and X-ray diffraction analysis revealing the stabilization of the highly Lewis acidic metal centers by secondary metal-fluorine contacts. The Lewis acidic properties of Al[N(C6F5)2]3 and Ga[N(C6F5)2]3 are demonstrated by reactions with Lewis bases resulting in the formation of metallates accompanied by crucial structural changes. The two metallates [Cs(Tol)3]+[FAl(N(C6F5)2)3]- and [AsPh4]+[ClGa(N(C6F5)2)3]- contain interesting weakly coordinating anions. The reaction of Al[N(C6F5)2]3 with trityl fluoride yielded [CPh3]+[FAl(N(C6F5)2)3]- which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(C6F5)2]3 and Ga[N(C6F5)2]3 was investigated by means of competition experiments for chloride ions in solution. DFT calculations yielded fluoride ion affinities in the gas phase (FIA) of 555 kJ mol-1 for Al[N(C6F5)2]3 and 472 kJ mol-1 for Ga[N(C6F5)2]3. Thus, Al[N(C6F5)2]3 can be considered a Lewis superacid with a fluoride affinity higher than SbF5 (493 kJ mol-1) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.

Probing steric influences on electrophilic phosphonium cations: a comparison of [(3,5-(CF3)2C6H3)3PF]+ and [(C6F5)3PF]+

The electrophilic phosphonium cation (EPC) salt [(3,5-(CF₃)₂C₆H₃)₃PF][B(C₆F₅)₄] (2) can display catalytic activity greater than its thermodynamic acidity would suggest. The role of steric factors is explored.

[HL2][P(1,2-O2C6Cl4)3] (L = THF, DMF): Brønsted acid initiators for the polymerization of n-butyl vinyl ether and p-methoxystyrene

The development of solid, weighable Brønsted acids featuring the hexacoordinated phosphorous(v) anion [TRISPHAT]⁻ are reported.