Similarities and differences of epoxide, aldehyde and peroxide initiators for Cp2TiCl-catalyzed styrene living radical polymerizations

Cp₂TiCl is the first example of a single electron transfer (SET) agent that both provides initiating radicals from three different types of functionalities (i.e. radical ring opening of epoxides and reduction of aldehydes and peroxides) and doubles as mediator for the living radical polymerization of styrene (St) by reversibly endcapping the growing polymer chains. An initiator (I) comparison was performed using 1,4-butanediol diglycidyl ether (BDE), benzaldehyde (BA) and benzoyl peroxide (BPO) as models. The investigation of the effect of reaction variables was carried out over a wide range of experimental conditions ([Cp₂TiCl₂]/[I] = 0.5/1-4/1; [Zn]/[Cp₂TiCl₂] = 0.5/1-3/1, [St]/[I] = 50/1-400/1 and T = 60-130 °C) to reveal living polymerization features such as a linear dependence of molecular weight on conversion and narrow molecular weight distribution (M_w/M_n) for each initiator class. However, progressively lower polydispersities and larger initiator efficiencies are obtained with increasing the [Cp₂TiCl₂]/[I] and [Zn]/[Cp₂TiCl₂] ratios and with decreasing temperature. Accordingly, optimum conditions correspond to [St]/[I]/[Cp₂TiCl₂]/[Zn] = [50-200]/[1]/[2-3]/[4-6] at 70-90 °C. By contrast to peroxides, aldehydes and the more reactive epoxides provide alcohol end groups useful in block or graft copolymers synthesis.     

Cp2TiCl‐catalyzed living radical polymerization of styrene initiated from peroxides

The effects of the reaction conditions and nature of the initiator were investigated in the Cp₂Ti(III)Cl‐catalyzed living radical polymerization of styrene initiated by benzoyl peroxide (BPO), tert‐butyl peroxide (TBPO), tert‐butyl peroxybenzoate (TBPOB), dicumyl peroxide (CPO), and tert‐butylperoxy 2‐ethylhexyl carbonate (TBPOEHC). The reversible termination of the growing chains with Cp₂Ti(III)Cl affords a linear dependence of molecular weight on conversion over a wide range of temperatures (60–120 °C) with an optimum in polydispersity (M_w/M_n < 1.2) for St/BPO/Cp₂TiCl₂/Zn = 100/1/3/6 at 60–90 °C. The similarity of the kinetic parameters from polymerizations initiated by peroxides with vastly different half‐life times (t = 1 h, t = 543 h) and the minimum peroxide/Ti = 1/2 ratio required for a living process indicate that initiation occurs primarily by the redox reaction of the peroxide with Cp₂Ti(III)Cl rather than peroxide thermal decomposition. This is consistent with one Ti equivalent consumed in the redox initiation and the second one utilized in the reversible termination of the growing chains. Qualitatively, based on the livingness of the process, these initiators ranked as BPO > TBPOB ～ TBPO > CPO > TBPOEHC. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1106–1116, 2006     

Fluorocarbon‐ended polymers: Metal catalyzed radical and living radical polymerizations initiated by perfluoroalkylsulfonyl halides

Perfluoroalkylsulfonyl chlorides and bromides initiate metal catalyzed free radical polymerization of both hydrocarbon and fluorocarbon monomers affording polymers with perfluoroalkyl end groups. In the case of styrene (S) and methyl methacrylate (MMA) with Cu‐based catalysts the process affords polymers with a relatively narrow molecular weight distribution and linear dependence of molecular weight on conversion, suggesting that a living radical polymerization mechanism occurs. The orders of reaction in monomer, initiator and catalyst for these polymerizations were determined. In the case of PMMA, the detailed structure of a perfluorobutane chain‐end was determined by NMR analysis. Perfluoroalkylsulfonyl chlorides are stable in neutral aqueous media. This permits their use as initators for fluoroolefin polymerizations in H₂O. Poly(tetrafluoroethylene‐co‐hexafluoropropylene) was obtained in good yield with few ionic end groups. The aqueous fluoroolefin polymerization appears to be catalyzed by metal zero species from the reactor walls. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3313–3335, 2000     

Macromolecular engineering by controlled/living ionic and radical polymerizations

Various methods for the synthesis of well‐defined (co)polymers with controlled dimension, polydispersity, topology, composition and functionality are discussed. They include controlled/living vinyl polymerization using anionic, cationic and radical intermediates being in equilibria with dormant species. Special emphasis is placed on the radical polymerization and on the needs for the comprehensive structure property correlation.     

[Cp2TiCl2] as polymerization catalyst in aqueous medium: polymerization of styrene in water

The contrary to the general idea, an early transition metal organometallic compound, [Cp₂TiCl₂] has been found to be active catalyst for aqueous medium polymerization of styrene.     

Formation of [Cp2TiSbMe2]2, [Cp2TiSb(SiMe3)2]2 and [Cp2TiCl]2·2Mes4Sb2

Reactions of [Cp₂Ti(btmsa)] (btmsa = bis(trimethylsilyl)acetylene) with R₄Sb₂ (R = Me, Me₃Si) give [Cp₂TiSbMe₂]₂ (1) or [Cp₂TiSb(SiMe₃)₂]₂ (2) respectively. [Cp₂TiCl]₂·2Mes₄Sb₂ (3) is serendipitously formed from [Cp₂Ti(btmsa)] and Mes₂SbH containing NH₄Cl traces.     

Tin-free radical chemistry using the persistent radical effect: alkoxyamine isomerization, addition reactions and polymerizations

In this tutorial review applications of alkoxyamines as C-radical precursors for the conduction of tin-free radical reactions are presented. These processes are controlled by the Persistent Radical Effect. A brief introduction on the Persistent Radical Effect is provided. In addition, the use of microwave irradiation to conduct thermal radical reactions is discussed. Finally, the use of alkoxyamines as initiators/mediators for the controlled/living radical polymerization is highlighted.     

Theoretical study on the reaction mechanisms of ethylene with Cp2TiR, Cp2Ti(Cl)R, and Cp2Ti(Cl:AlH2Cl)R (R=H and CH3)

The potential energy surfaces for reactions of ethylene with Cp₂Ti⁺R, Cp₂Ti(Cl)R, and Cp₂Ti(Cl:AlH₂Cl)R (R=H and CH₃) were calculated by ab initio molecular orbital methods. These six reaction mechanisms were compared. Of the two possible reaction paths, attack of ethylene at Ti and the Cl and R ligands (path IN) and that from the opposite side of the Cl ligand (path OUT), the former is found to be more favorable, with a very low activation energy for reaction of ethylene with Cp₂Ti(Cl)H. For reaction of ethylene with Cp₂Ti(Cl)CH₃, the insertion transition states on both paths have almost the same energy barrier height above the reactants. For reaction of ethylene with Cp₂Ti(Cl:AlH₂Cl)R, the bond alternation between Ti–Cl and Cl–Al plays an important role in the mechanisms.     

Block copolymers of styrene and p-acetoxystyrene with polyisobutylene by combination of living carbocationic and atom transfer radical polymerizations

Successful combination of quasiliving carbocationic (QLCP) and atom transfer radical polymerizations (ATRP) was carried out by initiating ATRP with polyisobutylene (PIB) macroinitiators obtained by QLCP. It has been found that 1-chloro-1-phenylethyl-telechelic PIBs with M̄_n of 7 800 and 30 700 are efficient macroinitiators for ATRP of styrene in bulk and in xylene solution and for p-acetoxystyrene (pAcOSt) in xylene. Size exclusion chromatography (SEC) traces clearly indicated quantitative initiation and the formation of the desired polystyrene-block-polyisobutylene-block-polystyrene (PSt-PIB-PSt) and PpAcOSt-PIB-PpAcOSt triblock copolymers. Experiments also revealed absence of thermal self-initiation of styrene under ATRP conditions.     

One-step facile synthesis of deuterium labeled aldehydes from tertiary amides using Cp2Zr(D)Cl

The synthesis of deuterium labeled aldehydes was accomplished in an expedient and facile manner through the reduction of amides using commercially available Cp₂Zr(D)Cl. The reactions proceeded rapidly (ca. 15 min) and in high yields (70-99%).     

Controlling polymer structures by atom transfer radical polymerization and other controlled/living radical polymerizations

Fundamentals of controlled/living radical polymerization (CRP) and Atom Transfer Radical Polymerization (ATRP), relevant to the synthesis of controlled polymer structures are described. Macromolecular brushes with star like structure are used as an example to illustrate synthetic power of ATRP.     

Titanium‐mediated living radical styrene polymerizations. VI. Cp2TiCl‐catalyzed initiation by epoxide radical ring opening: Effect of the reducing agents,temperature, and titanium/epoxide and titanium/zinc ratios

The effects of the reducing agent, temperature, and epoxide/Ti and Ti/Zn ratios were investigated for the Cp₂TiCl‐catalyzed living radical polymerization of styrene initiated by epoxide radical ring opening. No reduction of bis(cyclopentadienyl)titanium dichloride occurred with Cu, Devarda's alloy, Ni, Ce, Cr, Sn, Mo, and ascorbic acid, whereas Al, lithium nitride, Mn, Sm, and Fe led to free‐radical or poorly controlled polymerizations. The best results were obtained with Zn alloy, powder, or nanoparticles. Nano‐Zn provided the lowest polydispersity index values, highest initiator efficiency (IE), and fastest reaction rate while maintaining a well‐defined living polymerization. Progressively lower polydispersity was obtained with an increasing excess of Zn with an optimum at Cp₂TiCl/Zn = 1/2. This was rationalized through the heterogeneous nature of Zn and its possible involvement in the reversible termination step. The polymerization was insensitive to light or dark conditions, and a linear dependence of M_n on the conversion was observed at all temperatures in the 60–130 °C range with an optimum at 70–90 °C. A stoichiometric 1/2 epoxide/Ti ratio provided low polydispersity (weight‐average molecular weight/number‐average molecular weight < 1.2) and high IE, whereas increasing the epoxide/Ti ratio to 1/3 maintained a low polydispersity index but decreased the IE. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2156–2165, 2006     

Peptide-polymer bioconjugates: hybrid block copolymers generated via living radical polymerizations from resin-supported peptides

A novel strategy for the preparation of peptidic-synthetic bioconjugate block copolymers is based upon sequential condensation and living radical addition polymerizations, each performed upon a solid support.     

Titanium‐mediated living radical styrene polymerizations. V. Cp2TiCl‐catalyzed initiation by epoxide radical ring opening: Effect of solvents and additives

The effects of solvents, additives, ligands, and solvent in situ drying agents as well as catalyst and initiator concentrations have been investigated in the Cp₂TiCl‐catalyzed radical polymerization of styrene initiated by epoxide radical ring opening. On the basis of the solubilization of Cp₂Ti(III)Cl and the polydispersity of the resulting polymer, the solvents rank as follows: dioxane ≥ tetrahydrofuran > diethylene glycol dimethyl ether > methoxybenzene > diphenyl ether ≥ bulk > toluene ? pyridine > dimethylformamide > 1‐methyl‐2‐pyrrolidinone > dimethylacetamide > ethylene carbonate, acetonitrile, and trioxane. Alkoxide additives such as aluminum triisopropoxide and titanium(IV) isopropoxide are involved in alkoxide ligand exchange with the epoxide‐derived titanium alkoxide and lead to broad molecular weight distributions, whereas similarly to strongly coordinating solvents, ligands such as bipyridyl block the titanium active site and prevent the polymerization. By contrast, softer ligands such as triphenylphosphine improve the polymerization in less polar solvents such as toluene. Although mixed hydrides such as lithium tri‐tert‐butoxyaluminum hydride, sodium borohydride, and lithium aluminum hydride react with bis(cyclopentadienyl)titanium dichloride to form mixed titanium hydride species ineffective in polymerization control, simple hydrides such as lithium hydride, sodium hydride, and especially calcium hydride are particularly effective as in situ trace water scavengers in this polymerization. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 2015–2026, 2006     

Direct formation of cyclobutenylphosphonates from 1-alkynylphosphonates and Cp2ZrCl2/2EtMgCl/2CuCl

Zirconacycles 2 prepared from 1-alkynylphosphonates 1, zirconocene dichloride, and 2 equiv of EtMgCl are smoothly converted into cyclobutenylphosphonates 3 when treated with two equiv of CuCl in 65-81% isolated yield. The reaction is specific and general only for zirconacyclopentenyl phosphonates.     

Atom transfer radical polymerization from nanoparticles: a tool for the preparation of well-defined hybrid nanostructures and for understanding the chemistry of controlled/"living" radical polymerizations from surfaces

Structurally well-defined polymer--nanoparticle hybrids were prepared by modifying the surface of silica nanoparticles with initiators for atom transfer radical polymerization and by using these initiator-modified nanoparticles as macroinitiators. Well-defined polymer chains were grown from the nanoparticle surfaces to yield individual particles composed of a silica core and a well-defined, densely grafted outer polystyrene or poly(methyl methacrylate) layer. In both cases, linear kinetic plots, linear plots of molecular weight (M(n)) versus conversion, increases in hydrodynamic diameter with increasing conversion, and narrow molecular weight distributions (M(w)/M(n)) for the grafted polymer samples were observed. Polymerizations of styrene from smaller (75-nm-diameter) silica nanoparticles exhibited good molecular weight control, while polymerizations of methyl methacrylate (MMA) from the same nanoparticles exhibited good molecular weight control only when a small amount of free initiator was added to the polymerization solution. The difference in polymerization behavior for styrene and MMA was ascribed to the facts that styrene undergoes thermal self-initiation while MMA does not and that termination processes involving freely diffusing chains are faster than those involving surface-bound chains. The polymerizations of both styrene and MMA from larger (300-nm-diameter) silica nanoparticles did not exhibit molecular weight control. This lack of control was ascribed to the very high initial monomer-to-initiator ratio in these polymerizations. Molecular weight control was induced by the addition of a small amount of free initiator to the polymerization but was not induced when 5--15 mol % of deactivator (Cu(II) complex) was added.     

Direct through anionic,cationic, and radical active species: Terminal carbon–halogen bond for “controlled”/living polymerizations of styrene

In this work, we examined the synthesis of novel block (co)polymers by mechanistic transformation through anionic, cationic, and radical living polymerizations using terminal carbon–halogen bond as the dormant species. First, the direct halogenation of growing species in the living anionic polymerization of styrene was examined with CCl₄ to form a carbon–halogen terminal, which can be employed as the dormant species for either living cationic or radical polymerization. The mechanistic transformation was then performed from living anionic polymerization into living cationic or radical polymerization using the obtained polymers as the macroinitiator with the SnCl₄/n‐Bu₄NCl or RuCp^*Cl(PPh₃)/Et₃N initiating system, respectively. Finally, the combination of all the polymerizations allowed the synthesis block copolymers including unprecedented gradient block copolymers composed of styrene and p‐methylstyrene. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 465–473     

Cp2TiCl2 catalyzed one-pot synthesis of n-Bu3GeH from GeCl4

The reaction of GeCl₄ with n-BuMgCl in presence of a catalytic amount of Cp₂TiCl₂ gives n-Bu₃GeH and n-Bu₄Ge in ca. 70 and 25% yield, respectively. This method provides an industrially feasible one-pot synthesis for Bu₃GeH and Bu₄Ge. The reaction temperature and stoichiometry seem to be important in the distribution of the products. Apart from elemental analysis these compounds have been characterized by comparing their boiling points, NMR spectral data and GC assay with that of the authentic samples.     

Monomeric, dimeric and polymeric [Cp2MoH2] complexes with Ag---S bonds

The reaction of [Cp₂MoH₂] and AgBF₄ with the dithio ligands Na(S₂CPh) and K(S₂COⁱPr) afforded the complexes [(Cp₂MoH₂AgS₂CPh)₂] (1) and [(Cp₂MoH₂AgS₂COⁱPr)₂] (2). Using the monothio ligands Na(SC(O)Ph), K(SC(O)CH₃) and Na(S(NHPh)C=C(CN)₂) the complexes [(Cp₂MoH₂AgSC(O)Ph)₂] (3), [((Cp₂MoH₂)₂(AgSC(O)CH₃)₃)_n] (4) and [(Cp₂MoH₂)₂AgS(NHPh)C=C(CN)₂] (6) were formed. The reaction of thiobenzamide and [(Cp₂MoH₂)₂AgCl] gave the complex [(Cp₂MoH₂Ag(Cl)S(NH₂)CPh)₂] (5). Complexes 1 and 2 have a dimeric structure with the two dithio ligands bridging the two silver atoms. Complex 3 is also a dimer, however, the monothio ligands are coordinated with their single sulphur atoms to the silver atoms. In the polymer 4 the thioacetate ligand has the same bonding mode as in 3. The three-dimensional structure of 4 is built-up of parallel strings. In the dimer 5 the thiobenzamide ligands bind with the sulphur atom to a silver atom each. Complex 6 has a monomeric structure in which the silver atom is coordinated to two [Cp₂MoH₂] ligands and to the sulphur atom of the S(NHPh)C=C(CN)₂ ligand. Compounds 1–6 were characterised analytically and spectroscopically and the structures were determined by single crystal X-ray analyses.