Dithiadiazolium salts as initiators for the cationic ring-opening polymerization of tetrahydrofuran

Mono-, bis- and tris-(1,3,2,4-dithiadiazolium) salts [R-(CNSNS ⁺)_n]_n+[AsF^-₆]_n (R = aryl, n = 1, 2, 3) were found to initiate the cationic ring-opening polymerization of tetrahydrofuran (THF) at room temperature to give clear gels from which the pure polymer was precipitated. 1,3,2,4-Dithiadiazolium cations associated with the hard [AsF₆]^- anion thus constitute a new class of cationic polymerization initiators. The poly(THF) formed by initiation with 1,3,2,4-dithiadiazolium cation was characterized by gel permeation chromatography, infrared spectrophotometry, and ¹³C-NMR spectroscopy. Number-average molecular weights of 198 700 g mol^-1 (polydispersity 1.96) and 190 000 g mol^-1 (polydispersity 1.61) were obtained using [PhCNSNS ] [AsF₆] and [C₆H₃-1,3,5-(CNSNS )₃][AsF₆]₃, respectively, as initiators. The use of multifunctional dithiadiazolium salts as initiators suggests that they may be useful in the preparation of starburst and dendritic polymers. © 1992 John Wiley & Sons, Inc.     

Trimethylsilyl triflate as an initiator for cationic polymerization: Improved initiation through the use of promoters

Trimethylsilyl triflate (TMSOTf) can be used as an initiator for the cationic polymerization of alkenes and heterocycles. However, TMSOTf without additives and promoters acts inefficiently. Initiation in the cationic polymerization of tetrahydrofuran is slow because of unfavorable charge distribution in the trimethylsilyltetrahydrofuranium cation—a product of the reaction of monomer with TMSOTf. Acetone, 1,3-dioxolane, and 1,2-propylene oxide have been used as promoters to react with TMSOTf to create more reactive initiating species and to improve efficiency of initiation. Of these promoters, 1,2-propylene oxide has been the most successful. When TMSOTf has been used to initiate the polymerization of 2-methyl-2-oxazoline, unfavorable charge distribution in the N-trimethylsilyl-2-methyl-2-oxiranium cation has produced an unreactive imine dimeric cation which extends the time required for polymerization to several weeks. 1,2-Propylene oxide has been utilized to prevent formation of the imine dimeric cation by producing a more reactive initiating species. In the polymerization of isobutyl vinyl ether initiated by TMSOTf, 1,2-propylene oxide has been shown to be ineffective as a promoter, but acetone can be used successfully. © 1995 John Wiley & Sons, Inc.     

Synthesis of well-defined norbornenyl-terminated poly(alkyl methacrylate)s by group transfer polymerization and their grafting-through ring-opening metathesis polymerization

Highly efficient syntheses of poly(alkyl methacrylate)-based brush polymers were accomplished via a facile group transfer polymerization (GTP) and a consecutive grafting-through ring-opening metathesis polymerization. The GTP system, composed of the norbornenyl-methyl trimethylsilyl ketene acetal initiator and the N-(trimethylsilyl) bis(trifluoromethanesulfonyl)imide catalyst, rapidly and quantitatively generates norbornenyl-terminated poly(alkyl methacrylate) macromonomers with very narrow polydispersities (M_w/M_n < 1.10). The ring-opening metathesis polymerization of methacrylate macromonomers using Grubbs third generation catalyst successfully generated a group of methacrylate-based brush polymers, which assured the high quality of the macromonomers obtained from GTP.     

Reversible redox controlled acids for cationic ring-opening polymerization

Advancements in externally controlled polymerization methodologies have enabled the synthesis of novel polymeric structures and architectures, and they have been pivotal to the development of new photocontrolled lithographic and 3D printing technologies. In particular, the development of externally controlled ring-opening polymerization (ROP) methodologies is of great interest, as these methods provide access to novel biocompatible and biodegradable block polymer structures. Although ROPs mediated by photoacid generators have made significant contributions to the fields of lithography and microelectronics development, these methodologies rely upon catalysts with poor stability and thus poor temporal control. Herein, we report a class of ferrocene-derived acid catalysts whose acidity can be altered through reversible oxidation and reduction of the ferrocenyl moiety to chemically and electrochemically control the ROP of cyclic esters.

A class of ferrocene-derived acid catalysts whose acidity can be altered through reversible oxidation and reduction of the ferrocenyl moiety to chemically and electrochemically control the ROP of cyclic esters is reported.     

Heteroscorpionate rare-earth initiators for the controlled ring-opening polymerization of cyclic esters

A series of neutral rare-earth metal amides containing different achiral and chiral heteroscorpionate ligands was synthesized and characterized and these compounds were employed in the polymerization of cyclic esters. Thus, treatment of [Ln{N(SiHMe(2))(2)}(3)(thf)(2)] (Ln = Nd, Sm) with acetamide or thioacetamide heteroscorpionate ligands for 2 h at 0 °C afforded the α-agostic silylamido dimeric rare-earth compounds [Ln{N(SiHMe(2))(2)}(NNE)](2) (Ln = Nd and Sm; NNE = heteroscorpionate ligands, E = O, S) (1-8), some as enantiopure complexes. Complexes 1-8 contain dianionic heteroscorpionate pseudoallyl ligands resulting from C-H activation of the bridging methine group of the bis(pyrazol-1-yl)methane moiety and subsequent coordination to the metal center. However, when the reaction was carried out for 1 h at lower temperature new bis(silylamido) dimeric lanthanide compounds [Ln{N(SiHMe(2))(2)}(2)(NNE)](2) (Ln = Nd and Sm; E = O) (9 and 10) were obtained. The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structures of 1, and 4 were also established. Neodymium complexes are active initiators for the ring-opening polymerization (ROP) of lactide (LA) and lactones, giving rise to medium-high molar mass polymers under mild conditions and with narrow polydispersities. These complexes were well suited for achieving well-controlled polymerization through an insertion-coordination mechanism. Achiral and racemic complexes did not affect stereocontrol in the polymerizarion of rac-LA but the enantiomerically pure complex 1 was found to exhibit a homosteric preference for one of the two enantiomers of rac-LA at low conversions.     

Synthesis of optically inactive cellulose via cationic ring-opening polymerization

Cellulose, which comprises D-glucose and L-glucose (D,L-cellulose), was synthesized from D-glucose (1D) and L-glucose (1L) via cationic ring-opening polymerization. Specifically, the ring-opening copolymerization of 3-O-benzyl-2,6-di-O-pivaloyl-β-D-glucopyranoside (2D) and 3-O-benzyl-2,6-di-O-pivaloyl-β-D-glucopyranoside (2L), synthesized from compounds 1D and 1L, respectively, in a 1:1 ratio, afforded 3-O-benzyl-2,6-di-O-β-D,L-glucopyranan (3DL) with a degree of polymerization (DPn) of 28.5 (Mw/Mn?=?1.90) in quantitative yield. The deprotection of compound 3DL and subsequent acetylation proceeded smoothly to afford acetylated compound 4DL with a DPn of 18.6 (Mw/Mn?=?2.08). The specific rotation of acetylated compound 4DL was?+?0.01°, suggesting that acetylated compound 4DL was optically inactive cellulose triacetate. Furthermore, before acetylation, compound 4DL was an optically inactive cellulose comprising an almost racemic mixture of D-glucose and L-glucose. Compound 4DL was an amorphous polymer. This is the first reported synthesis of optically inactive D,L-cellulose.

     

The mechanism of TBD-catalyzed ring-opening polymerization of cyclic esters

Triazabicyclodecene (TBD) has recently been shown to be an effective organocatalyst for the ring-opening polymerization (ROP) of cyclic esters. Using DFT methods, we have studied possible mechanisms of this reaction. Our studies explain not only the narrow polydispersity index (PDI) observed in the ROP of six-membered ring lactones, but also the surprising failure of the ROP for the more reactive butyrolactone.     

Mechanism of cationic polymerization of glycidic esters

The kinetics of cationic polymerization of glycidyl esters (glycidyl acetate, glycidyl benzoate, glycidyl naphthoate) were studied. The kinetic parameters of this process were determined. It was shown that the rate of polymerization is anomalously dependent on the polarity of the medium. The existence of an internal complexation and an equilibrium between two types of active centers, determining the rate of the process, has been suggested.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 722–726, November–December, 1985.     

Cationic ring-opening polymerization of tetrahydrofuran with Keggin-type heteropolycompounds as solid acid catalysts

<正>Three Keggin-type heteropolyanions,namely H_3PMo_(12)O_(40)·13H_2O,(NH_4)_3PMo_(12)O_(40)·4H_2O and H_3PW_(12)O_(40)·13H_2O were prepared and tested in the ring-opening polymerization reaction of tetrahydrofuran.The effects of the counter-cation (H~+,NH_4~+) and the peripheral atoms(Mo,W) on the polymerization were investigated.It has been found that when the protons of H_3PMo_(12)O_(40)·13H_2O were replaced by the ammonium cations the polymerization rate decreased dramatically. Whereas,when the peripheral atoms(Mo) were replaced by their homologous(W),the polymerization rate increased twofold.As for the viscosity average molecular weight(M_v) of polymer products,it was found that the high molecular weight(7930) was obtained by using H_3PW_(12)O_(40)·13H_2O.The molecular weight(M_v) obtained by H_3PMo_(12)O_(40)·13H_2O and (NH_4)_3PMo_(12)O_(40)·4H_2O was 6470 and 6810,respectively.     

Sterically controlled propagation in the cationic ring-opening polymerization of bicyclic acetals

This paper deals with sterically controlled propagation in the cationic ring-opening polymerization of bicyclic acetals in relation to the chemical synthesis of structurally regulated polysaccharides, particularly focusing on the following two topics: 1) Chemical synthesis of (1→6)-β-linked polysaccharides by the ring-opening polymerization of 1,6-anhydrosugar derivatives. 2) Asymmetric selective copolymerization of racemic bicyclic acetal with optically active monomer.     

Stereochemical aspects of the controlled ring-opening polymerization of chiral cyclic esters

The stereochemical aspects of controlled polymerization of lactide and β-butyrolactone are discussed. The physicochemical and thermal properties of these polymers are shown to be depending on the stereochemical structure of the macromolecular chain. Different types of processes involving change of enantiomeric composition in the course of the polymerization reaction are examined in function of different initiators used. The formation of stereocomplexes from stereocopolymers of various enantiomeric compositions is reported.     

Stereoregular functionalized polysaccharides via cationic ring-opening polymerization of biomass-derived levoglucosan

We report the facile synthesis and characterization of 1,6-α linked functional stereoregular polysaccharides from biomass-derived levoglucosan via cationic ring-opening polymerization (cROP). Levoglucosan is a bicyclic acetal with rich hydroxyl functionality, which can be synthetically modified to install a variety of pendant groups for tailored properties. We have employed biocompatible and recyclable metal triflate catalysts – scandium and bismuth triflate – for green cROP of levoglucosan derivatives, even at very low catalyst loadings of 0.5 mol%. Combined experimental and computational studies provided key kinetic, thermodynamic, and mechanistic insights into the cROP of these derivatives with metal triflates. Computational studies reveal that ring-opening of levoglucosan derivatives is preferred at the 1,6 anhydro linkage and cROP proceeds in a regio- and stereo-specific manner to form 1,6-α glycosidic linkages. DFT calculations also show that biocompatible metal triflates efficiently coordinate with levoglucosan derivatives as compared to the highly toxic PF₅ used previously. Post-polymerization modification of levoglucosan-based polysaccharides is readily performed via UV-initiated thiol–ene click reactions. The reported levoglucosan based polymers exhibit good thermal stability (T_d > 250 °C) and a wide glass transition temperature (T_g) window (<−150 °C to 32 °C) that is accessible with thioglycerol and lauryl mercaptan pendant groups. This work demonstrates the utility of levoglucosan as a renewably-derived scaffold, enabling facile access to tailored polysaccharides that could be important in many applications ranging from sustainable materials to biologically active polymers.

We demonstrate the facile synthesis and characterization of stereoregular polysaccharides from the biomass-derived platform molecule levoglucosan via metal-triflate mediated cationic-ring opening polymerization.     

Synthesis of (adamantylimido)vanadium(V)-alkyl complexes containing aryloxo ligands and their use as the catalyst precursors for ring-opening metathesis polymerization of norbornene, and ring-opening polymerization of tetrahydrofuran

Various (adamantylimido)vanadium(V) dialkyl complexes containing aryloxo ligands, V(NAd)(CH₂SiMe₃)₂(OAr) [Ad = 1-adamantyl (1); Ar = Ph (a), 4-FC₆H₄ (b), 2,6-F₂C₆H₃ (c), 2,6-Me₂C₆H₃ (d), C₆F₅ (e)], have been prepared and identified. These complexes were employed as the catalyst precursors for ring-opening metathesis polymerization (ROMP) of norbornene (NBE) in the presence of PMe₃ at 80 °C. The activity was strongly affected by the aryloxo substituent and increased in the order: C₆H₅ < 4-FC₆H₄ < 2,6-Me₂C₆H₃ << 2,6-F₂C₆H₃, C₆F₅. The same trend was observed in the ROMPs by the arylimido-aryloxo analogues, V(NAr′)(CH₂SiMe₃)₂(OAr) (2a-e; Ar′ = 2,6-Me₂C₆H₃), under the same conditions, and the activities by the arylimido analogues were generally higher than the adamantylimido analogues in most case. The (imido)vanadium(V) complexes containing O-2,6-F₂C₆H₃ (1,2c) or OC₆F₅ (1,2e) exhibited high catalytic activities, and these results strongly suggest that electronic as well as steric factors play a role. Living ring-opening polymerization of THF proceeded in the presence of V(NAd) (CH₂SiMe₃)(OAr)₂ (Ar = 2,6-Me₂C₆H₃, C₆F₅) and [Ph₃C][B(C₆F₅)₄], affording high molecular weight polymers with narrow molecular weight distributions (ex. M_n = 2.11 × 10⁵, M_w/M_n = 1.18).     

On the nature of active species in cationic ring-opening polymerization of cyclodimethylsiloxanes

Contrarily to cationic ring-opening polymerization of cyclic ethers and of some other cyclic monomers, for which direct identification of the various types of active centres has been made in a few cases, the nature of the species active in the polymerization of cyclo-dimethylsiloxanes is not yet known. However, some provisional conclusions about the possible mechanisms may be deduced from the wide variation in the types of products and in the kinetics observed according to either the size of the cyclic monomer (D₃, D₄, D₅, D₆) and to the type of initiation (chemically, or radiation induced). For polymerizations with either protonic or non-protonic initiators, made in CH₂Cl₂ near room temperature, the smaller cycle D₃ behaves quite differently from D₄, D₅ and D₆. D₃ is more reactive in both homo- and copolymerizations. It gives small cycles of other types and the effect of water on the reaction may be quite different. A discussion of the data leads to the conclusion that polymer growth for most cyclosiloxanes involves activated esters, while it may occur for D₃ on different sites such as oxonium or silanol groups. Polymerization of D₃, D₄ and D₅ initiated in bulk at 90°C by high energy radiation, in high purity conditions, has also been shown to be cationic but the active centres concentration is much lower, and the propagation rate constants much higher, than in chemically initiated polymerizations. The global rates, the monomer reactivities in copolymerization and the types of cycles are similar for D₃, D₄ and D₅, which is attributed to propagation occurring on very reactive silicenium ions, either free or in the same solvation state.     

Determination of concentration of propagating species in cationic polymerization of tetrahydrofuran

A spectroscopic method is described for the determination of the concentration of propagating species, [P^*], in the polymerization of tetrahydrofuran catalyzed by a mixture of AlEt₃?H₂O (1:0.5) and epichlorohydrin. A phenyl ether group was introduced at the polymer chain end by the quantitative reaction of the propagating species with excess sodium phenoxide. From the amount of phenyl ether groups in the polymer and of the remaining sodium phenoxide, [P^*] was determined by means of ultraviolet spectroscopy. The [P^*] value so determined was found to be in good agreement with that calculated from the amount and molecular weight of polymer based on a stepwise addition mechanism without chain transfer or termination. The present method of [P^*] determination was employed to examine the course of polymerization. It has now been found that [P^*] increases progressively during an induction period and remains unchanged in the subsequent period of polymerization.     

The initiation process in the polymerization of tetrahydrofuran with carbonium ion salts

The initiation process for the polymerization of tetrahydrofuran with (C₆H₅)₃C⁺SbCl₆^? has been studied. Two mechanisms have been considered: a cation-addition process, and a process in which tetrahydrofuran donates a hydride ion to the cation of the initiator to form triphenylmethane. The biscarbonium salt [(C₆H₅)₂C⁺C₆H₄CH₂]₂(SbCl₆^?)₂ has been synthesized and used to initiate the polymerization of tetrahydrofuran. The results are consistent with the hydride-ion mechanism but may be inconclusive because of chain transfer. NMR experiments with 0.05–0.2M solutions of initiator in tetrahydrofuran show that triphenylmethane is rapidly produced in an amount equal to the molar amount of initiator originally present. Some NMR evidence for the presence of an acetal end group in the polymer has been obtained. It is concluded that the initiation process in this system definitely involves the formation of triphenylmethane, although a detailed, unique mechanism cannot be selected at this time.