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.

     

Branched polymers by cationic ring-opening polymerization

Two methods for the synthesis of branched (co)polymers by cationic ring-opening polymerization are presented. The first method is based on the spontaneous intermolecular termination that is observed in the polymerization of the four-membered cyclic sulfides (thietanes). The branching points in these polymers are sulfonium ions. This method has been extended to polyether - polysulfide block copolymers obtained by sequential polymerization of THF and a thietane. In the thus obtained AB block polymers, the branching points are concentrated in the sulfide segments only. By similar techniques, ABA types of block copolymer networks have been obtained making use of bifunctional initiators. The second method consists of copolymerizing a cyclic acetal such as 1,3-dioxolane (DXL), with a “monofer”, which is a monomer that contains also a chain-transfer function. As monofers for the DXL polymerization glycidol and glycerol formal were used. The end products are polyacetal-polyols which contain a hydroxyl group at each of the chain ends. Reaction of these polyols with di-isocyanates leads to the corresponding polyacetal polyurethanes.     

Living photolytic ring-opening polymerization of amino-functionalized [1]ferrocenophanes: synthesis and layer-by-layer self-assembly of well-defined water-soluble polyferrocenylsilane polyelectrolytes

Facile synthetic routes have been developed that provide access to cationic and anionic water-soluble polyferrocenylsilane (PFS) polyelectrolytes with controlled molecular weight and narrow polydispersity. Living photolytic ring-opening polymerization of amino-functionalized [1]ferrocenophane (fc) monomers [fcSiMe{C[triple chemical bond]CCH(2)N(SiMe(2)CH(2))(2)}] (3), [fcSi{C[triple chemical bond]CCH(2)N(SiMe(2)CH(2))(2)}(2)] (10), [fcSiMe(C[triple chemical bond]CCH(2)NMe(2))] (14), and [fcSiMe(p-C(6)H(4)CH(2)NMe(2))] (20) yielded the corresponding polyferrocenylsilanes [(fcSiMe{C[triple chemical bond]CCH(2)N(SiMe(2)CH(2))(2)})(n)](5), [(fcSi{C[triple chemical bond]CCH(2)N(SiMe(2)CH(2))(2)}(2))(n)] (11), [{fcSiMe(C[triple chemical bond]CCH(2)NMe(2))}(n)] (15), and [{fcSiMe(p-C(6)H(4)CH(2)NMe(2))}(n)] (21) with controlled architectures. Further derivatization of 5, 15, and 21 generated water-soluble polyelectrolytes [(fcSiMe{C[triple chemical bond]CCH(2)N(CH(2)CH(2)CH(2)SO(3)Na)(2)})(n)] (6), [{fcSiMe(C[triple chemical bond]CCH(2)NMe(3)OSO(3)Me)}(n)] (7), and [{fcSiMe(p-C(6)H(4)CH(2)NMe(3)OSO(3)Me)}(n)] (22), respectively. The polyelectrolytes were readily soluble in water and NaCl aqueous solutions, with 6 and 22 exhibiting long-term stability in aqueous media. The PFS materials 6 and 22, have been utilized in the layer-by-layer (LbL) self-assembly of electrostatic superlattices. Our preliminary studies have indicated that films made from controlled low molecular-weight PFSs possess a considerably thinner bilayer thickness and higher refractive index than those made from PFSs that have an uncontrolled high molecular-weight. These results suggest that the structure and optical properties of LbL ultra-thin films can be tuned by varying polyelectrolyte chain length. The water-soluble low molecular weight PFSs are also useful materials for a range of applications including LbL self-assembly in highly confined spaces.     

Living ring-opening polymerization of cyclic carbonate using cationic zirconocene complex as catalyst

Living ring-opening polymerization of the cyclic carbonate 1,3-dioxepan-2-one was achieved by using the cationic zirconocene complex [Cp₂ZrMe]⁺[B(C₆F₅)₄]⁻ as catalyst at room temperature. A linear relation between conversion and molecular weight of the obtained polymer was observed. Furthermore, block copolymerization of the cyclic carbonate and ε-caprolactone was successfully performed.     

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.     

Living ring-opening polymerization of cyclic thiocarbonate

This work deals with the cationic ring-opening polymerization of a cyclic thiocarbonate, 5,5-dimethyl-1,3-dioxane-2-thione (1). The polymerization was carried out with 2 mol% of trifluoromethanesulfonic acid, methyl trifluoromethanesulfonate, boron trifluoride etherate, or triethyloxonium tetrafluoroborate as an initiator to afford the polythiocarbonate with the narrow molecular weight distribution (M_n = 11200-31000, M_w/M_n = 1.04-1.15). The molecular weight of the obtained polymer could be controlled by the feed ratio of the monomer to the initiator and increased when the second monomer was added to the polymerization mixture after quantitative consumption of 1 in the first stage, supporting that the cationic ring-opening polymerization of 1 proceeded via a living process.     

Living ring-opening polymerization of N-sulfonylaziridines: synthesis of high molecular weight linear polyamines

Sulfonylaziridines have been identified as excellent monomers for living ring-opening polymerization initiated by nucleophilic sulfonylamides. The resulting polymers exhibit low polydispersities and controllable molecular weights. The enantiopurity of the monomer plays a key role: racemic samples yield soluble polymers of target molecular weights, while enantiopure samples produce insoluble polymers with molecular weights significantly below theoretical values. Dynamic light scattering and kinetics of polymerization are discussed.     

Living anionic ring-opening polymerization of 1,1-dipropylsilacyclobutane

Synthesis and living anionic ring-opening polymerization of 1,1-dipropylsilacyclobutane are reported. High molecular weight poly(dipropylsilylenepropylene) up to M _n = 83900 g/mol (SEC/PS standards) with low polydispersity (M _w/M _n = 1.11 to 1.22) was obtained at −20°C. End functionalization of poly-(dipropylsilylenepropylene) with chlorodimethylvinylsilane and synthesis of block copolymers with styrene was achieved. The polymers were characterized with NMR, SEC, MALDI-TOF and DSC.     

Allyl sulfonium salt as a novel initiator for active cationic polymerization of epoxide by shooting with radicals species

A rapid cationic polymerization of cyclohexene oxide that completed within a few minutes was achieved by a new initiation system that involves (1) a copper‐catalyzed reduction of benzoyl peroxide by an ascorbic acid derivative that generates free radicals and (2) capture of the radicals by allyl sulfonium salt having hexafluoroantimonate (SbF) as a counter anion, followed by fragmentation of sulfonium radical cation, from which a super acid HSbF₆ was produced to initiate the rapid polymerization. The key factor in designing an efficient allyl sulfonium salt was attachment of an electron withdrawing ester group at the allyl group, of which ability to stabilize the formed radical can enhance the efficiency in trapping radicals by the allylic salt. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4178–4183, 2010     

Vinyl ethers with a functional group: Living cationic polymerization and synthesis of monodisperse polymers

The living cationic polymerization of vinyl ethers (VEs) having a (polar) functional pendant has been achieved by the hydrogen iodide/iodine (HI/I₂) initiating system to give polymers with a very narrow molecular weight distribution (MWD) (M_w/M_n ≤ 1.2). The functional pendants include benzyl, saturated or unsaturated ester, (poly) oxyethylene, and substituted phenoxyl groups. Although these polar groups often disturb cationic vinyl polymerization by inducing chain transfer and termination, the HI/I₂ initiator cleanly polymerized the “functionalized” VEs without side reactions, mostly in nonpolar media at low temperatures below −15 °C. The HI/I₂-initiated living polymerization also provided facile methods to synthesize new functional polymers, including water-soluble polymers, macromolecular amphiphiles, and macromers, all having a narrow MWD. The simplest example is the living polymerization of VEs carrying an oxyethylene chain [-(CH₂CH₂O)_n-R] as pendant, which directly yields water-soluble polymers. The debenzylation of poly(benzyl VE) prepared with HI/I₂ led to poly(vinyl alcohol). Polymers of the saturated ester-containing monomers (2-acetoxyethyl and 2-benzoyloxyethyl VEs) were readily hydrolyzed into poly (2-hydroxyethyl VE), soluble in water and swellable in methanol. This lead was extended to the synthesis of a new amphiphile, poly(cetyl VE-b-2-hydroxyethyl VE), from a block copolymer of cetyl and 2-acetoxyethyl VEs prepared by their sequential living polymerization initiated with HI/I₂. An adduct between HI and 2-vinyloxyethyl methacrylate [CH₃-CH(I)-OCH₂CH₂OCOC(CH₃) =CH₂] was found to initiate living polymerizations of VEs in the presence of iodine; the products were methacrylate-type macromers carrying a poly(VE) side chain with a narrow chain-length distribution.     

Living cationic polymerization of vinyl monomers: Mechanism and synthesis of new polymers

This paper reviews the recent progress in our research on the living cationic polymerization of vinyl compounds by the hydrogen iodide/iodine (HI/I₂) initiating system, with emphasis on its scope, mechanism, and applications to new polymer synthesis. The scope of the living cationic polymerization has been expanded to include vinyl ethers, propenyl ethers, unsaturated cyclic ethers, and styrene derivatives as monomers. The initiation/propagation mechanism was discussed on the basis of recent direct analysis on the living system by NMR and UV/visible spectroscopy. The proposed mechanism involves a quantitative formation of Hl-vinyl ether adduct [CH₃-CH(OR)-I; l] that is by itself incapable of initiating polymerization. In the presence of iodine, however, the CH-I bond of l is electrophilically activated by iodine and living propagation occurs via the insertion of vinyl ether to the activated CH-I bond. Such living polymerizations were found to proceed in not only nonpolar but polar solvents (CH₂Cl₂) as well. Quenching the living end with amines gave polymers capped with an amino group that in turn enabled us to determine the living end concentration. Applications of the HI/I₂-initiated living process to the synthesis of new bifunctional and block polymers were also described.     

Synthesis and ring-opening polymerization of optically pure mesogenic malolactonates

Optically pure malolactonate monomers containing biphenyl mesogenic groups with either an ethylene or a hexamethylene spacer were prepared from optically pure malic acid and polymerized with alkylaluminoxane catalysts to form a series of new chiral side chain liquid-crystalline polymers, which contained the chiral centres in the backbone. The mesogenic malolactonate monomers were determined to be optically pure by ¹H NMR spectroscopy of the β-lactone complexed with a chiral europium shift reagent. Both the methylaluminoxane and isobutylaluminoxane catalysts gave polymers having bimodal molecular weight distributions, the latter catalyst yielded a larger amount of the higher molecular weight fraction than the former. The polymers showed high optical rotations, high degrees of isotactic stereoregularity, and enantiotropic liquid-crystalline properties, all of which were influenced by the molecular weight distribution. Copolymers of malolactonate monomers with different spacers were also prepared and characterized.     

Living cationic polymerization of vinyl monomers: New initiators and functional polymer synthesis

This paper focuses on two recent topics in living cationic polymerization of vinyl monomers, i.e., (a) Development of new initiating systems: RCOOH/Lewis acid for vinyl ethers; CH₃CH(C₆H₅)Cl/SnCl₄/nBu₄NCl for styrene. (b) Synthesis of shape-controlled poly(vinyl ethers): Tri-armed star polymers; Multi-armed spherical polymers. For the RCOOH-based systems, a generalized concept of living cationic polymerization was discussed on the basis of the effects of the counteranions (or R) and Lewis acids (ZnCl₂ and EtAlCl₂). The CH₃CH(C₆H₅)Cl-based system permitted a truly living cationic polymerization of styrene. The tri- and multi-armed poly(vinyl ethers) included new amphiphilic polymers of unique topology, solubility, etc., all of which were prepared by living cationic polymerization.     

Living cationic polymerization of vinyl ethers

The cationic polymerization of vinyl ethers initiated by CH₃-CH(OR)(I) / R₄N⁺A⁻ (R = Alkyl, A⁻ = ClO₄⁻, BF₄⁻, PF₆⁻, I⁻, NO₃⁻) shows the characteristics of a living polymerization. The rate of polymerization is a function of the solvent polarity, the temperature, the type and concentration of the ammonium salt. The experimental data can be explained on the basis of the secondary salt effect overlapped by some dipol-dipol interactions of the chain end and the added salt. Functionalization of the chain end with thermolabile azo functions yields polymeric initiator which was applied for the synthesis of blockcopolymers. Vinyl ethers functionalized with furylacrylic ester groups were polymerized and crosslinked via [2+2] cycloaddition.     

Living cationic polymerization of vinylnaphthalene derivatives

The living cationic polymerization of 6‐tert‐butoxy‐2‐vinylnaphthalene (tBOVN), a vinylnaphthalene derivative with an electron‐donating group, was achieved with a TiCl₄/SnCl₄ combined initiating system in the presence of ethyl acetate as an added base at –30 °C. The absence of side reactions at low temperature was confirmed by ¹H NMR analysis of the resulting polymer. In contrast to this controlled reaction at –30 °C, reactions performed at higher temperature, such as 0 °C, frequently involved unwanted intramolecular or intermolecular Friedel–Crafts reactions of naphthalene rings due to the high electron density of these rings. The cationic polymerization of 6‐acetoxy‐2‐vinylnaphthalene, a derivative with an acetoxy group, was also controlled under similar conditions, but chain transfer reactions were not completely suppressed during the polymerization of 2‐vinylnaphthalene. The glass transition temperature (T_g) of the obtained poly(tBOVN) was 157 °C, a value higher by 94 °C than that of the corresponding styrene derivative. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4828–4834     

Synthesis and characterization of a double photochromic initiator for cationic polymerization

A novel cationic photoinitiator namely, 2-benzyl-2-(N,N-dimethyl-2-oxo-2-phenylethyl) ammonium hexafluoroantimonate-1-(4-morpholinophenyl)-butane-1-one (BDMPP⁺ ), carrying two photochromophoric groups was synthesized and characterized. Theoretical absorption characteristics of the salt were studied and compared with those obtained experimentally. Photoinitiation activity of this salt was demonstrated by polymerization of various monomers at λ = 350 nm. Upon irradiation by UV light, cationic species formed from homolytic dissociation followed by electron transfer or directly by heterolytic scission initiate cationic polymerization.     

Living cationic polymerization of p-alkoxystyrenes by free ionic species

In contrast to the common view, living cationic polymerization of p-methoxy- and p-t-butoxystyrenes proceeded in polar solvents such as EtNO₂/CH₂Cl₂ mixtures, and involvement of free ionic growing species therein was examined. For example, the two alkoxystyrenes were polymerized with the isobutyl vinyl ether-HCl adduct/ZnCl₂ initiating system at −15°C in such polar solvents as CH₂Cl₂ or EtNO₂/CH₂Cl₂ [1/1 (v/v)], as well as toluene. The number average molecular weight (M̄_n) of the polymers increased in direct proportion to the monomer conversion, even after sequential monomer addition, and the molecular weight distribution (MWD) stayed very narrow throughout the reaction. In addition, the M̄_n agreed with the calculated values, assuming that one adduct molecule generates one living polymer chain. In these polar media the addition of a common ion salt retarded the polymerization, indicating that dissociated ionic species are involved in the propagating reaction. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3694–3701, 1999