Polymer networks via cationic ring-opening polymerization

Three methods for the formation of polymer networks from bifunctionally growing polymers obtained by cationic ring-opening polymerization are described. The first method is based on the irreversible inter-molecular termination reaction of thietane polymerizations. Starting from bifunctionally living poly(THF) a new kind of polymer structure consisting of ABA block copolymers with cross-linked A-segments is obtained. The second method is the direct coupling of active species with primary amines or ammonia. The third method consists in transformation of the living end groups of poly(THF) into triethoxysilane end groups, followed by cross-linking by addition of water and a trace of acid.     

Acidic polysaccharide mimics via ring-opening metathesis polymerization

An efficient and general synthetic strategy for the preparation of high-molecular-weight hydrophilic polymers bearing both carboxylic acid and hydroxyl pendant groups is described. Specifically, poly(5,6-dihydroxyoxanorbornane carboxylic acid) with molecular weight ranging from ～100?000 to 5?000?000 g/mol was prepared by ring-opening metathesis polymerization of methyl 5-oxanorbornene-2-carboxylate in the presence of Grubbs catalyst II and subsequently modified to tune the hydrophobic/hydrophilic properties by the introduction of either hydroxyl or carboxylic acid functionalities. These polymers mimic the natural acidic polysaccharide alginate and form hydrogels with polylysine. These polymers belong to a class of carbohydrate-like polymers, which are of interest for investigating the relationships between chemical structure and rheological properties as well as for providing new synthetic polysaccharide substitutes for applications in the biotechnology and pharmaceutical industries.     

High-density doxorubicin-conjugated polymeric nanoparticles via ring-opening metathesis polymerization

High-density doxorubicin-conjugated polymeric nanoparticles are prepared via ring-opening metathesis polymerization and sustained release of nearly 50% of the anticancer agent is observed after 24 h in mildly acidic aqueous solution.     

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.

     

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.     

Alkene insertion reactivity of a o-carboranyl-substituted 9-borafluorene

The synthesis of 9-borafluorene with an electron-withdrawing o-carboranyl substituent and its reactions with a series of alkenes are described. The o-carboranyl substituent is bonded via one of the cluster carbon atoms to the boron atom of the 9-borafluorene moiety. In all cases, the reactions afford partly saturated analogs of borepins (i.e. 6,7-dihydroborepins) by unprecedented alkene insertion into the endocyclic B–C bond of the borole ring. Comparative studies with 9-bromo-9-borafluorene illustrate the superior insertion reactivity of the carboranyl-substituted derivative. A suite of experimental and computational techniques disclose the unique properties of the 9-borafluorene and provide insight into how the 9-carboranyl substituent affects its chemical reactivity.

A 9-carboranyl-substituted 9-borafluorene is reported, which is capable of undergoing efficient ring expansion to 6,7-dihydroborepins by a previously unknown alkene insertion.     

Synthesis of hybrid block copolymers via integrated ring-opening metathesis polymerization and polymerization of NCA

Linear hybrid block copolymers with well controlled molecular weights and narrow polydispersities were synthesized via ring-opening metathesis polymerization (ROMP) followed by ring-opening polymerization of amino acid N-carboxyanhydrides.     

Shape-tunable polymer nodules grown from liposomes via ring-opening metathesis polymerization

A new inisurf (acting as surfactant and initiator) molecule for ring-opening metathesis polymerization (ROMP) was synthesized and used in aqueous solution in order to control the size and shape of polymer nodules grown from liposomes. Nodules were observed to grow in size with conversion of monomer, and depending on the monomer used, they adopted either a spherical or comet-like shape. Here, we investigate polymer production from a liposome surface. We use a hydrophobic derivative of the Grubbs catalyst positioned at the liposome surface to allow for ROMP of monomers dissolved in the aqueous outer phase. We obtain nodules of polymer that can grow up to tens of micrometers, unveiling new efficient possibilities of polymerization from a membrane in an aqueous solution.     

Facile synthesis of fluorescent dye labeled biocompatible polymers via immortal ring-opening polymerization

A highly efficient strategy for synthesizing "clean" fluorescent dye-labeled biocompatible polymers was established by employing a rare-earth metal catalyst via immortal ROP.     

Lipase-catalyzed ring-opening polymerization of lactide

The six-membered lactide was polymerized using the ring-opening polymerization with lipase as a catalyst at a temperature between 80 and 130°C in bulk to yield the corresponding polylactide with weight-average molecular weights of up to 126000. The most preferable conditions with respect to the molecular weight of the polylactide are the bulk polymerization using lipase PS at a temperature of 100°C. The D ,L -lactide gave higher molecular weight compared to the D ,D - and L ,L -lactide.     

Preparation of crystalline and solvent resistant polycarbonates via ring-opening polymerization of cyclic oligomers

A route to solvent-resistant polycarbonates which may also be crystalline has been achieved through the intermediacy of cyclic oligomeric carbonates. Mixed oligomeric cyclics of a variety of bisphenols were prepared via the carefully controlled hydrolysis-condensation of their bischloroformates. Following ring-opening polymerization, the polymers were evaluated for potential solvent resistance. Copolymers of hydroquinone and bisphenol A containing ratios of hydroquinone to BPA between 50/50 and 70/30 were found to be insoluble in CH₂Cl₂, tetrahydrofuran, and CHCl₃, as well as other organic solvents. Polymers containing ratios of hydroquinone/BPA greater than 60/40 show a crystalline melting point by DSC at about 310°C and a glass transition temperature at 154°C.     

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.     

Stereoselectivity of the cyclooctene ring-opening polymerization

The stereoselectivity of the cyclooctene ring-opening polymerization in the presence of the tungsten hexachloride–tetraisobutylalumoxane (TBAO) catalyst system has been studied. The reaction conditions have been established in which the polymer chain isomerization is reduced to a minimum. Under these conditions the stereoselectivity is determined by an individual act of the monomer molecule insertion. The dependence of stereoselectivity on monomer concentration, number of active sites, and temperature has been investigated. The results obtained suggest that the cyclooctene polymerization is selective in favor of the geometrical configuration of the starting cycloolefin (the “retention” effect) by analogy with the previously reported metathesis of acyclic olefins. Considerations which support the idea that cis and trans units are reproduced by the same type of active sites are discussed. The experimental evidence indicates that in this case the stereoselectivity is determined by the difference in kinetics of two reaction paths of the monomer molecule insertion that lead to the formation of cis and trans units.     

Regio-controlled ring-opening polymerization of perfluoroalkyl-substituted epoxides

Highly fluorinated epoxides were polymerized under mild conditions regioregularly, which gave easy access to isotactic polymers by using optically pure epoxides.     

Anionic ring-opening polymerization of silacyclobutane derivatives

Anionic polymerizations of 1,1-dimethylsilacyclobutane, 1,1-diethylsilacyclobutane and 1-methyl-1-phenylsilacyclobutane were investigated. Addition of 5 mol % of butyllithium to a solution of 1,1-dimethylsilacyclobutane in THF-hexane (1 : 1) at −48°C provided poly(1,1-dimethylsilabutane) in 99% yield. M_n and M_w/M_n of the obtained polymer were 2400 and 1.10. This polymerization proceeded with a living nature. M_n increased in proportion as the yield of polymer increased. Addition of the second fresh feed of the monomer to the reaction mixture restarted polymerization of the second monomer at the same rate as in the initial stage. Addition of styrene to the living poly(1,1-dimethylsilabutane) provided a poly(1,1-dimethylsilabutane-b-styrene) block copolymer. It was also found that a polymerization of 1,1-diethylsilacyclobutane in THF-hexane at −48°C showed a living nature. In contrast, a polymerization of 1-methyl-1-phenylsilacyclobutane in THF at −78°C did not show a living nature. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3207–3216, 1997     

Synthesis of degradable molecular brushes via a combination of ring-opening polymerization and click chemistry

Acid-degradable molecular brushes with polycarbonate backbone and densely grafted side chains (∼1.9 SCs per backbone repeating unit) were synthesized for the first time using the grafting-onto method. Extremely efficient copper-catalyzed azide-alkyne cycloaddition click reactions between the polycarbonate backbone containing two pendant azido groups per backbone unit and alkynyl-terminated poly (methyl acrylate) (ay-PMA₇₂, average degree of polymerization DP = 72) SCs were demonstrated to finish in 10 min with a quantitative conversion of the azido groups. Similar grafting efficiencies were also achieved when using alkynyl-terminated polystyrene (ay-PS), poly(ethylene oxide) (ay-PEO), and poly (t-butyl acrylate)-b-polystyrene (ay-PtBA-b-PS) to successfully prepare molecular brushes with high grafting density (>1.8 SCs per backbone repeating unit). Under acidic condition, the polycarbonate backbones were completely degradable and the final degraded product of the molecular brushes was a linear polymer chain with molecular weight two times of the SCs. When a mixture of hydrophobic ay-PS and hydrophilic ay-PEO chains was used, amphiphilic heterobrushes PC-g-(PS-co-PEO) were synthesized, which could self-assemble into micelles or vesicles in selective solvents, depending on the ratio of the two SCs in the brush. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 239–248