Mechanism aspects of th e ring-opening polymerization of the episulfides compared to epoxides

The cis- and trans-2-butene episulfides polymerize with cationic catalyts differently than reported for the corresponding oxides. Where the cis-oxide gave amorphous disyndiotactic polymer, the cis-sulfide gives crystalline racemic diisotactic polymer since this polymer could be asymmetrically synthesized in optically active form. Also the same crystalline polymer was obtained with coordination catalysts. Where the trans-oxide gave only crystalline, meso-diisotactic polymer, the trans-sulfide gives mainly amorphous polymer which, in one case, did slowly crystallize. The difference between the trans forms appears due to the longer C? S bond which lowers steric hindrance and thus isomer selection in the attack of episulfide on the growing sulfonium ion to give less steroregular polymer. The difference in the cis forms may result from the sulfur atom in the last chain unit coordinating with the counterion. The greater hindrance around oxygen in the comparable oxide polymers may prevent the same mechanism from being utilized. The cationic polymerization of isobutylene sulfide gives both crystalline and amorphous polymer. NMR evidence indicates that the amorphous polymer results from substantial head-to-head, tail-to-tail polymerization, along with the expected head-to-tail polymerization. The same phenomenon occurs, but to a lesser extent, in cationic isobutylene oxide polymerizations. The preparation and properties of high molecular weight, head-to-tail isobutylene oxide and sulfide polymers from R₂Mg-NH₃ coordination catalysis are described.     

The use of platinum and rhodium catalysts for the preparation and cationic ring-opening polymerization of silicon-containing epoxides

Regioselective platinum and rhodium catalysts were employed for the preparation of novel silicon-containing polyethers. First, silicon-containing monomers bearing cycloaliphatic epoxy groups were prepared by a rhodium-catalyzed regioselective hydrosilation. Then these monomers were polymerized using a platinum-catalyzed, cationic, ring-opening polymerization to give the linear polyethers. The obtained polyethers were compared with polymers prepared by photoinitiated cationic ring-opening polymerization using onium salt photoinitiators and found to be identical. © 1993 John Wiley & Sons, Inc.     

Influence of multidentate N-donor ligands on highly electrophilic zinc initiator for the ring-opening polymerization of epoxides

A set of multidentate ligands have been synthesized and used to stabilize the putative highly electrophilic zinc species initiating ring-opening polymerization (ROP) of cyclohexene oxide (CHO) and propylene oxide (PO). Reaction of the bidentate C₂-chiral bis(oxazoline) ligand (^R2,R3BOX: R₂ = (4S)-tBu, R₃ = H (a); R₂ = (4S)-Ph, R₃ = H (b); R₂ = (4R)-Ph, R₃ = (5S)-Ph (c)) with Zn(R₁)₂ (R₁ = Et (1), Me (2)) led to the heteroleptic three-coordinate complexes (^R2,R3BOX)ZnR₁, 1a-c and 2a, which were isolated in 92-96% yield. Next, two pyridinyl-functionalized N-heterocyclic carbene (NHC) ligands have been designed and synthesized: the 1,3-bis(2-pyridylmethyl)imidazolinium salt (d) and the protected NHC adduct 2-(2,3,4,5,6-pentafluorophenyl)-1,3-bis(2-pyridylmethyl)imidazolidine (e). The reaction of ligands d and e with ZnEt₂ led directly to the formation of (NHC)ZnEt(Cl) 3d complex with ethane elimination and the adduct (NHC-C₆F₅(H))ZnEt₂4e, respectively, in high yield. In situ combinations of selected complexes 1a-c, 3d and 4e with B(C₆F₅)₃ (1 or 2 equivalents) give active systems for ROP, with high productivity (3.3-5.9 10⁶ g_polym. mol_Zn⁻¹ h⁻¹) and high molecular weight (M_n up to 132 10³ g mol⁻¹) for CHO polymerization. Although the in situ B(C₆F₅)₃-activated zinc species were not isolated, the sterically demanding BOX ligands (1c > 1b > 1a) and functionalized NHC ligands seem to enhance the stability of highly electrophilic zinc complexes over ligand redistribution, allowing a better control of the cationic ROP as reflected particularly for 3d and 4e complexes by their respective efficiency (42-88%).     

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.     

Straightforward synthesis of thiodisaccharides by ring-opening of sugar epoxides

3,4-Anhydro hexopyranosides have been prepared by diastereoselective epoxidation of derivatives of 2-propyl 3,4-dideoxy-alpha-D-erythro-hex-3-enopyranoside (5), selectively protected at HO-2 and HO-6. The allylic group at C-2, in 5 and derivatives, plays a critical role in the facial selectivity of the epoxidation reaction. Thus, the free HO-2 in 3 (the 6-O-acetyl derivative of 5) directs the attack of m-chloroperbenzoic acid from the more hindered alpha face of the molecule to give 2-propyl 6-O-acetyl-3,4-anhydro-alpha-D-allopyranoside (7) accompanied by the beta epoxide 6 as a very minor product. Reverse diastereoselectivity has been obtained when the HO-2 in 3 was substituted by a bulky tert-butyldimethylsilyl (TBS) group. In this case, the major isomer was the 2-O-TBS derivative of 6 (alpha-D-galacto configuration). The ring-opening of sugar epoxides by nucleophilic per-O-acetyl-1-thio-beta-D-glucopyranose (11) was employed as a convenient approach to the synthesis of (1-->3)- and (1-->4)-thiodisaccharides. For example, ring-opening of the oxirane 7 by 11 led to the expected regioisomeric per-O-acetyl thiodisaccharides beta-D-Glc-S-(1-->3)-4-thio-alpha-D-Glc-O-iPr (12) and beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Gul-O-iPr (13). Regioselectivity in the construction of the (1-->4)-thioglycosidic linkage could be achieved by hindering C-3 of the 3,4-anhydro sugar with a bulky silyloxy group at the vicinal C-2. For instance, coupling of the 2-O-TBS derivative of 7 with 11 led regioselectively to the protected thiodisaccharide beta-D-Glc-S-(1-->4)-4-thio-alpha-D-Glc-O-iPr (27). The utility of the approach was demonstrated through the synthesis of sulfur-linked analogues of naturally occurring (laminarabiose and cellobiose) and non-natural disaccharides (i.e., beta-D-Glc-(1-->4)-alpha-D-Gul).     

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.     

Copper(II) tetrafluoroborate-catalyzed ring-opening of epoxides by amines

Copper(II) tetrafluoroborate catalyzes efficiently the selective opening of epoxides by amines leading to the synthesis of β-amino alcohols. The reaction works well with aromatic and aliphatic amines in high yields under solvent-free conditions.     

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     

Tributylphosphine-catalyzed ring-opening reaction of epoxides and aziridines with acetic anhydride

Reactions of aziridines and epoxides with acetic anhydride catalyzed by an organophosphine provided the corresponding esters of β-amino alcohols and 1,2-diols in high yields, respectively.     

The molecular weight limiting step in the ring-opening polymerization of 1,1-disubstituted epoxides catalyzed by bis(diisobutylaluminum)oxide

The ring-opening polymerization of 1,2-epoxy-4,4,4-trifluoro-2-methylbutane ( 2 ) and its unfluorinated analogue, 1,2-epoxy-2-methylbutane ( 3 ), gives the expected polyethers when catalyzed by bis(diisobutylaluminum)oxide. However, the molecular weights are low because of a competing deprotonation/ring-opening of the epoxide that produces all three possible alkene end groups and their stereoisomers. With 2 , deprotonation occurs mainly at the methyl group to give the terminal double bond and only minor amounts of the internal double bond. However, 3 gives the opposite regiochemistry. A model is proposed where this reaction occurs within an aluminum(polyether oxide) bound to the least hindered face of the epoxide ring. In this model, the different regiochemistries originate from a concerted and highly directed deprotonation/ring-opening of 2 versus a step-wise deprotonation of 3 through its ring-opened, tertiary-cation.     

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.     

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.     

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.     

Organocatalytic living ring-opening polymerization of cyclic carbosiloxanes

[reaction: see text] An organocatalytic route to narrowly dispersed poly(carbosiloxanes) of predictable molecular weight and end group fidelity is described. N-Heterocyclic carbenes (NHC) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyze the ring opening of cyclic carbosiloxanes. The pK(b) of the catalyst is important in preventing adverse transetherification reactions and obtaining well-defined polymers. Mechanistic studies indicate that hydrogen bonding to TBD or the NHC activates alcohols or silanols for ring-opening reactions.     

Novel ring-opening polymerization of lactide by lipase

Six-membered D,L-, L,L- and D,D-lactides were polymerized by lipase over a temperature range of 80 to 130 °C to yield the polylactide with a molecular weight (M_w) of greater than 270000. Among the lipases tested, lipase PS gave the greatest molecular weight of polylactide. The polymerization of D,L-lactide by lipase was better than that of L,L- and D,D-lactides. The polymerization of lactide by lipase showed the characteristic features, such as induction period for the initiation of polymerization, formation of oligomer and subsequent formation of high molecular weight polylactide, which may imply the characteristic polymerization by lipase. Immobilization of lipase on celite significantly enhanced the polymerization of lactide particularly with respect to the low concentration of the enzyme and the M_w of the resultant polymer. It was found that there is no clear relationship between enzymatic polymerizability and enzymatic degradability with respect to the enzyme origin and the stereochemistry of lactide.     

Synthesis of degradable copolymers by ring-opening polymerization

Hydrolysable copolymers made from different cyclic monomers have been studied. The monomers involved are 1,5-dioxepan-2-one (DXO), L-dilactide, 1,3-dioxan-2-one (TMC), oxepan-2,7-dione (AA) and oxepan-2-one (ϵ-CL). The hydrolysis of the DXO/L-dilactide copolymer showed great differences in degradation rate depending on composition. A statistical copolymer made from TMC and ϵ-CL was amorphous with a glass transition temperature of −48°C. TMC and AA could form a blockcopolymer with n-BuLi as initiator in toluene, 0°C.     

Cationic ring-opening polymerization of bicyclic amide acetals

Various bicyclic amide acetals were synthesized from the cycloaddition reactions of 2-substituted-2-oxazolines with styrene oxide. Ring-opening polymerization of the bicyclic amide acetals occurred upon heating in the presence of methyl tosylate. Characterization of the bicyclic amide acetals and their polymers was accomplished by NMR and elemental analysis. Vapor pressure osmometry showed the highest polymer molecular weight was only 2,400. The mechanisms for cycloaddition and polymerization are discussed. © 1994 John Wiley & Sons, Inc.     

Redox control of a ring-opening polymerization catalyst

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and M?ssbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.