Block copolymers of polyacetals

The polymerization of 1,3-dioxolane was carried out using trifluoromethanesulfonic acid (triflic acid) as catalyst, in the presence of an oligomeric diol, α-hydro-ω-hydroxypoly(oxyethylene) (PEOG) or α-hydro-ω-hydroxypoly(oxytetramethylene) (poly-THF-diol). When PEOG was used, a linear increase of the number average molecular weight versus conversion was observed. A triblock copolymer was obtained as well as small cycles in their usual equilibrium concentration. When poly-THF-diol was used, fast transacetalization provoked the “polycondensation” of the diol through acetal linkages at the very onset of the reaction, leading to a very inhomogeneous mixture. After equilibrium was reached, through reactions involving all the various species present, a triblock copolymer was also obtained. In conclusion, this method is well suited to prepare α,ω-diol block copolymers in which the initial oligomeric diol constitutes the center block, while the outside blocks are made of polyacetal.     

Unzipping and scrambling reaction-induced sequence control of copolymer chains via temperature changes during cationic ring-opening copolymerization of cyclic acetals and cyclic esters

A temperature change-dependent sequence transformation of copolymer chains was demonstrated by a method based on tandem depolymerization and transacetalization reactions during the cationic ring-opening copolymerization of cyclic acetals and cyclic esters. In this study, the position of polymerization-depolymerization equilibrium was controlled by the reaction temperature rather than by the decrease in monomer concentration under vacuum conditions, as in our previous study. First, the conditions for efficient copolymerization were optimized, with a particular focus on the structures of cyclic acetals and cyclic esters. Subsequently, sequence transformation induced by temperature change was examined during the copolymerization of 2-methyl-1,3-dioxepane (generated in situ from 4-hydroxybutyl vinyl ether) and δ-valerolactone using EtSO₃H. The homosequence length of cyclic acetals decreased during depolymerization (unzipping) at the oxonium chain ends upon increasing the temperature from 30 to 90 °C, while transacetalization (scrambling) of the main chain transferred midchain cyclic acetal homosequences to the oxonium chain ends. As a result of the cycle of unzipping and scrambling reactions, an alternating-like copolymer was obtained. Interestingly, the possibility of reversible sequence transformation upon heating and cooling was also demonstrated.     

Unexpected highly chemoselective deprotection of the acetals from aldehydes and not ketones: TESOTf-2,6-lutidine combination

Acetal functions are recognized as good protecting groups of carbonyl groups. Although many deprotecting methods of acetals to carbonyl functions have already been developed, there is no methodology which can deprotect acetals in the presence of ketals because the usual acidic or radical reactions occur more easily via the more stable cationic or radical intermediates from the ketals. On the other hand, this new method can proceed in a reverse manner to that described in previous reports. That is, the method can deprotect aliphatic acetals in the presence of ketals. The reaction condition is common for silylation, i.e., the TESOTf-2,6-lutidine combinations. Although the TMSOTf-2,6-lutidine combination can also deprotect acetals, it lacks chemoselectivity in deprotection of the acetals from aldehydes and ketones. The treatment of acetals with TESOTf and 2,6-lutidine in CH2Cl2 followed by a H2O workup gave the corresponding aldehydes. Of course, the compounds, which have both acetal and hydroxyl functions afforded the compounds obtained by the usual silylation of an alcohol and deprotection of an acetal without any problem. However, deprotection of the ketals from ketones was not observed during the conversion reaction of acetals from aldehydes. This chemoselectivity was confirmed in the reactions of the compounds that have the acetal and ketal in the same molecule. In both cases, the acetal functions were deprotected to give aldehydes with intact ketals. Furthermore, under the conditions described here, many functional groups such as methoxy, acetoxy, allyl alcohol, and silyloxy ether are intact. This method is very mild and available for many compounds.     

Ketene acetal monomers: Synthesis and characterization

A new, facile, and high-yield synthesis of ketene acetals derived from readily available and inexpensive starting materials has been developed. For example, an α,β-unsaturated aldehyde can be condensed with an alkane diol to afford a 2-vinyl substituted cyclic acetal. This latter compound can be converted to the desired cyclic ketene acetal by isomerization of the double bond in the presence of tris(triphenylphosphine)ruthenium(II) dichloride. Good to excellent yields of cyclic ketene acetals were obtained employing this method. The novel monomers were fully characterized by IR, NMR, and by elemental analysis. © 1996 John Wiley & Sons, Inc.     

Copolymerization of 2-methylene-4-phenyl-1,3-dioxolane with electrophilic vinyl monomers through zwitterionic mechanism

Spontaneous copolymerization of cyclic ketene acetal, 2-methylene-4-phenyl-1,3-dioxolane ( I ) with common electrophilic vinyl monomers, such as methyl α-cyanoacrylate (MCA), acrylonitrile (AN), and methyl methacrylate (MMA) were investigated to further explore zwitterion polymerization method with cyclic ketene acetals. In the reaction of I with MCA and AN, spontaneous copolymerization took place at ambient temperature. The copolymers of I with MCA gave low molecular weight polymers, but copolymers obtained with I and AN were high molecular weight polymers. In the reaction of I and MMA, high molecular weight copolymer was obtained only at temperatures above 80°C. Thus, obtained polymers were not the alternating copolymers and possessed high I content in all the cases. From the above results, macrozwitterionic mechanism was suggested as discussed.     

An Efficient Photoinduced Deprotection of Aromatic Acetals and Ketals

A novel type of photodeprotection reaction of simple aromatic acetals and ketals is described. The deprotection is highly efficient under optimized conditions. The aromatic ring confers the photoreactivity to the compounds. The efficiency of the process depends on the structure of the acetal moiety. The common minor side reaction, the photooxidation, becomes the major reaction pathway in the cases of cyclic acetals. The use of photon as only reagent makes this procedure especially attractive for acetal deprotection.     

Cyclic Acetal-Photosensitized Polymerization. XII. Photopolymerizations of Styrene and Methyl Methacrylate in the Presence of Polycyclic Acetals

Styrene (St) and methyl methacrylate (MMA) were photopolymerized in the presence of poly-2-vinyl-1,3-dioxolane (PVDO), poly-2-vinyl-4-hydroxymethyl-1,3-dioxolane (PVHDO), or the terpolymer of vinyl formal/vinyl acetate/vinyl alcohol (PVFAcA) at 30 or 40°C. The ability to accelerate the photopolymerization increased in the order PVFAcA < PVHDO < PVDO; the ability of pendent cyclic acetal was larger than that of cyclic acetals which are not pendent. Moreover, the promoting ability per cyclic acetal increased with the increase in the number of cyclic acetal cyclic acetal group in the molecule.     

Synthesis and reactions of telechelic polyacetals

Telechelic polyacetals have been synthesized by endcapping of bifunctionally living polyacetals with tertiary amines. Bifunctionally living poly-(1, 3-dioxolane) (poly-DXL) was obtained by polymerization of the cyclic acetal initiated with terephthaloyl-bis-triflate. Attempts to prepare well defined bifunctionally growing poly-DXL with trifluoro-methane sulfonic acid anhydride failed. Low molecular weight dialkyl formals containing functional groups were used as transfer agents for the polymerization of cyclic acetals to control the molecular weight of the polymer and to introduce functional endgroups. Polytelechelic hydroxy-terminated poly-DXL was synthesized by copolymerization of DXL and glycidol. The latter monomer reacts as comonomer through its epoxy ring and as transfer agent by its hydroxyl group.     

Cyclic Acetal-Photosensitized Polymerization. XI. Photoirradiations onto Polycyclic Acetals

Photoirradiations onto polycyclic acetals, i. e., polymers containing cyclic acetal groups in the molecule, were carried out at 30 or 40°C. The terpolymer of vinyl formal/vinyl acetate/vinyl alcohol (PVFAcA) was decomposed by means of irradiation, while poly-2-vinyl-1,3-dioxolane (PVDO) and poly-2-vinyl-4-hydroxy-methyl-1,3-dioxolane (PVHDO) were crosslinked. These results indicate the possibility of control of the decomposition or the crosslinking of polymer.     

Indirect electrochemical α-methoxylation of aliphatic ethers and acetals - reactivity and regioselectivity of the anodic oxidation using tris(2,4-dibromophenyl)amine as redox catalyst

The technically important α-methoxylation of aliphatic ethers and acetals to form mixed acetals respectively aldehydes or ortho-esters can be performed electrochemically at low potentials in methanol solution using an undivided cell and tris(2,4-dibromophenyl)amine as redox catalyst. The regioselectivity is usually considerably higher as compared with direct electrolysis in the absence of a catalyst. Especially valuable is the method for the regioselective methoxylation of secondary carbon atoms in presence of primary or tertiary ones and of the acetal carbon in 1,3-dioxolanes. The redox catalyst is stable under the reaction conditions so that more than thousand turnovers could be obtained.     

A new design of cleavable acetal‐containing amphiphilic block copolymers triggered by light

We report a new design of photolabile acetal‐containing amphiphilic block copolymers. Acetals as protecting groups for carbonyls or diols can be hydrolyzed under acidic condition but very stable with respect to hydrolysis at pH > 7. When combining light‐capturing chromophores with acetals, the hydrolysis of acetals can be activated by light to design dual responsive acetal‐containing polymers. Using acetalization reaction of 2,3‐dihydroxypropyl methacrylate with benzaldehyde derivatives, two new acetal‐containing photolyzable monomers have been designed. Comparable to commonly used photolabile monomers containing nitrobenzyl esters, the two acetal‐containing monomers are easy to polymerize using atom transfer radical polymerization with excellent molecular weight and dispersity control. We studied the cleavage kinetics and mechanism of acetal groups in both monomers and polyethylene oxide (PEO)‐containing amphiphilic block copolymers using ¹H NMR and UV–vis spectroscopy. o‐Nitrobenzaldehyde acetal showed a Norrish Type II rearrangement to form benzoic ester; while, 2,5‐dimethoxy benzaldehyde acetal was photolabile to completely release 2,3‐dihydroxypropyl methacrylate. The photocleavage of acetals is a zero‐order reaction in regardless of molecular states of acetals; while, the acid‐cleavage of acetals proves to be a first‐order kinetics and the cleavage becomes much slower for polymers. The self‐assembly of acetal‐containing amphiphilic block copolymers and the acid‐/light‐controlled dissociation of their vesicles have been investigated. We demonstrate that those acetal‐containing polymers are potentially useful as smart drug delivery systems where the release kinetics of payloads is tunable using light and pH as triggers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1815–1824     

Influence of the chain composition on the thermodynamic properties of binary and ternary polymer solutions

Thermodynamic properties of binary and ternary polymer solutions (one or two uncharged polymers in one solvent) were studied. Poly(vinyl pyrrolidone) (PVP), fully hydrolyzed poly(vinyl alcohol) (PVA) homopolymers, and water-soluble poly(vinyl alcohol-co-vinyl acetal, -vinyl propional, and -vinyl butiral) copolymers with various acetal content and chain structure, respectively, were used in the experiments. The hydrophilic/hydrophobic character of the PVA-based macromolecules and their compatibility with the PVP homopolymer were systematically regulated by changing the chemical structure of the copolymers (acetal content and/or length of side chains). The water activities in binary and ternary solutions of the chemically different polymers were determined by a gel-deswelling method developed here for ternary solutions. On the basis of the Flory-Huggins theory, the relevant solvent-segment and segment-segment pair interaction parameters (chi) have been calculated. The chi12 segment-solvent interaction parameters proved to be sensitive indicators for changes in the chemical structure of the copolymers. With increase of either the acetal content or the length of side chains in the copolymer, chi12 approached the value characteristic of a theta condition. No significant differences could be revealed in the segment-segment interaction parameters obtained for the PVP-copolymer mixtures with various acetal derivatives, when the chi12 and chi13 interaction parameters determined in binary solutions were used in the calculations for chi23. Determination of the parameters chi1,23 as suggested by Panayiotou, however, showed that increasing the acetal content or the length of the hydrophobic side chains in the copolymer resulted in a reduction in the interaction between the PVA "acetals" and PVP molecules.     

Atomic force microscopy imaging and dilute solution properties of cyclic and linear polystyrene combs

Large macrocyclic poly(chloroethyl vinyl ether)s (PCEVE)s of controlled ring size and narrow distribution were synthesized by a ring-closure process involving the intramolecular formation of acetal linkages between the two external blocks of linear ABC triblock precursors prepared by living cationic polymerization. The corresponding shape-persistent ring P(CEVE-g-PS) combs having macrocyclic poly(chloroethyl vinyl ether) backbones and polystyrene side chains were then synthesized by a "grafting onto" technique and characterized by size exclusion chromatography (SEC) analysis and atomic force microscopy (AFM) imaging of isolated molecules. Quantitative hydrolysis of the acetal linkages of the macrocyclic PCEVE backbone in acidic conditions yields the linear poly(chloroethyl vinyl ether)-g-polystyrene) homologue and allows a direct comparison of the characteristics and dimensions of cyclic and linear comb architecture. The influence of the chain architecture and PS graft dimensions on the dilute tetrahydrofuran (THF) solution properties, radius of gyration, and hydrodynamic radius of the comb copolymers is also studied and compared to data reported for linear and cyclic polystyrene chains.     

13C NMR study of the structures of some acyclic and cyclic ketene acetals

A ¹³C NMR study of the spatial structures of some acyclic and cyclic ketene acetals (for example, ketene dimethyl acetal and 2-methylene-1,3-dioxolane) has been carried out. The conclusions obtained are based on observation of the effect of structural changes on the ¹³C NMR chemical shift of the β carbon on the ketene moity. Since the extent of p-π conjugation and hence the ¹³C chemical shift of this carbon depend on the spatial orientation of the alkoxy groups about the O-C(sp²) bonds, the shift concerned may be used as a measure of the planarity of the system. The most stable retamers of ketene dimethyl acetal are s-cis,s-cis (planar) and s-cis,gauche (slightly nonplanar), in the order of decreasing stability. For ketene dialkyl acetals, the relative stability of the planar s-cis,s-cis form decreases with increasing bulkiness of the alkyl groups, but at least for primary and secondary alkyl groups, the s-cis,s-cis rotamer appears to be the most favored species. The conformations of 5- to 8-membered cyclic ketene acetals are discussed and compared with those of the corresponding cyclic vinyl ethers and hydrocarbons.     

Tetrabutylammonium tribromide (TBATB) as an efficient generator of HBr for an efficient chemoselective reagent for acetalization of carbonyl compounds

Acyclic and cyclic acetals of various carbonyl compounds were obtained in excellent yields under a mild reaction condition in the presence of trialkyl orthoformate and a catalytic amount of tetrabutylammonium tribromide (TBATB) in absolute alcohol. Chemoselective acetalization of an aldehyde in the presence of ketone, unsymmetrical acetal formation, shorter reaction times, mild reaction conditions, the stability of acid-sensitive protecting groups, high efficiencies, facile isolation of the desired products, and the catalytic nature of the reagent make the present methodology a practical alternative.     

Syntheses and properties of ABA,CBA, and CBC triblock copolymers based thermoplastic elastomers with glassy (A), elastomeric (B), and crystalline (C) blocks

In this contribution, we describe the syntheses, characterization, and properties of ABA, CBA and CBC triblock copolymers with glassy (A), elastomeric (B), and crystalline (C) blocks. These three hard-soft-hard triblock copolymers were prepared via living ring-opening metathesis copolymerization by use of Grubbs third generation catalyst through one-pot sequential monomer addition and subsequent hydrogenation. These hard-soft-hard triblock copolymers based thermoplastic elastomers have been analyzed by proton NMR, differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), IR, and transmission electron microscopy (TEM). The mechanical properties of these triblock copolymers were also measured by monotonic and step cyclic tensile test. Compared with the ABA triblock copolymer, the CBC triblock copolymer containing highly crystalline hard end-blocks shows enhanced tensile strength and the best elastic recovery of 90.8%. The hybrid CBA triblock copolymer displays much improved elongation, which is almost twice as long as the other two TPEs, and excellent elastic recovery of 87.0%.     

Reaction of Acetals with Various Carbon Nucleophiles under Non‐Acidic Conditions: C?C Bond Formation via a Pyridinium‐Type Salt

Mild substitution reactions of acetals with carbon nucleophiles via the pyridinium‐type salts generated by the treatment of acetals with TESOTf‐2,4,6‐collidine or 2,2′‐bipyridyl have been developed. Various carbon nucleophiles, such as organocuprates, silyl enol ethers, enamines, etc., reacted with the pyridinium‐type salts to give the corresponding substituted products in good yields. The reactions proceeded under very mild conditions (non‐acidic conditions) and thus acid‐sensitive functional groups can be tolerated during the reaction. In addition, only an acetal can form the pyridinium‐type salt and react with nucleophiles in the presence of a ketal. This unusual selectivity is in contrast to general methods conducted under acidic conditions.     

A Precautionary Note Regarding Derivatization of Secondary Amines with Dialkylacetals of Formamide Other Than the Diethyl Derivative

Abstract

We have extended our earlier work on alkylation of secondary amino compounds with dimethylformamide diethyl acetal to other alkyl acetals, such as dimethyl and dipropyl acetal. All the alkylating DMT-dialkyl acetal reagents tested gave only N-ethyl derivatives of the secondary amines. The reagent purity was tested by preparing the esters of carboxylic acids, where the appropriate alkyl (methyl or propyl) ester is obtained. It is concluded that all dimethylformamide dialkyl acetals yield a single N-ethyl derivative of secondary amine. The reaction mechanism is not understood.     

(Triisopropylsilyl)acetaldehyde acetal as a novel protective group for 1,2-diols

(Triisopropylsilyl)acetaldehyde dimethyl acetal (TIPS-ADMA) was synthesized from chlorotriisopropylsilane in three steps. Cyclic and acyclic 1,2-diols can be transformed to (triisopropylsilyl)ethylidene acetals (TIPS-AA). Removal of the acetal by LiBF₄ regenerates the starting diol in excellent yield even in the presence of an acetonide of 1,2-diol. The TIPS-AA group can survive under the deprotection conditions of the acetonide in acetic acid at 80 °C. Selective protection of 2,3- and 4,6-diols for O-methyl d-mannoside with TIPS-ADMA and selective deprotection of the acetals have been achieved.     

The first synthesis of cyclopropanone acetals from the reaction of Fischer carbene complexes with ketene acetals

The reaction of iso-propoxy stabilized Fischer carbene complexes with ketene acetals gives moderate to excellent yields of cyclopropanone acetals when carried out under a carbon monoxide atmosphere. This is in contrast to the known reaction of methoxy substituted complexes which give cyclic ortho esters under the same conditions. A mechanism is proposed which involves a branch point between the two products as the zwitterionic intermediate resulting from nucleophilic addition of the ketene acetal to the carbene carbon. A 1,3-migration of the methoxyl group to the cationic center leads to the ortho ester and a ring closure by backside attack leads to the cyclopropanone acetal. A double-labeling experiment shows that the 1,3-migration occurs by an intramolecular process that is proposed to involve a bridging oxonium ion. The effect of the isopropoxy group is thus interpreted to be to sterically hinder the formation of a bridged oxonium ion.