Search

A series of chiral 1,2-cycloalkanediols including (1S,2S)-1,2-cyclopentanediol, (1S,2S)-1,2-cyclohexanediol, (1S,2S)-1,2-cycloheptanediol, and (1S,2S)-1,2-cyclooctanediol (a, b, c, and d) were used as chiral templates in the cyclocopolymerization of bis(4-vinylbenzoate)s (1a−d) with styrene. The copolymerizations of 1a with styrene produced an insoluble gel at a high mole fraction of 1a in the feed. On the other hand, the copolymerization of 1b−d with styrene proceeded homogeneously to yield polymers 2b−d, which were soluble in chloroform and tetrahydrofuran. After the removal of the chiral template from the resulting polymers 2a−d, polymers 3b−d exhibited optical rotations ([α]435, c 1.0, CHCl3) of −2.2 to −17.9° whose signs were opposite to those of polymers 2b−d, while polymer 3a showed little or no optical activity. Hence, the efficiency of templates on the chirality induction improved by expansion of the ring size of the chiral template; i.e., 1a ≪ 1b < 1c < 1d. The A values of the CD spectra of monomers 1a−d related to the dihedral angle between two 4-vinylbenzoyl groups of the monomers which decreased with increasing ring size. Therefore, the results indicated that the dihedral angle affected the chiral inductivity of the template.

Two diastereomeric chiral cycloalkanediols such as (1S,2R)-2-hydroxymethyl-1-cyclohexanol (a) and (1S,2S)-2-hydroxymethyl-1-cyclohexanol (b), which were conformational restriction models for (S)-1,3-butanediol (c), were used as chiral templates to investigate the conformational effect of the 1,3-diol skeleton of the template moiety in the cyclocopolymerization of bis(4-vinylbenzoate)s (1) with styrene. The specific rotation for the resulting template-free polymer 3a was almost twice that for 3b. In comparison with chiral template c, the efficiency of the chirality induction increases in the order of b < c < a. These characteristics of chiral templates were discussed in terms of the conformational distribution of the monomer 1a−c.

The cyclopolymerizations of two meso dianhydrohexitols, 1,2:5,6-dianhydro-3,4-di-O-methylallitol (1) and -galactitol (3), were examined to form a polymer with six-membered ring repeating units. Distinct from the cases of 1,2:5,6-dianhydro-3,4-di-O-methyl-d-mannitol (5), -l-iditol (6), and -d-glucitol (7) in which the five-membered ring units were formed with high regio- and stereoselectivity, six-membered ring units were found in polymers 2b, 4a, and 4b obtained by the polymerizations of 1 and 3. The anionic polymerization of 1 with t-BuOK was highly regio- and stereoselective to form polymer 2a consisting of five-membered ring units, but that of 3 was different in the scission mode of the epoxy groups to form polymer 4a consisting essentially of six-membered ring units. The cationic polymerization of 1 with BF3·OEt2 produced polymer 2b having a structure consisting of the six-membered ring units as major constituents and five-membered ring units as minor ones. The structure of polymer 4b from the cationic polymerization of 3 was somewhat complex, comprising unknown units and uncyclized units along with the five- and six-membered ring units. The steric hindrance in the growing ion was an advantage in forming the six-membered ring unit.

Radical cyclocopolymerization of the dimethacrylate 1 (M1), which was derived from 1,2:5,6-di-O-isopropylidene-d-mannitol, with styrene (M2) induced new chirality in the polymer chain. The removal of the chiral template in the copolymer 2 followed by treatment with diazomethane gave optically active poly(methyl methacrylate-co-styrene) (4). Polymer 4 had more active chirality upon increasing the M1 unit, thus indicating that the new chirality should be induced by the M1 diad unit rather than the isolated M1 unit. Characteristics of monomer 1 include the considerably higher fraction of tactic triad rr in poly(methyl methacrylate) (PMMA) derived from the homopolymerization and the larger probability of forming the racemo diad of the M1 unit during the intermolecular reaction of the copolymerization. In polymer 4, the origin of chirality was assigned to the chiral tetrad sequences of the MMA unit which were induced by participation of the racemo configuration.

The cyclopolymerizations of (2S,5S)-1,2:5,6-diepoxyhexane using BF3·OEt2 and tert-BuOK have been studied. The polymer yield and molecular weight of the polymer using tert-BuOK were higher than those using BF3·OEt2. For the polymers using BF3·OEt2, the mole fractions of the cyclized units were 0.76−0.90, and the cyclic constitutional repeating units were five- and six-membered rings. The polymer using tert-BuOK essentially consisted of the five-membered cyclic repeating unit.

1,2:5,6-Di-O-isopropylidene-3,4-bis-O-(4-vinylbenzoyl)-d-mannitol (1a) and -d-glucitol (1b) with styrene (St) were polymerized using AIBN in toluene at 60 °C to yield copolymers (2a and 2b, respectively) consisting essentially of the cyclic repeating and styrene units. In order to confirm asymmetric induction in 2, the removal of the template from 2 was carried out using KOH in aqueous MeOH, and then the resulting hydrolyzed copolymer was treated with diazomethane for conversion into poly[(methyl 4-vinylbenzoate)-co-styrene] (3). Copolymers 2a and 2b showed a negative specific rotation, whereas a positive rotation was observed for 3a and 3b. The CD spectrum of 1a exhibited a negative Cotton effect at 300.0 nm and a positive one at 267.3 nm as well as a negative exciton coupling of 1b. According to the exciton chirality method, 1a and 1b had a negative chirality and its benzoate chromophores, the two 4-vinylbenzoyl groups, were twisted counterclockwise. On the other hand, the CD spectrum of 3a (1a/St = 0.30/0.70) and 3b (1b/St = 0.27/0.73) showed a split Cotton effect with a positive one at 254.9 nm and a negative one at 237.1 nm for 3a and with a positive Cotton effect at 254.7 nm and a negative one at 232.7 nm for 3b. This result indicated that 3 had a positive chirality in which two 4-vinylbenzoyl groups were twisted clockwise, i.e., the vicinal (methyl 4-vinylbenzoate) groups in the main chain were an (S,S)-configuration.

The copolymerizations of (S)-1,3-butanediyl and (2S,4S)-2,4-pentanediyl bis(4-vinylbenzoate)s (1a and 1b) (M1) with styrene (M2) were carried out using AIBN in toluene at 60 °C. The obtained copolymers (2) consisted essentially of the cyclic repeating and styrene units, because the characteristic absorption due to the vinyl protons disappeared in the ¹H-NMR spectra. The copolymerization reactivities of 1a and 1b were higher than that of styrene, resulting in the mole fraction of the M1 unit in the copolymer 2 being higher than that of the M2 unit for every monomer feed. The monomer reactivity ratio, which was calculated by the Kelen−Tüdös method, was r1 = 4.39 and r2 = 0.20 for 1a and r1 = 3.81 and r2 = 0.28 for 1b. The specific rotation ([α]435, c 1.0 in CHCl3) changed from +283° to +182° for 2a and from +434° to +319° for 2b. After removing the chiral templates from 2a and 2b, poly[(methyl 4-vinylbenzoate)-co-styrene]s (3a and 3b) were obtained. The specific rotation ([α]435, c 1.0 in CHCl3) varied from −0.5° to −24.0° for 3a and from −0.4° to −46.3° for 3b, which indicated that the (2S,4S)-2,4-pentanediyl template approximately doubled in effect for the asymmetric induction by comparison with the (S)-1,3-butanediyl one. A split Cotton effect with a negative Cotton effect and a positive one was observed for the CD spectrum of 1 and the reversed one for 3. According to the CD exciton chirality method, the clockwise twist of the two 4-vinylbenzoates of 1 transmitted their chirality to 3 in which two 4-vinylbenzoyl groups twisted counterclockwise, i.e., an (R,R)-configuration of vicinal (methyl 4-vinylbenzoate) units in the main chain.

The anionic cyclopolymerizations of 1,2:5,6-dianhydro-3,4-di-O-methyl-d-mannitol (1) and 1,2:5,6-dianhydro-3,4-O-isopropylidene-d-mannitol (2) have been carried out using potassium tert-butoxide (t-BuOK) and potassium hydroxide (KOH). For the polymerization of 1, a well-defined carbohydrate polymer consisting of (1→6) linked 2,5-anhydro-3,4-di-O-methyl-d-glucitol units was synthesized through a regio- and stereoselective mechanism. When a [1]/[t-BuOK] molar ratio of 20 was used in toluene for 48 h, the yields and number-average molecular weights (Mn) of the polymers gradually increased with polymerization time. The Mn of the polymer varied with the molar ratio of monomer to initiator, and a linear relationship between them was found. The degree of polymerization was larger than that estimated from the molar ratio, resulting in an initiator efficiency of about 55%. KOH was also effective for converting monomer 1 to a gel-free polymer but was not as active as t-BuOK. The rate of polymerization was rather slow, and the polymerization was not complete, even after 100 h. The presence of a crown ether, 18-crown-6, in the cyclopolymerization allowed the Mn of the polymer to approach the value estimated from the [1]/[t-BuOK] molar ratio. On the other hand, monomer 2 tended to form a gel in the polymerization process, so soluble polymers were isolated only at early stages of the polymerization.

The anionic cyclopolymerization of 1,2:5,6-dianhydro-3,4-di-O-methyl-l-iditol (3) was carried out using t-BuOK and KOH. The resulting polymer 4 consisted of 2,5-anhydro-3,4-di-O-methyl-d-glucitol as a five-membered repeating unit, which was identical with that of polymer 2 obtained from 1,2:5,6-dianhydro-3,4-di-O-methyl-d-mannitol (1). The copolymerization of 1 with 3 produced a random copolymer consisting of (1→6)-, (6→1)-, (1→1)-, and (6→6)-linked 2,5-anhydro-3,4-di-O-methyl-d-glucitol. These facts elucidated the presence of two linkages during the homopolymerization of 1 and 3, i.e., the (1→6) and (6→1)-bonded 2,5-anhydro-3,4-di-O-methyl-d-glucitol units in polymers 2 and 4, respectively.

Optically active poly(methyl 4-vinylbenzoate-co-styrene)s (3a−c) were synthesized through the cyclocopolymerizations of (2S,3S)-2,3-butanediyl, (2S,4S)-2,4-pentanediyl, and (2S,5S)-2,5-hexanediyl bis(4-vinylbenzoate)s (1a, 1b, and 1c, respectively) with styrene. The extent of chirality induction increased in the order of 1a ≅ 1c < 1b, and the cyclized units predominately possessed an (R,R)-configuration. The MM2 calculation derived the conformer distributions, in which monomers 1a and 1b predominately had a counterclockwise gauche form (“g^-”) and a combination of trans form and clockwise gauche form (“tg⁺”), respectively. Monomer 1c had a wide distribution into “tg⁺g⁺”, “g⁺tg⁺”, and “ttt” forms because of flexibility. The semiemprical molecular orbital calculations were performed for the model reaction of the radical cyclization. The calculated population difference between (R,R)- and (S,S)-configurations indicated that the template in 1b was more effective for the chirality induction than that in 1a, similar to the experimental results.

The effect of 18-crown-6 ether (18C6) in the anionic cyclopolymerization of 1,2:5,6-dianhydro-3,4-di-O-methyl-d-mannitol (1) using potassium tert-butoxide (t-BuOK) was studied. The addition of 18C6 did not affect the regio- and stereoselectivity in the polymerization of 1, and the resultant polymer was (1→6)-2,5-anhydro-3,4-di-O-methyl-d-glucitol (2). The complexation of t-BuOK with 18C6 increased the initiator efficiency from 0.3 to 1.0, which was accompanied by enhancement of the apparent polymerization rate. The rate constant ratios of the transfer reaction to the propagation (ktr/kp) in the systems with and without 18C6 were 1 order of magnitude lower than the value in the polymerization of monoepoxides. The “living” nature of the t-BuOK/18C6 initiating system was evaluated by the two-step monomer resumption experiment.

Stereoregular polymerization of (o-ethoxyphenyl)acetylenes (OEPA) was performed using a [Rh(norbornadiene)Cl]2 catalyst in the presence of triethylamine to give poly((o-ethoxyphenyl)acetylene) (POEPA) bearing a cis-transoid form under mild conditions. The polymer underwent isomerization from the predominant cis form to the trans form when compressed at 200 kg/cm² at room temperature. The polymers, obtained before and after the compression, were characterized in detail using laser Raman, diffuse reflective UV, ESR, and X-ray diffraction. The data suggested that the isomerization of the resulting polymer from the cis-transoid to the trans-transoid form was induced to give rise to the conjugated trans-planar zigzag structures, where unpaired electrons are stabilized as mobile electrons called solitons. The formation of the soliton radicals was confirmed by the motional narrowing of the line width observed in the ESR spectra of the compressed polymer. The trans conjugation length, n, (C&dbd;C)n generated by the compression was estimated as n = ca. 22−45 using UV and laser Raman. The studies show that the compression of the cis-transoid POEPA form is the preferred method to isomerize it to the trans-transoid form. The compressed polymer contains a higher degree of conjugation compared with that of the thermal isomerization of the pristine polymer.