Monomodal Molecular Weight Distribution Research Articles

Efficiency of n-butyllithium/m-diisopropenylbenzene diadduct as a dicarbanion initiator in the making of α,ω-hydroxyl terminated polybutadiene using oxetane as the capping agent

The diadduct made from a 2:1 ratio of n-butyllithium to m-diisopropenylbenzene has been evaluated as a diinitiator for synthesizing α,ω-hydroxyl terminated polybutadiene in cyclohexane solution. While the g.c./m.s. analysis results showed the predominant formation of the diadduct at 20°C, an incomplete reaction at 0°C led to a mixture containing a variety of initiating species of varying number of carbon-lithium ends. The observed narrow monomodal molecular weight distribution resulting from the use of this mixture indicated that this diadduct did not react as a diinitiator but rather as a monoinitiator. Although one of the two carbon-lithium ends of the diadduct was inactive during the polymerization, both ends were able to react with oxetane, thus forming the α,ω-hydroxyl terminated polybutadiene. This monofunctionality during polymerization and difunctionality towards the capping reaction has been corroborated by 1H n.m.r., 13C n.m.r. and hydroxyl titration. The capping reaction of the carbon-lithium end was as efficient for poly(butadienyl)dilithium, synthesized using the diadduct as the initiator, as for poly(butadienyl)lithium, synthesized using n-BuLi as the initiator. The inactivity of one of the two carbon-lithium ends during the polymerization did not prevent the diadduct from being an effective initiator for making telechelic HTPB.

Poly[poly(isobornyl methacrylate-co-methyl methacrylate) (poly(IBMA-co-MMA))-b-polybutadiene-b-poly(IBMA-co-MMA)] Copolymers: Synthesis, Morphology, and Properties

Anionic random and block copolymerization of isobornyl methacrylate (IBMA) and methyl methacrylate (MMA) has been studied in THF at −78 °C by using (1,1-diphenyl-3,3-dimethylbutyl)lithium (DDBLi) as initiator in the presence of LiCl. The random copolymerization of MMA and IBMA has also been carried out at 0 °C, all the other conditions being kept unchanged. Poly[poly(IBMA)-b-poly(BD)-b-poly(IBMA)] (IBI), poly[poly(IBMA-co-MMA)-b-poly(BD)-b-poly(MMA-co-IBMA)] (I/MBM/I), and poly[poly(IBMA)-b-poly(MMA)-b-poly(BD)-b-poly(MMA)-b-poly(IBMA)] (IMBMI) block copolymers have been synthesized by sequential anionic polymerization of butadiene, MMA, and IBMA initiated by the m-diisopropenylbenzene (m-DIB)/tert-butyllithium (t-BuLi) diadduct. These block copolymers of a monomodal and narrow molecular weight distribution (M̄w/M̄n = 1.1) have been analyzed by size exclusion chromatography (SEC), NMR, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). Stereocomplexation of IMBMA and I/MBM/I with iPMMA has also been studied by DSC. Although IBI triblock copolymers show a lamellar morphology even for relatively low hard block content (33 wt %), cylindrical and lamellar morphologies have been observed for the other block copolymers under consideration. These new block copolymers exhibit high ultimate tensile strength (30 MPa), elongation at break (1000%), and upper service temperature (140−200 °C).

Poly[methyl methacrylate (M)-b-styrene (S)-b-Butadiene (B)-b-S-b-M] Pentablock Copolymers: Synthesis, Morphology, and Properties

A series of poly[methyl methacrylate (M)-b-styrene (S)-b-butadiene (B)-b-S-b-M], or MSBSM, pentablock copolymers have been successfully synthesized by sequential living anionic polymerization initiated with the diadduct of tert-butyllithium (t-BuLi) onto m-diisopropenylbenzene (m-DIB) in a cyclohexane/diethyl ether mixture for the butadiene and styrene polymerization at room temperature and in a cyclohexane/THF mixture for the MMA polymerization at −78 °C. All the pentablock copolymers have a monomodal and narrow molecular weight distribution (Mw/Mn < 1.20), and their weight composition varies from 11 to 55% M, 18 to 55% S, and 15 to 64% B. Toluene-cast films of these copolymers have been analyzed by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and transmission electron microscopy (TEM). All these materials show a phase separation of the soft B component from the hard S and M blocks, which cannot, however, be distinguished one from each other by DSC or DMA. Indeed, a single transition is observed for the binary hard phase at a temperature intermediate between the glass transition temperature of polystyrene (PS) and poly(methyl methacrylate) (PMMA). In addition to classical phase morphologies, such as cylindrical and lamellar phase organization, two nonclassical morphologies, i.e., catenoid−lamellar and strut phase structures have been observed by TEM. The phase morphology strongly depends on the pentablock composition and any chemical modification of blocks. For instance, the cylindrical morphology, characteristic of an MSBSM copolymer containing equal amounts of hard and soft phases, is changed into a lamellar morphology upon hydrogenation of the B midblock. Copolymers of a relatively low hard phase content typically behave as thermoplastic elastomers of high ultimate tensile strength (ca. 30 MPa) and elongation at break (ca. 900%). These mechanical properties, however, depend on the casting solvent.

Polyelectrolytes with various charge densities: Synthesis and characterization of diallyldimethylammonium chloride‐acrylamide copolymers

AbstractThe synthesis of defined polyelectrolyte models by radical copolymerization of diallyldimethylammonium chloride (DADMAC, M1) and acrylamide (AAM, M2) in aqueous solution is impeded by a large difference of the reactivity ratios (r1 ≪ r2) leading to a strong conversion dependence of the copolymer composition. For the synthesis of normally distributed polyelectrolytes of the same molecular weight with various charge densities, a general algorithm based on a feeded polymerization with dosage of the more reactive monomer was therefore developed.Various copolymer compositions predictable by the Mayo‐Lewis equation and determined by chloride potentiometry were obtained. The sequence lengths distributions investigated by 13C NMR spectroscopy followed a Markov statistic of first order. The 13C NMR spectroscopy is shown as alternative method to gravimetry in determining reactivity ratios. The partial specific volumes and the refractive index increments of the copolymers correspond to the additivity principle. Further molecular and dynamic parameters are determined by membrane osmometry, static and dynamic light scattering, analytical ultracentrifugation, gel permeation chromatography, and viscometry. Monomodal molecular weight distributions, small polydispersities only for copolymers with high content of DADMAC are obtained at given comonomer and initiator concentration. The number‐average molecular weight is almost constant for a broad copolymer composition range.Rg‐M and [η]‐M relations could be established for practical use at high ionic strength, despite the various chemical compositions. Some indications for a higher stiffness of copolymers with high contents of DADMAC despite the shielding of the electrostatic interactions are, however, given. At low ionic strength the electrostatic interactions prevail. Their influence with increasing charge density is stronger on the Huggins constants (intermolecular interactions) than their effect on the intrinsic viscosities (intramolecular interactions).

New Thermosensitive Rubbery Polymers. Synthesis of Poly(siloxyethylene glycol) and Its Aqueous Solution Properties

Poly(siloxyethylene glycol) (PSEG), which consists of alternating oligo(dimethylsiloxane) and oligo(ethylene glycol) (OEG) units in the main chain, was synthesized through polycondensation reactions between oligosiloxane having two diethylamino groups and OEG. The molecular weight of the polymer was in the range 3500−17 700 g/mol with a monomodal molecular weight distribution. The glass transition temperature of the polymer thus obtained was controlled by changing the silicon content in the polymer. Actually, with increasing Si content, the Tg of the polymer decreased from −53 (PEG) to −100 °C. PSEG having a silicon content of <15 wt % was soluble in aqueous media. With increasing solution temperature, phase separation occurred and the solution become turbid. The temperature at the phase separation is known as a lower critical solution temperature (LCST). The LCST was controlled by the silicon content in the polymer. The hydrolytic stabilities of the PSEG in aqueous solution were also examined.

A New Soluble Poly(p-phenylene) with Tetrahydropyrene Repeating Units

The first poly(p-phenylene) (PPP) based on repeating 4,5,9,10-tetrahydropyrene units has been synthesized starting from 2,7-dibromo-4,9-di-n-octyl-4,5,9,10-tetrahydropyrene (3a) as the monomer in a polycondensation reaction. The new polymer is soluble in common organic solvents such as CHCl 3 , CH 2 Cl 2 , THF, and toluene. By contrast with other PPP's, the solubilizing alkyl substituents do not induce torsion about the aryl-aryl single bonds. GPC analysis indicates a monomodal molecular weight distribution, and the average degree of polymerization, based on tetrahydropyrene units, is 40 (polystyrene standards), corresponding to 80 phenylene rings. The NMR spectra reveal that the poly(tetrahydropyrene) (PTHPy, 4) obtained possesses a regular structure. The fluorescence spectrum measured in THF solution shows an emission maximum at 425 nm, while for the film, the maximum is shifted to 457 nm. The above characteristics qualify the new PPP as an attractive material for use in light-emitting diodes.

Molecular Weight Distribution of Living Polymerization Involving Chain-Transfer Agents: Computational Results, Analytical Solutions, and Experimental Investigations Using Ring-Opening Metathesis Polymerization

The number-average degree of polymerization and polydispersity index of living polymerization in the presence of chain-transfer agents are calculated numerically and analytically to determine how the molecular weight of the polymer can be regulated. The solutions reveal that un-steady-state polymerization exists in certain cases. The Mayo plot is nearly linear, but (as opposed to a nonliving system) its slope is not equal to k tr /k p in general (where k i , k p , and k tr are the specific rate constants of initiation, propagation, and chain transfer, respectively), the former differing from the latter by about 0.5-1 order of magnitude when k tr k p = 0.01-1.0. Plots of the slopes of Mayo plot versus k tr k p for different values of k p /k i reveal that only when k tr is equal to or greater than k p by an order of magnitude can the molecular weight be effectively controlled by addition of chain-transfer agent. A sufficient amount of chain-transfer agent is a necessary but not a sufficient condition to ensure monomodal molecular weight distribution. An analytical expression for the number-average degree of polymerization of the dead chains when k tr < k p has been derived and shows excellent agreement with numerical results. An analytical expression relating the slope of the Mayo plot to k tr /k p has also been obtained. These equations hold exactly when the concentration of the catalyst is much less than that of the chain-transfer agent. An experimental investigation of the kinetics of ring-opening metathesis polymerization (ROMP) of norbornene by Mo(=CHCMe 2 Ph)(NAr)(OCMe 3 ) 2 (Ar = 2,6-diisopropylphenyl) (1) in the presence of neohexene suggests that this ROMP system is adequately described by a relatively simple polymerization scheme. As measured from NMR spectroscopy, the specific rate constants of initiation, propagation, and chain transfer at 22° C are 0.57, 17, and 0.00003 M -1 s -1 , respectively.

Salt and solvent effects in “living” carbocationic polymerization

AbstractCarbocationic polymerization is strongly affected by medium and salt present in the system. The increase of solvent polarity enhances overall rates of cationic polymerization because it increases the proportion of carbocations in equilibrium with dormant covalent species. Higher solvent polarity might also increase molecular weights due to the reduction of transfer to counterions and may broaden polydispersity by slowing down an exchange between active and dormant species. Added salts may increase the proportion of ionic species by the ionic strength effect. The salts with common ions suppress free ions and lead to a monomodal molecular weight distribution by eliminating free ionic species with long lifetimes. The addition of tetrabutylammonium salts in the polymerization of styrene, initiated by 1‐phenylethyl chloride and catalyzed by tin tetrachloride is a typical example. The special salt effect based on exchange of counterions may enhance or reduce the proportion of ionic species and may accelerate the exchange between active and dormant species, affecting the polymerization rates and polydispersities.

A hydrocarbon‐soluble difunctional orgaolithium anionic initiator. Synthesis of 2,2‐bis[3‐(1‐lithio‐2,3‐dimethylpentyl)‐4‐methoxyphenyl]propane

AbstractThe reaction of stoichiometric amounts of sec‐butyllithium with 2,2‐bis[3‐(1‐propenyl)‐4‐methoxyphenyl]propane (4) produces a new difunctional organolithium initiator, 2,2‐bis[3‐(1‐lithio‐2,3‐dimethylpentyl)‐4‐methoxyphenyl] propane, which is soluble in hydrocarbon solvent in the absence of any polar additive. Following isolation and purification by gas liquid chromatography, the structures of (4) and the methanolysis product of the difunctional initiator were characterized by 1H NMR, FTIR and mass spectroscopy and elemental analysis. The dilithium initiator is effective for the polymerization of 1,3‐butadiene and allows the preparation of polybutadienes with predictable molecular weights, narrow, monomodal molecular weight distribution and low 1,2‐diene microstructure.

Ring-opening polymerization of cycloolefin induced by tungsten porphyrinates

Polymerization reactions of cyclopentene, cyclooctene and cyclododecene have been carried out in presence of catalytic systems based on tungsten tetraphenylporphyrinate, under inert atmosphere at room temperature, in aromatic solvents. The catalyst precursor was prepared by interaction of WCl 6 with free tetraphenylporphyrin in CCl 4 and characterized by spectroscopic methods (e.g. UV—Vis etc.). Spectroscopic studies on the interaction between the catalytic components or with the monomer, as well as kinetic results, molecular weight distribution and polymer microstructure indicated a high stability and efficiency of the catalytic systems employed. This catalyst allowed polyalkenamers with monomodal and narrow molecular weight distribution to be obtained. The polypentenamer displayed a block and/or alternate distribution of the carbon—carbon double bond dyads and high trans configuration. Data obtained under the above conditions gave information concerning the nature of active species during the initiation and propagation reactions.

Synthesis of polyphosphazene block copolymers bearing alkoxyethoxy and trifluoroethoxy groups

Block copolymers were formed by the anionically initiated copolymerization of (CH 3 OCH 2 -CH 2 O)(CF 3 CH 2 O) 2 P=NSi(CH 3 ) 3 or (CH 3 OCH 2 CH 2 OCH 2 CH 2 O)(CF 3 CH 2 O) 2 P=NSi(CH 3 ) 3 , followed by the subsequent addition of (CF 3 CH 2 O) 3 P=NSi(CH 3 ) 3 . These materials were characterized by 31 P and 1 H NMR, SEC, and DSC. SEC traces show that these block copolymers exhibit monomodal molecular weight distributions

Structure and optical properties of polyacetylene prepared via an improved Durham-route precursor polymer

Unoriented and stretch-oriented polyacetylene samples prepared via an improved Durham precursor polymer have been investigated. The standard catalysts for the Durham precursor polymerization have been replaced by a Schrock-type molybdenum-based initiator, which gives a narrower distribution in molecular weight and better stereoregularity in the precursor polymer. We have looked at the process of orientation achieved by stretching free-standing films during thermal conversion. Mixtures of high and lower molecular weight precursor polymers, giving a bimodal distribution, are more readily stretched than polymers with a monomodal molecular weight distribution, and we find that these oriented films show significantly higher levels of orientation and larger crystallite sizes than films prepared from the standard Durham-route precursor. Unoriented films of polyacetylene show optical absorption similar to those of the standard Durham polyacetylene, though there are significant differences in the photoinduced absorption spectra.

Viscoelastic properties of propylene‐ethylene block copolymer melts (abstract)

Stress relaxation and creep behavior of 16 commercial propylene‐ethylene block copolymers was measured in a molten state. The relaxation moduli of samples with monomodal molecular weight distribution show plateau or shoulder at long times. The bimodality of molecular weight distribution in the latter samples is due to the ethylene‐propylene copolymer component with high molecular weight. The plateau at long times is more noticeable as the bimodality is stronger. The height and width of the plateau is independent of the ethylene content. A PP‐HDPE‐EFR blend which was made as a model material of propylene‐ethylene block copolymer does not show the plateau. The relaxation modulus at high temperature is always lower than that at low temperature, the time‐temperature superposition can be performed well in all the measured range, and the shift factor obeys the Arrhenius equation. It was assumed from the above experimental results that appearance of the plateau at long times originates mainly from the bimodality of molecular weight distribution, not from the dispersion state of the ethylene‐propylene copolymer particles.

A new difunctional anionic initiator soluble in non‐polar solvents

AbstractThe reaction between 1,5‐diethenylnaphthalene and sec‐butyllithium produces a new difunctional organolithium initiator which is soluble in non‐polar solvents and effective in the synthesis of styrene‐isoprene‐styrene triblock copolymer. Monomodal molecular weight distributions are observed for block copolymers and styrene homopolymers. The microstructure of polyisoprene blocks is similar to those polymers initiated by butyllithium in the same solvent.

Anionic synthesis of polystyrene and polybutadiene heteroarm star polymers

AbstractThe use of living linking reactions of poly(styryl)lithium with 1,3‐bis(1‐phenylvinyl)benzene followed by crossover reactions with styrene or butadiene monomers has been used to prepare four‐armed heteroarm, star‐branched polymers. Bimodal molecular weight distributions have been observed for crossover reactions with both styrene and butadiene. Addition of THF ([THF]/[Li]=14–32) for crossover to styrene and lithium sec‐butoxide for crossover to butadiene produces monomodal molecular weight distributions. Symmetrical, four‐armed star polystyrenes have been synthesized; properties have been compared with a corresponding polymer prepared via a silicon tetrachloride linking reaction. Heteroarm, star‐branched polymers with two polystyrene arms and two polybutadiene arms with high 1,4‐microstructure have been prepared.

Mixed (electronic and ionic) conductive solid polymer matrix, 1. Synthesis and properties of poly(2,5,8,11,14,17,20,23‐octaoxapentacosyl methacrylate)‐block‐poly(4‐vinylpyridine)

AbstractPoly(2,5,8,11,14,17,20,23‐octaoxapentacosyl methacrylate)‐block‐poly(4‐vinylpyridine) semicomb polymers were synthesized using anionic living polymerization techniques. The polymers are white, powdery materials and were characterized by 1H and 13C nuclear magnetic resonance spectroscopy and gel‐permeation chromatography. The polymers display monomodal molecular weight distributions which are relatively narrow. The block copolymers show microphase separation as indicated by the presence of two glass transition temperatures (Tg), a soft (oxyethylene) phase Tg is observed between −60°C and −45°C and a hard (4‐vinylpyridine) phase Tg is observed between 135°C and 150°C. The soft oxyethylene phase has been doped with LiClO4 to obtain ionic conductors with electrical conductivities around 5 · 10−6 S · cm−1 at 25°C and the hard 4‐vinylpyridine phase has been complexed with 7,7,8,8‐tetracyano‐1,4‐quinodimethane (TCNQ, 2,5‐cyclohexadiene‐1,4‐diylidenedimalonitrile) to obtain electronic conductivities around 10−6 S · cm−1 at 25°C. The mixed (electronic and ionic) conductivities are intermediate between the ionic and electronic conductivities. Higher electronic conductivities (≈ 10−5 S · cm−1 at 30°C) are obtained for the polycation TCNQ−/TCNQ0 complexes.

Crystallization and mesomorphic disordering of di-n-hexylsilylene/di-n-pentylsilylene copolymers

A series of di-n-hexyl/di-n-pentylsilylene copolymers with monomodal molecular weight distribution have been prepared by Wurtz-coupling of dichlorosilanes using isooctane as a co-solvent. Crystallization and mesomorphic disordering were studied with regard to thermochromic effects and the silicon-chain conformation by DSC, MAS29Si-NMR, and UV-spectroscopy. The results show that the formation of a columnar mesomorphic phase is little affected by copolymerization of DHS and DPS units. Due to the preordering in the mesophase, three-dimensional ordering in the crystal is not hindered by topological constraints and isodimorphic mixed crystal formation was observed over the whole copolymer composition range. The predominantly all-trans structure, which is observed for crystalline poly(di-n-hexylsilylene), is maintained for copolymers containing up to 20%n-pentyl substituents.

Influence of molecular weight and polydispersity on phase behaviour of polyesters with laterally fixed or cross-shaped mesogens

Abstract To supplement previous studies of polyesters with laterally attached and cross-shaped mesogens the influence of molecular weight and molecular weight distribution on the phase behaviour has been investigated. For that purpose two polyesters have been fractionated by preparative gel permeation chromatography under high pressure and observed by polarizing microscopy and DSC measurements. A monotropic nematic polyester with laterally attached mesogens shows changing phase transitions up to a molecular weight of 10 000 (Mw); at higher molecular weight only the clearing transition is still slightly influenced. The molecular weight distribution at an average molecular weight of 15 000 (Mw) has no influence on the melting and clearing temperatures, but does effect recrystallization. The tendency to recrystallize decreases with increasing polydispersity, with increasing aberration from a monomodal molecular weight distribution. The recrystallization and the melting enthalpy are most distinguished at mole...

Dilithium initiators based on 1,3‐bis(1‐ phenylethenyl)benzene. Tetrahydrofuran and lithium sec‐butoxide effects

AbstractThe adduct (3) from 2 moles of sec‐butyllithium with 1,3‐bis(1‐phenyl‐ethenyl)benzene has been prepared and evaluated as a hydrocarbon‐soluble, dilithium initiator for butadiene and styrene polymerization in cyclohexane or benzene solution. Under high vacuum conditions with purified reagents, bimodal molecular weight distributions are observed for polybutadienes with Mn &lt; 150 × 103 g/mol and for polystyrenes with Mn &lt; 50 × 103 g/mol; monomodal distributions are observed only for higher molecular weights. Addition of tetrahydrofuran ([THF]/[Li] = 14 to 32) or preformed lithium sec‐butoxide ([LiOBu]/[3] = 1·1) produces narrow, monomodal molecular weight distributions even at Mn = 26·2 × 103 g/mol polybutadiene and at Mn = 10 × 103 g/mol for polystyrene. Added lithium sec‐butoxide is the preferred additive since high 1,4‐polybutadienes are obtained. These results provide an explanation for the contradictory results reported previously; under less rigorous experimental conditions, oxygen and hydroxylic impurities can form lithium alkoxides in situ.

Gradient high performance liquid chromatography of polymers, 2. Characterization of block copolymers of decyl and methyl methacrylate synthesized via group transfer polymerization

AbstractStatistical and block copolymers of decyl (DMA) and methyl methacrylate (MMA) were synthesized via group transfer polymerization (GTP) and characterized by size exclusion chromatography (SEC) and gradient HPLC. For HPLC a normal‐phase solvent gradient of isooctane and THF was used; the chromatograms were recorded by evaporative light scattering detection (ELSD). The block copolymers were shown to contain substantial amounts of precursor homopolymer by gradient elution, even in those cases where SEC furnished a perfectly monomodal molecular weight distribution (MWD). Comparison of the respective HPLC elution characteristics demonstrated that block copolymers are more strongly retained than statistical copolymers of identical composition. Whereas the latter are eluted according to chemical composition, the elution of the block copolymers seems to be controlled by the length of the poly(MMA) block.