Steric effects on excimer formation in isotactic and atactic polystyrene and poly(ar-methylstyrene)s in solution

Effects of tacticity and steric hindrance on excimer formation were investigated in isotactic and atactic polystyrene, poly(o-methylstyrene), poly(m-methylstyrene), and poly(p-methylstyrene) in the presence and absence of a quencher (CCl₄). The calculated rate constants for excimer formation in the isotactic polymers except for poly(o-methylstyrene) were almost the same and larger than those in the corresponding atactic polymers. These results indicate that excimer formation was due to not only rotational sampling but also energy migration to trapping sites. It was found that steric hindrance on excimer formation was intimately related to the excition diffusion length in the polymer chain.     

Dynamics of macromolecular chains. VI. Carbon 13 and proton nuclear magnetic relaxation of polystyrene in solution

Spin-lattice ¹H and ¹³C nuclear magnetic relaxation (NMR) times T₁ have been measured for solutions of polystyrene in hexachlorobutadiene at two different frequencies. Some nuclear Overhauser enhancements and linewidths have also been determined. At 15 and 25 MHz the relaxation times T₁ of the ortho and meta carbons show two different dependences on temperature. These measurements indicate internal motion of phenyl groups around the C_α—C_para axis. A single isotropic correlation time is inadequate to explain the relaxation data for the para carbon. Use of a diamond-lattice motional model reveals that segmental reorientation of the chain backbone of polystyrene can be described in terms of two correlation times, ρ characterizing the three-bond motion process, and θ reflecting either isotropic motions of subchains or departure from an ideal lattice. Data on low-molecular-weight polystyrene indicate the participation of overall rotatory diffusion in the relaxation process. This motion is no longer efficient in high-molecular-weight polymers, where relaxation is due to segmental reorientation.     

Preparation,fractionation, and dilute-solution behavior of ABA poly(methyl methacrylate-b-α-methylstyrene) block copolymers

The preparation by anionic polymerization of six ABA poly(methyl methacrylate-b-α-methylstyrene) block copolymers and of sixteen poly(α-methylstyrene)s is described. The block copolymers, of similar molecular weight but with different chemical compositions, were fractionated by preparative gel permeation chromatography and their behavior in dilute solution was investigated using viscometry. The results obtained indicate that the intramolecular phase separation does not occur under the conditions utilized, the block copolymers assuming randomcoil configurations in all of the copolymer/solvent systems studied. Consequently the block copolymer molecules are more expanded than homopolymers of the same molecular weight. The series of poly(α-methylstyrene)s covered the molecular weight range 2.7 × 10³–1.3 × 10⁶ and enabled the determination of Mark–Houwink–Sakurada constants for poly(α-methylstyrene) in the solvents chosen for the block copolymer studies.     

Spectres ultrasonores de solutions diluées de polystyrène et de quelques derivés—I. Poly(α-Méthylstyrène) et poly(o-bromostyrène)

Ultrasonic absorption data were obtained over the frequency range 0·3–150 MHz for dilute solutions of polystyrene, poly(α-methylstyrene) and poly(o-bromostyrene). We have shown that for polystyrene and poly(α-methylstyrene) the spectra are very similar and can be resolved into two relaxations. A bromine atom ortho in the benzene ring affects the ultrasonic spectrum which shows three relaxation times at least. Investigation of the temperature dependence also has shown a different behaviour for poly(o-bromostyrene).     

Ultrasonic Modification of Polymers. II. Degradation of Polystyrene,Substituted Polystyrene,and Poly(n-vinyl Carbazole) in the Presence of Flexible Chain Polymers

Abstract

Ultrasonic (20 kHz, 70 W) solution degradations of polystyrene, substituted polystyrenes, and poly(n-vinyl carbazole) have been carried in toluene and tetrahydrofuran at 27 and -20°C in the presence of flexible chain polymers. Polystyrene formed block copolymers at 27°C with stiff-chain polymer PVCz; however, in the presence of flexible chain polymers, e.g., poly(vinyl methyl ketone) or poly(vinyl methyl ether), there were no block copolymers formed. Poly(n-vinyl carbazole) does not seem to form any block copolymers at 27°C with flexible chain polymers, e.g., poly(octadecyl methacrylate) and poly(ethyl methacrylate). Poly(p-chlorostyrene) and poly(p-methoxystyrene) also do not form block copolymers at 27°C with poly(octadecyl methacrylate) but do so with poly(hexadecyl methacrylate). It is quite possible that these may only be blends of two homopolymers. Poly(octa-decyl methacrylate) does yield a block copolymer when sonicated at -15°C with poly(p-isopropyl α-methylstyrene).     

Composition and Microstructure Determination of the Syndiotactic Copolymer of para-Methylstyrene and Styrene by Pyrolysis Gas Chromatography

The composition and microstructure of syndiotactic para-methylstyrene/styrene copolymer was determined by a pyrolysis gas chromatography (Py-GC) method. This method uses the styrene and para-methylstyrene monomer peak intensities to determine the styrene and para-methylstyrene composition in the copolymer. The number average sequence length of styrene was calculated by using the triad peak intensities. Because of the low concentration of para-methylstyrene in the copolymer, the number average sequence length of para-methylstyrene was determined with formulas that incorporate the copolymer composition and the number average sequence length of styrene. The distribution of para-methylstyrene defined by the terms “percent of single units” and “percent of desired distribution” was calculated by the number average sequence of para-methylstyrene. This method has been tested with copolymers containing up to 24 mole% of para-methylstyrene. The composition results from Py-GC of para-methylstyrene and styrene copolymers used in this study were in excellent agreement with ¹H-NMR results.     

The heat capacity of solid and liquid polystyrene,p-substituted polystyrenes,and crosslinked polystyrenes

Heat capacities were measured for poly(4-methylstyrene) [300–500K], poly(4-fluorostyrene) [130–350K], poly(4-chlorostyrene) [300–550K], poly(4-bromostyrene) [300–550K], poly(4-iodostyrene) [300–550K] and poly(styrene-co-divinylbenzene) with 1, 2, 4, 8, and 12 wt.% divinylbenzene (technical grade) [300–550K]. Polystyrene and poly(α-methylstyrene) data were found to match the ATHAS data bank collections. Crosslinking causes no significant change in heat capacity, but substitution does. The heat capacities in the solid state are evaluated using approximate group and skeletal vibration spectra. Glass transitions are discussed, and full thermodynamic functions (C_p, H, S, G) can be calculated for amorphous, crystalline, and deuterated polystyrene as well as poly(α-methylstyrene). Glassy polystyrene has an entropy of 7.5 J K^?1 mol^?1 at absolute zero. Changes of the heat capacity at the glass transition are explained and are predicted to go to zero for 50% poly(styrene-co-divinylbenzene) at about 550K.     

Head-to-head vinyl polymers. X. Cyclopolymerization of o-dimethacryloyloxybenzene

To clarify the possibility of preparing a polymer that contained head-to-head (h–h) methyl methacrylate (MMA) unit radical cyclopolymerization of o-dimethacryloyloxybenzene (o-DMB) was investigated. p-Dimethacryloyloxybenzene (p-DMB) was also used in comparison. When the polymerizations were carried out with higher monomer concentrations than ca. 0.5 mol/L, benzene-insoluble polymer was produced. The extent of cyclization (f_c) in benzene-soluble polymer increased with a decrease in the monomer concentration and an increase in the polymerization temperature. The ¹H-NMR and ¹³C-NMR spectra of poly(MMA) derived from poly(o-DMB) by hydrolysis and methylation were in fairly good agreement with those of ordinary head-to-tail (h–t) poly(MMA). Therefore, it was concluded that the intramolecular propagation in cyclopolymerization of o-DMB was mainly performed by a h–t mechanism. However, the initial and maximum degradation temperatures of the poly(MMA) were observed to be somewhat higher than those of the ordinary poly(MMA), which suggests that a minor amount of the h–h unit was formed.     

Kinetic and microstructure studies of poly(ethylene-co-p-methylstyrene) copolymers prepared by metallocene catalysts with constrained ligand geometry

This paper discusses the poly(ethylene-co-p-methylstyrene) copolymers prepared by metallocene catalysts, such as Et(Ind)₂ZrCl₂ and [C₅Me₄(SiMe₂N^tBu)]-TiCl₂, with constrained ligand geometry. The copolymerization reaction was examined by comonomer reactivity (reactivity ratio and comonomer conversion versus time), copolymer microstructure (DSC and ¹³C-NMR analyses) and the comparisons between p-methylstyrene and other styrene-derivatives (styrene, o-methylstyrene and m-methylstyrene). The combined experimental results clearly show that p-methylstyrene performs distinctively better than styrene and its derivatives, due to the cationic coordination mechanism and spatially opened catalytic site in metallocene catalysts with constrained ligand geometry. A broad composition range of random poly(ethylene-co-p-methylstyrene)copolymers were prepared with narrow molecular weight and composition distributions. With the increase of p-methylstyrene concentration, poly(ethylene-co-p-ethylstyrene)copolymer shows systematical decrease of melting point and crystallinity and increase of glass transition temperature. At above 10 mol % of p-methylstyrene, the crystallinity of copolymer almost completely disappears. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1017–1029, 1998     

Synthesis of tritium labelled monomers and polymers

The use of P₂O₅ for promoting the tritiation of various monomers and polymers has been investigated. Methyl methacrylate and vinyl acetate may be labelled at ambient temperatures by this procedure which is also applicable to labelling polystyrene and poly(α-methylstyrene). Exchange labelling of polymer substrates is most conveniently carried out in chlorinated hydrocarbons. The rate of tritium exchange increases with solvent polarity and temperature. Monomers of high radiochemical purity may be derived from the thermal depolymerization of tritiated polystyrene, poly(α-methylstyrene) and poly(methyl methacrylate).     

Poly(thiaheterohelicene) derived from the long‐alkylated polysulfonium precursor

Poly[4,6‐bis(dodecylthio)‐1,3‐phenylene‐alt‐2‐methyl‐1,3‐phenylene] (Poly‐S) was synthesized via Suzuki–Miyaura coupling of 1,3‐dibromo‐2,6‐bis(dodecylthio)benzene and 2‐methyl‐1,3‐phenylenebis(pinacol borate). After quantitative oxidation of the pendant sulfide moiety to the sulfoxide derivative (Poly‐SO), the m‐linked benzene rings were fused via intramolecular ring‐closing condensation with excess triflic acid to form the corresponding poly(sulfonium cation) (Poly‐S⁺) with a helical structure. Poly‐S⁺ was quite soluble in non‐polar solvents due to the long alkyl dodecyl chain. The Poly‐S⁺ film, especially the film cast from its acetone solution, gave a polarized optical micrograph, suggesting an oriented structure of the helical poly(sulfonium) derivative. The X‐ray diffraction pattern of the film supported a hexagonal columnar packing of the polymer. The polysulfonium derivative in the film state was converted to the corresponding poly(thiaheterohelicene) ( Poly‐TH ) via the dealkylation with a basic potassium hydroxide methanol solution or pyridine. The Poly‐TH film was doped with iodine. Its conductivity was enhanced about 30 times when compared to that of the undoped film. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of PP graft copolymers via anionic living graft-from reactions of polypropylene containing reactive p-methylstyrene units

In this article, we discuss a new chemical route for preparing polypropylene (PP) graft copolymers containing a PP backbone and several (polar and nonpolar) polymer side chains, including polybutadiene, polystyrene, poly(p-methylstyrene), poly(methyl methacrylate), and polyacrylonitrile. The new PP graft copolymers had a controlled molecular structure and a known PP molecular weight, graft density, graft length, and narrow molecular weight distribution of the side chains. The chemistry involves an intermediate poly(propylene-co-p-methylstyrene) copolymer containing few p-methylstyrene (p-MS) units. The methyl group in a p-MS unit could be lithiated selectively by alkylithium to form a stable benzylic anion. Because of the insolubility of the PP copolymer at room temperature, the excess alkylithium could be removed completely from the lithiated polymer. By the addition of the anionically polymerizable monomers, including polar and nonpolar monomers, the stable benzylic anions in PP initiated a living anionic graft-from polymerization at ambient temperature to produce PP graft copolymers without any significant side reactions. The side-chain length was basically proportional to the reaction time and monomer concentration. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4176–4183, 1999     

Metathesis polymerization of N-aryl norbornene dicarboximides. I

Metathesis polymerization of N-phenyl-exo-norbornene dicarboximide and ortho/meta/para methyl substituted phenyl nadimides was carried out using WCl₆/tetramethyltin. Structural characterization was done by FTIR, ¹H- and ¹³C-NMR. A mixture of cis and trans double bond structures were introduced in the backbone during polymerization. The cis content was higher (52 to 65%). In the DSC scan of poly(N-o-tolyl nadimide), two exotherms were observed at 240 and 270°C while in other samples only one exothermic transition was observed above 240°C. These exotherms disappeared in the second heating cycle. The T_g of the polymers, as determined in the second heating cycle, was highest in poly(N-o-tolyl nadimide) and lowest in poly(N-m-tolyl nadimide). The polymers were stable up to 443 ± 3°C and decomposed above this temperature in a single step. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2917–2924, 1997     

Photoisomerization of Trans Ortho‐, Meta‐, Para‐Nitro Diarylbutadienes: A Case of Regioselectivity

A series of ortho‐, meta‐ and para‐substituted trans‐nitro aryl (phenyl and pyridyl) butadienes have been synthesized and characterized. The effect of substitution and positional selectivity on their fluorescence and photoisomerization were systematically investigated. Among all dienes, meta‐ and para‐nitro phenyl‐substituted derivatives exhibit remarkable solvatochromic emission shifts due to intramolecular charge transfer. On the other hand, ortho derivatives undergo regioselective isomerization upon photoexcitation in contrast to inefficient isomerization of para and meta nitro‐substituted dienes. Single crystal X‐ray analysis revealed existence of intramolecular hydrogen bonding between the nitro group and the hydrogen of the proximal double bond. This restricts the rotation of the proximal double bond thereby allowing regioselective isomerization. The observations were also supported by NMR spectroscopic studies.     

Dipole moments of some substituted benzaldehydes. Conformational preference of substituents ortho to the aldehyde group

The dipole moments of a number of substituted benzaldehydes are measured in benzene solution. The angle which the dipole axis of the CHO group makes with the axis of rotation of the group is determined. The observed moments of the ortho-substituted benzaldehydes are compared with the moments calculated for free rotation as well as fors-trans ands-cis orientations of the -CHO group.o-Fluorobenzaldehyde exists mostly in thes-trans conformation.o-Chloro-,o-bromo-ando-nitro-benzaldehydcs also exist in thes-trans conformation; their observed dipole moments are even lower than the values calculated fors-trans forms, indicating mutual induction of the ortho substituents. Though 2,5-dichlorobenzaldehyde is expected to have the same dipole moment as benzaldehyde, the observed moment is significantly lower due to mutual induction of the ortho substituents. 2,5-Dimethylbcnzaldehyde has, however, almost the same moment as benzaldehyde. The dipole moment ofo-methoxybcnzaldchyde is considerably higher than the values calculated for boths-cis ands-trans conformations. An explanation is given for this.o-Hydroxybenzaldehyde exists exclusively in thes-cis form due to internal H-bonding.     

Modulating light‐tunable acid sensitivity of a bioinspired polymer simply by adjusting the position of a single methoxy substituent

This article describes a rhodopsin‐inspired photosensitive polymer whose light‐tunable acid sensitivity can be widely modulated simply by adjusting the position of a single methoxy substituent in the aromatic rings of cinnamyl groups. The well‐defined poly(5‐ethyl‐5‐methacryloyloxymethyl‐2‐(p‐methoxystyryl)‐[1,3]dioxane) (PEMpMSD) and poly(5‐ethyl‐5‐methacryloyloxymethyl‐2‐(o‐methoxystyryl)‐[1,3]dioxane) (PEMoMSD) as well as poly(5‐ethyl‐5‐methacryloyloxymethyl‐2‐styryl‐[1,3]dioxane) were synthesized via reversible addition‐fragmentation chain transfer (RAFT) process. The results demonstrated that the para‐methoxy substitution of EMpMSD monomer led to the more shortened initialization period and rapid chain propagation of RAFT process than 5‐ethyl‐5‐methacryloyloxymethyl‐2‐styryl‐[1,3]dioxane monomer under mild visible light radiation at 25 °C. The ortho‐methoxy substitution of PEMoMSD led to high degree of photoinduced Z‐isomerization over 80%. Moreover, the para‐methoxy substitution of PEMpMSD led to the rapid hydrolysis of the cyclic acetal linkages in ambient acid media, while the ortho‐methoxy substitution of PEMoMSD slowed down this hydrolysis. This hydrolysis slowed down on Z‐isomerization particularly in PEMoMSD. These effects widely broadened the tunability of the light‐modulated acid sensitivity. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Comparative analysis of the clathrate forms of syndiotactic polystyrene,poly (p-methylstyrene) and poly (m-methylstyrene)

The crystal structures of the clathrate forms of syndiotactic polystyrene (s-PS), poly (p-methylstyrene) (s-PPMS) and poly (m-methylstyrene) (s-PMMS) containing guest molecules having widely different steric hindrance are compared in detail. Common features and differences concerning the packing of the chains, the shape and the dimensions of the cavities and the stability of the forms deprived of the guest molecules are pointed out. A new clathrate form of s-PPMS containing CS₂ is also described.     

Block and graft copolymers by selective cationic initiation. IV. Physical properties of bigraft copolymers

The physical properties of bigraft copolymers, i.e., Nordel-g-polystyrene-g-poly(α-methylstyrene) and Nordel-g-polystyrene-g-polyisobutylene, have been studied in terms of stress strain behavior, glass transition temperature, dynamic mechanical data, intrinsic viscosity versus temperature profile and solubility properties. These products are thermoplastic elastomers and show the presence of incompatible domains. T_g and dynamic-mechanical data indicate an aggregation of the polystyrene and poly(α-methylstyrene) phases.     

Dynamic birefringence of poly(styrene-4-sulfonate sodium) macromolecules in aqueous solutions at high ionic strengths

For samples of various molecular masses, the flow birefringence of poly(styrene-4-sulfonate sodium) macromolecules in aqueous solutions at various ionic strengths is studied in relation to the concentration of NaCl. It is shown that the sign of birefringence changes from negative to positive with an increase in the ionic strength of a solution. Application of the macroform theory to the Maxwell effect makes it possible to estimate the intrinsic optical anisotropy of the repeating unit of poly(styrene-4-sulfonate sodium) (a||- a⊥) = −17 × 10^-25 cm³), which nearly coincides with similar values known for atactic polystyrene and poly(κ-methylstyrene).