Effect of styrene oligomers on syndiospecific polymerization of styrene

Styrene oligomers, preferentially consisting of styrene dimers and trimers, are formed by a free radical mechanism at the thermal polymerization of stabilizer-free styrene during storage and at higher polymerization temperatures. The identity of several dimer and trimer fractions formed in such a free radical polymerization, their influence on a coordinative polymerization reaction, the syndiospecific polymerization of styrene, as well as their effect on the properties of the resulting polymers has been investigated.Styrene dimers and styrene trimers reduce the polymerization activity of the transition metal catalyst significantly, especially at low amounts of oligomers added to the styrene. This behavior is discussed with respect to a proposed mechanism involving complexation of the active transition metal species with the specific oligomer instead of the styrene monomer, resulting in increased steric hindrance towards insertion of a styrene molecule to the active site.Both oligomers reduce the molecular weight of the syndiotactic polystyrene, by acting as chain-transfer agents. The constancy of the polydispersity over the whole concentration range of added dimer or trimer indicates that the uniformity of the active sites of the coordinative polymerization is not significantly influenced by the presence of the oligomers.The thermal properties of the polymers demonstrate that the oligomers do not affect the high syndiospecificity of the active catalytic sites, whereas the increase in crystallization temperature with increasing amounts of styrene dimer or trimer is comparable to effects observed by the addition of crystallization nucleators to semicrystalline polymers.     

Formation Mechanism of Polyaniline: Evidence from the Post-Reactions of Isolated Intermediates

The formation mechanism of Polyaniline was derived from the post reactions of the polymerization intermediate. The intermediate, RPAN, (3000 Å thickness film, purplish-blue in color) was isolated on silane modified substrates after polymerization for 10 minutes. Under air, RPAN converts to emeraldine salt within a couple of seconds. In the inert atmosphere, the conversion occurred at a relatively low speed. In acidic media, the intermediate was believed to be a very reactive radical cation, which oxidized then coupled with aniline oligomers to start the auto-catalytic chain growing process. When the radical cation was quenched in water or acid, a violet species, IPAN, with a structure composed mostly of pernigraniline units, was formed. IPAN reacts slowly with aniline oligomers in the presence of protons and oxidants to emeraldine. The conversion processes were monitored by IR and Uv/Vis/NIR spectroscopy.     

Synthesis and properties of DNA oligomers containing 2'-deoxynucleoside N-oxide derivatives

Cytosine and adenine N-oxide derivatives have long been known as products resulting from the oxidative damage of DNA by peroxides such as hydrogen peroxide. Although the synthesis and properties of 2'-deoxynucleoside N-oxide derivatives have been well described, little has been reported about the chemical and biochemical behavior of initially formed DNA oligomers containing these N-oxide bases. In this study, we established a convenient method for the solid-phase synthesis of oligodeoxynucleotides incorporating 2'-deoxycytidine N-oxide (dC O) or 2'-deoxyadenosine N-oxide (dA O) by using the postsynthetic oxidation of N-protected DNA oligomers except for the target dC or dA site with m-CPBA in MeOH in a highly selective manner. In this strategy, the benzoyl, phthaloyl, and (4-isopropylphenoxy)acetyl groups proved to serve as base protecting groups to avoid oxidation of adenine, cytosine, and guanine, respectively, at the unmodified sites.     

Mechanism for radical cation transport in duplex DNA oligonucleotides

We investigated the photoinduced one-electron oxidation of a series of DNA oligomers having a covalently linked anthraquinone group (AQ) and containing [(A)(n)GG](m) or [(T)(n)GG](m) segments. These oligomers have m GG steps, where m = 4 or 6, separated by (A)(n) or (T)(n) segments, where n = 1-7 for the (A)(n) set and 1-5 for the (T)(n) set. Irradiation with UV light that is absorbed by the AQ causes injection of a radical cation into the DNA. The radical cation migrates through the DNA, causing chemical reaction, primarily at GG steps, that leads to strand cleavage after piperidine treatment. The uniform, systematic structure of the DNA oligonucleotides investigated permits the numerical solution of a kinetic scheme that models these reactions. This analysis yields two rate constants, k(hop), for hopping of the radical cation from one site to adjacent sites, and k(trap), for irreversible reaction of the radical cation with H(2)O or O(2). Analysis of these findings indicates that radical cation hopping in these duplex DNA oligomers is a process that occurs on a microsecond time scale. The value of k(hop) depends on the number of base pairs in the (A)(n) and (T)(n) segments in a systematic way. We interpret these results in terms of a thermally activated adiabatic mechanism for radical cation hopping that we identify as phonon-assisted polaron hopping.     

Molecular mass distribution of oligomers in products of tetrafluoroethylene radical telomerization

The molecular mass distributions (MMD) of perfluorinated oligomers in products of tetra-fluoroethylene (TFE) radical polymerization in various solvents (telogens) were determined from an analysis of differential thermogravimetric curves and data of gel permeation chromatography and mass spectrometry. Radiolysis of the telogens generates radicals initiating polymer chain growth. The choice of the solvent and TFE concentration makes it possible to obtain oligomers with the controlled average chain length from 4 for 40 CF₂ fragments and specified terminal groups. The polymerization of TFE in THF and propylene oxide affords oligomers with cyclic terminal groups capable of further polymerization due to ring opening. The appearance of two MMD maxima (low-molecular-weight at n ₁ ～6–8 and high-molecular-weight at n ₂ > 10 shifting towards high n with an increase in the TFE concentration) is caused by the formation of colloidal solutions of oligomers.     

Radical ion states of platinum acetylide oligomers

The ion radicals of two series of platinum acetylide oligomers have been subjected to study by electrochemical and pulse radiolysis/transient absorption methods. One series of oligomers, Ptn, has the general structure Ph-C[triple bond]C-[Pt(PBu3)2-C[triple bond]C-(1,4-Ph)-C[triple bond]C-]n-Pt(PBu3)2-C[triple bond]C-Ph (where x=0-4, Ph=phenyl and 1,4-Ph=1,4-phenylene). The second series of oligomers, Pt4Tn, contain a thiophene oligomer core, -C[triple bond]C-(2,5-Th)n-C[triple bond]C- (where n=1-3 and 2,5-Th=2,5-thienylene), capped on both ends with -Pt(PBu3)2-C[triple bond]C-(1,4-Ph)-C[triple bond]C-Pt(PBu3)2-C[triple bond]C-Ph segments. Electrochemical studies reveal that all of the oligomers feature reversible or quasi-reversible one-electron oxidation at potentials less than 1 V versus SCE. These oxidations are assigned to the formation of radical cations on the platinum acetylide chains. For the longer oligomers multiple, reversible one-electron waves are observed at potentials less than 1 V, indicating that multiple positive polarons can be produced on the oligomers. Pulse-radiolysis/transient absorption spectroscopy has been used to study the spectra and dynamics of the cation and anion radical states of the oligomers in dichloroethane and tetrahydrofuran solutions, respectively. All of the ion radicals exhibit two allowed absorption bands: one in the visible region and the second in the near-infrared region. The ion radical spectra shift with oligomer length, suggesting that the polarons are delocalized to some extent on the platinum acetylide chains. Analysis of the electrochemical and pulse radiolysis data combined with the density functional theory calculations on model ion radicals provides insight into the electronic structure of the positive and negative ion radical states of the oligomers. A key conclusion of the work is that the polaron states are concentrated on relatively short oligomer segments.     

Reaction of SO4˙⁻ with an oligomer of poly(sodium styrene sulfonate). Probing the mechanism of damage to fuel cell membranes

Clarification of the mechanism of degradation of model compounds for polymers used in polymer electrolyte fuel cells may identify intermediates that propagate damage; such knowledge can be used to improve the lifetime of fuel cell membranes, a central issue to continued progress in fuel cell technology. In proton-exchange membranes based on poly(styrene sulfonic acid), hydroxycyclohexadienyl radicals are formed after reaction with HO˙ and thought to decay to short-lived radical cations at low pH. To clarify subsequent reactions, we generated radical cations by reaction of SO(4)˙(-) with oligomers of poly(styrene sulfonic acid) (MW ≈ 1100 Da). At 295 K, this reaction proceeds with k = (4.5 ± 0.6) × 10(8) M(-1) s(-1), both at pH 2.4 and 3.4, and yields benzyl radicals with an estimated yield of ≤60% relative to [SO(4)˙(-)]. The radical cation is too short-lived to be observed: based on a benzyl radical yield of 60%, a lower limit of k > 6.8 × 10(5) s(-1) for the intramolecular transformation of the aromatic radical cation of the oligomer to a benzyl radical is deduced. Our results show that formation of the benzyl radical, an important precursor in the breakdown of the polymer, is irreversible.     

Optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers

The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n=1-4) and their beta,beta'-unsubstituted analogues (Tn, n=1-4) are described. Both series are end capped with phenyl groups to prevent irreversible alpha-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E(PA1) and E(PA2)) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E(PA1) and E(PA2) versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E(PA1) and E(PA2) is reached already at relatively short chain lengths ( approximately 5 EDOT units) in contrast to the Tn series (>10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2.+, T3.+, EDOT2.+, and EDOT3.+ revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy.     

Tailored covalent grafting of hexafluoropropylene oxide oligomers onto silica nanoparticles: toward thermally stable, hydrophobic, and oleophobic nanocomposites

The modification of silica nanoparticles with hexafluoropropylene oxide (HFPO) oligomers has been investigated. HFPO oligomers with two different average degrees of polymerization (DPn = 8 and 15) were first prepared by anionic ring-opening polymerization, deactivated by methanol, and in some cases postfunctionalized by aminopropyl(tri)ethoxysilane or allylamine. The "grafting onto" reactions of these oligomers were then carried out either on bare silica (reaction between a silanol surface and ethoxy-silanized HFPO) or on silica functionalized by amino groups (in an amidation reaction with methyl ester-ended HFPO) or mercapto groups (via the radical addition of allyl-functionalized HFPO). Hybrid nanoparticles thus obtained were characterized by solid-state (29)Si NMR and FTIR spectroscopies as well as elemental and thermogravimetric analyses. The results assessed a significant yield of covalent grafting of HFPO oligomers when performing the hydrolysis-condensation of ethoxylated HFPO on the bare silica surface, compared to the other two methods that merely led to physically adsorbed HFPO chains. Chemically grafted nanohybrids showed a high thermal stability (up to 400 °C) as well as a very low surface tension (typically 5 mN/m) compared to physisorbed complexes.     

Short range order at the amorphous TiO(2)-water interface probed by silicic acid adsorption and interfacial oligomerization: an ATR-IR and 29Si MAS-NMR study

Adsorption and oligomerization of H(4)SiO(4) at the amorphous TiO(2)-aqueous interface were studied using in situ Attenuated Total Reflectance Infrared (ATR-IR) and ex situ solid state (29)Si nuclear magnetic resonance (NMR). The ATR-IR spectra indicate that a monomeric silicate species is present at low silicate surface concentration (Γ(Si)). Above a threshold Γ(Si) linear silicate oligomers are formed and these oligomers dominate the surface at high Γ(Si). Interestingly the ATR-IR spectra of H(4)SiO(4) on the TiO(2) surface are very similar to those previously observed on the poorly ordered iron oxide phase ferrihydrite. The (29)Si NMR spectrum of silicate on the TiO(2) surface shows the presence of Si in three states with chemical shifts corresponding to isolated monomers (Q(0)), the ends of linear oligomers (Q(1)) and the middle of linear oligomers (Q(2)). The ratio of the area of the Q(1) and Q(2) peaks was ≈2:1 which is consistent with the proposed formation of linear silicate trimers by insertion of a solution H(4)SiO(4) between adjacent suitably orientated adsorbed silicate monomers. A structural interpretation indicates that the observed interfacial silicate oligomerization behavior is a general phenomenon whereby bidentate silicate monomers on oxide surfaces are disposed towards forming linear oligomers by condensation reactions involving their two terminal Si-OH groups. The high surface curvature of nanometer sized spheres inhibits the formation of interfacial silicates with a higher degree of polymerization.     

N‐Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli‐Responsive Cyclophanes by Self‐Assembly

Dynamic covalent bonds and their chemistry have been of particular interest both from a fundamental and materials science aspect. Demonstrated herein is that triphenylamine (TPA) and carbazole (Cz), substituted with a dicyanomethyl radical, are useful motifs for dynamic covalent chemistry as they have the appropriate bond strength between monomer units as well as high stability and synthetic simplicity. TPA and Cz units substituted by two dicyanomethyl radicals formed macrocyclic oligomers classified as novel types of azacyclophanes, and in particular, the TPA‐based diradical gave a cyclic dimer in almost quantitative yield. The cyclic oligomers exhibited thermo‐ and mechanochromic behavior resulting from the generation of radical species by intermonomer C?C bond cleavage.     

Radical polymerization of tetrafluoroethylene in solutions of trimethoxysilanes: Formation of colloidal solution and gel of oligomers

Radical polymerization of tetrafluoroethylene (TFE) in solutions of trimethoxysilanes leads to the formation of fluoroalkoxysilane oligomers and the products of their subsequent hydrolysis and dimerization that occur when methoxyl groups are replaced by hydroxyl groups and Si–O–Si links to bind the oligomers are subsequently formed. The chain length of the oligomers increases with the initial TFE concentration, thereby leading to the formation of colloidal solutions. Colloid particles contain oligomers and solvent molecules, the number of which per TFE unit decreases as the chain length grows to 4–6. Partial replacement of the starting solvents, which are also capable of creating a silicone skeleton during polycondensation, makes it possible to control the number of fluoroalkyl chains attached to this skeleton.     

Conjugated chromophore arrays with unusually large hole polaron delocalization lengths

We report variable temperature X-band EPR spectroscopic data for the cation radical states of meso-to-meso ethyne-bridged (porphinato)zinc(II) (PZnn) oligomers. These [PZn2-PZn7]+ species span an average 18-75 A length scale and display peak-to-peak EPR line widths (DeltaBp-p) that diminish with conjugation length. Analysis of these EPR data show that PZnn+ structures possess the largest hole polaron delocalization lengths yet measured; experiments carried out over a 4-298 K temperature domain demonstrate remarkably that the charge delocalization length remains invariant with temperature. These cation radical EPR data are well described by a stochastic, near barrierless, one-dimensional charge hopping model developed by Norris for N equivalent sites on a polymer chain, where the theoretical EPR line width is given by DeltaBp-p(N-mer) = (1/N1/2)DeltaBp-p(monomer); PZnn+ oligomers are the first such systems to verify a Norris-type hole delocalization mechanism over a substantial ( approximately 75 A) length scale. Given the time scale of the EPR measurement, these data show that either (i) Franck-Condon effects are incapable of driving charge localization in [PZn2-PZn7]+, resulting in cation radical wave functions which are globally delocalized over a spatial domain that is large with respect to established benchmarks for hole-doped conjugated materials, or (ii) polaron hopping rates in these oligomers exceed 107 s-1, even at 4 K. Finally, this study demonstrates that polymeric building blocks having low magnitude inner sphere reorganization energies enable the development of electronic materials having long polaron delocalization lengths.     

Isolation and identification of the linear and cyclic oligomers of poly(ethylene terephthalate) and the mechanism of cyclic oligomer formation

Chromatographic techniques are described which can be used to isolate and identify the linear and the cyclic oligomers of poly(ethylene terephthalate). Extraction of the oligomers from high molecular weight polymer produces at least eight different cyclic species, some of which are isolated and identified. The cyclic dimer, the cyclic trimer, and the cyclic tetramer of poly(ethylene terephthalate) have also been prepared by acid chloride esterification and transesterification. Similar materials can be isolated from the ethylene glycol distillate obtained from the polymer melt. The mechanism of cyclic oligomer formation has been studied by determining the rate of formation of the cyclic oligomers during polymerization and during melt extrusion of polyesters which did not initially contain cyclic oligomers. The rate of formation depends upon the concentration of hydroxyl groups; hence, the cyclic oligomers are formed by transesterification from the chain ends or cyclodepolymerization. Therefore oligomers are inevitably produced during polymerization.     

Complete ozonolysis of alkyl substituted ethenes at -60 degrees C: distributions of ozonide and oligomeric products

The distribution of ozonide and oligomeric structures formed on complete ozonolysis of alkenes in a non-participating solvent at -60 degrees C is governed by the alkyl substitution around the carbon-carbon double bond. The ozonolysis of a 1,1-alkyl substituted ethene generally favours the formation of an ozonide (a 1,2,4-trioxolane). Whereas the ozonolysis of a 1,1,2-alkyl substituted ethene also produces ozonide, a considerable amount of the ozonised products are oligomeric in nature. For example, the ozonolysis of 3-methylpent-2-ene in solution to high conversion in pentane yields oligomers with structural units derived from the fragmentation products of the primary ozonide (a 1,2,3-trioxolane) which are namely butanone carbonyl oxide and acetaldehyde; these can be characterised by electrospray ionisation mass spectroscopy (ESI-MS) under soft ionisation conditions. The predominant oligomers formed are rich in carbonyl oxide units (80 + mol%) and are cyclic in nature. A small proportion of the oligomers formed are open chain compounds with end groups that suggest that chain termination is brought about either by water or by hydrogen peroxide. Residual water in the solvent will react with the carbonyl oxides to produce 2-methoxybut-2-yl hydroperoxide, which we propose readily decomposes generating hydrogen peroxide. A significant yield of oligomers also is obtained from the ozonolysis of a 1,2-alkyl substituted ethene. The ozonolysis of trans-hex-2-ene in pentane yields oligomers containing up to four structural units and are predicted to be mainly cyclic.     

Synthesis of new hybrid monomers and oligomers containing cationic and radical polymerizable vinyl groups and their photoinitiated polymerization

New hybrid vinyl monomers with both cationic- and radical-polymerizable vinyl groups were synthesized by the reaction of bis[1(chloromethyl)-2-(vinyloxy)ethyl]terephthalate ( 3 ) with unsaturated carboxylic acids using 1,8-diazabicyclo[5.4.0]-undecene-7 (DBU) as a base. The reaction of 3 with methacrylic acid 4a was carried out using DBU in DMSO at 70°C for 24 h to give an 86% yield of the hybrid vinyl monomer ( 5a ). Polycondensation of 3 with unsaturated dicarboxylic acids was also performed using DBU to give hybrid vinyl oligomers with radical polymerizable C (DOUBLE BOND) C groups (V_R) in the main chain and cationic polymerizable vinyl ether moieties (V_C) on the side chain. The photopolymerization of these hybrid vinyl compounds proceeded smoothly in bulk using either a cationic photoinitiator such as a sulfonium salt or a radical photoinitiator such as acyl phosphine oxide under UV irradiation. © 1996 John Wiley & Sons, Inc.     

Synthesis of C3' modified nucleosides for selective generation of the C3'-deoxy-3'-thymidinyl radical: a proposed intermediate in LEE induced DNA damage

DNA damage pathways induced by low-energy electrons (LEEs) are believed to involve the formation of 2-deoxyribose radicals. These radicals, formed at the C3' and C5' positions of nucleotides, are the result of cleavage of the C-O phosphodiester bond through transfer of LEEs to the phosphate group of DNA oligomers from the nucleobases. A considerable amount of information has been obtained to illuminate the identity of the unmodified oligonucleotide products formed through this process. There exists, however, a paucity of information as to the nature of the modified lesions formed from degradation of these sugar radicals. To determine the identity of the damage products formed via the 2',3'-dideoxy-C3'-thymidinyl radical (C3'(dephos) sugar radical), phenyl selenide and acyl modified sugar and nucleoside derivatives have been synthesized, and their suitability as photochemical precursors of the radical of interest has been evaluated. Upon photochemical activation of C3'-derivatized nucleosides in the presence of the hydrogen atom donor tributyltin hydride, 2',3'-dideoxythymidine is formed indicating the selective generation of the C3'(dephos) sugar radical. These precursors will make the identification and quantification of products of DNA damage derived from radicals generated by LEEs possible.     

Formation of hyperbranched polymers in atom transfer radical copolymerization of MMA and DVB

Hyperbranched poly(methyl methacrylate)s (HPMMAs) have been successfully prepared by atom transfer radical copolymerization of MMA and divinylbenzene (DVB). Kinetic study shows complete consumption of the initiator in 0.5 h, and relatively low polymerization rate when DVB content in the feed was high. By analyzing MALDI-TOF spectra of the resulting copolymers, the linear A _n B* (n = 0, 1, 2, 3) oligomers were formed in 0.5 h of polymerization, and then the oligomers reacted each other to form dimers, further reactions produced HPMMA. The SEC and NMR spectroscopies were used to trace the polymerization, and the results demonstrate that small amount of the branching reactions occur in the initial polymerization, and the branched polymers are significantly generated past a certain conversion depending upon the feed ratios. Raising the content of DVB in the monomer mixture can increase the pendent vinyl groups of the linear oligo-inimers, leading to gelation at low MMA conversion.     

Oligomeric Products in Plasma=Polymerized Organosilicones

Plasma polymers were deposited from a number of methylsilazane and methylsiloxane monomers of linear and cyclic structure. The oligomeric phase evolved from the polymers by a mild thermal treatment was analyzed using gas chromatography/mass spectrometry combined techniques. The oligomeric phase was found to be composed of the low-molecular-weight products with the weight being mostly no higher than two monomer units. The results suggested that the oligomers might be formed in the gas phase and then they diffuse to the surface of the growing polymer film. The structure of the oligomeric products indicated that their formation must proceed via the Si-N and Si-O bonds fission in silazane and siloxane monomers, respectively. An ionic nature of the primary active species involved in this process was assumed owing to the high susceptibility of these bonds toward heterolytic fission. The postulated ionic mechanism, considered to account for the formation of the observed oligomers, seems to be more reasonable than the radical mechanism. Pyrolysis/gas chromatography and infrared investigations of plasma polymers revealed that methylsilyl groups in the monomers were highly susceptible to form disilymethylene and disilymethylene and disylyethylene cross-linkages. This process was assumed to proceed via homolytic fission of the Si-C and C-H bonds in methylsilyl groups and subsequent recombination of formed radical species. Based on these results, the elementary reactions contributing to the overall plasma polymerization processes of both ionic and radical mechanisms were formulated.     

Analysis and purification of peptide nucleic acids by denaturing PAGE

A flexible and convenient protocol for the analysis and purification of peptide nucleic acid (PNA) oligomers and PNA-peptide chimeras by denaturing PAGE is described. Vertical slab gel electrophoresis, 26% in polyacrylamide and 8 M urea at pH 3, was suitable for analysis of oligomers ranging in size from tetramers (4-mers) to tetradodecamers (24-mers). Single-base resolution of oligomers was achieved and separations are generally superior to those given by standard RP-HPLC techniques. The separation of a related series of PNA oligomers showed the distance migrated was linearly dependent on the logarithm of the molecular weight. The migration of oligomers through the gel is dependent on the number of basic functional groups present, such as amino groups, and the A and C content of the oligomer. PNAs are amenable to detection by UV-shadowing technique illuminated at 260 nm or Coomassie blue staining, both with similar, sub-microgram per band detection limits.