Structure Of End Groups Research Articles

Synthesis, structures and thermal properties of new class epoxide-terminated telechelic poly(2,6-dimethyl-1,4-phenylene oxide)s

Dihydroxyl telechelic poly(2,6-dimethyl-1,4-phenylene oxide)s ( 1) with regiocontrolled end-group structures have been synthesized by oxidative polymerization of 2,6-dimethylphenol with various aromatic diols in the presence of CuBr/dibutylamine catalysts. The novel one- or two-armed telechelic derivatives based on aromatic diols as core and 1 as arms were subsequently epoxidized with epichlorohydrin and a series of new epoxidized poly(2,6-dimethyl-1,4-phenylene oxide)s ( 2) were accessible with number average molecular weight ranged from 3500 to 14,000. The end-group structures and regioselectivity of polymers were controlled by the CuBr/dibutylamine catalysts under different reaction conditions, and the structures and properties were studied by nuclear magnetic resonance spectroscopy, gel permeation chromatography, thermogravimetric analysis and differential scanning calorimetry. Upon heating in the presence of oxygen, the one-armed dihydroxyl telechelic polymer was converted via intermolecular coupling and redistribution reaction to the two-armed derivative with significant increase in its molecular weight and elimination of diol monomer. Treatment of dihydroxyl telechelic derivatives with epichlorohydrin and NaOH afforded the epoxide-terminated telechelic derivatives in 77–94% yields.

Effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives

This paper describes the effects of ultra-thin liquid lubricant films on contact slider dynamics in hard-disk drives. In the experiments, the contact slider dynamics as well as ultra-thin liquid lubricants behavior are investigated using three types of lubricants, which have different end-groups and molecular weight as a function of lubricant film thickness. The dynamics of a contact slider is mainly monitored using acoustic emission (AE). The disks are also examined with a scanning micro-ellipsometer before and after contact slider experiments. It is found that the lubricant film thickness instability occurs as a result of slider–disk contacts, when the lubricant film thickness is thicker than one monolayer. Their unstable lubricant behavior depends on the chemical structure of functional end-groups and molecular weight. In addition, it is also found that the AE RMS values, which indicate the contact slider dynamics, are almost same, independent of the end-groups and molecular weight for the lubricants, when the lubricant film thickness is approximately one monolayer. The molecular weight, however, affects the contact slider dynamics, when the lubricant film thickness is less than one monolayer. In other words, the AE RMS values increase remarkably as the molecular weight for the lubricant increases. When the lubricant film thickness is more than one monolayer, the AE RMS values decrease because of the effect of mobile lubricant layer, while the lubricant instability affects the contact slider dynamics. Therefore, it may be concluded that the lubricant film thickness should be designed to be approximately one monolayer thickness region in order to achieve contact recording for future head–disk interface.

The formation of aromatic end group structures in O-alkyl substituted celluloses under alkaline pulping conditions

Abstract UV resonance Raman (UVRR) spectroscopy was used to study changes in the structure of reducing end groups of O-alkyl substituted celluloses under conditions simulating the initial phase of soda pulping. The derivatives studied were carboxymethyl cellulose (CMC) with a degree of substitution (DS) of 0.58 and methyl cellulose (MC) with DS of 1.64–1.95. Hot alkaline treatments were carried out in solutions of sodium hydroxide (0.1–1 M) at 95°C in the presence of creosol as a lignin model compound. Alkali-stable aromatic end groups were generated in both cellulose ethers. Moreover, the end groups of methyl cellulose were extensively condensed with creosol. The results demonstrate that aromatisation and condensation depend on the pH and reaction time.

Metallocene Catalysts for Olefin Oligomerization

AbstractMetallocene complexes with a cocatalyst such as methylalumoxane, MAO can be used as catalysts in the dimerization or oligomerization of olefins (alkenes) including the (co‐/hydro‐)oligomerization of ethene, of the α‐olefins propene, 1‐butene, 1‐pentene, 1‐hexene, and higher α‐olefins, branched α‐olefins, cyclic olefins and α,ω‐dienes. Possible applications of α‐olefin oligomers are summarized. Oligo‐olefins can be analyzed by gel permeation chromatography (GPC) or 1H NMR spectroscopy for molecular weight determinations and by 1H and 13C NMR spectroscopy or MALDI‐TOF mass spectrometry for their end group structure.

A Computational Study of End‐Group Conformational Energy Barriers in Carotenoids

Introduction Carotenoids possessing different end-groups are known to be selectively metabolized in biological systems and there is evidence demonstrating that end-groups affect the ability of carotenoids to adopt specific geometries within larger biological structures such as membranes. In order to further assess the importance of the end-group structure in carotenoids we have investigated the influence of the end-group type and functionality on the conformational energy barrier. Methods We used the density functional method and the program GAUSSIAN to calculate the conformational energy curves for rotation of the beta-ring or the epsilon-ring relative to short polyene chains around the C6-C7 single bond. Results Epsilon rings have been shown to possess a dramatically larger conformational energy barrier to rotation than beta rings. The rotational energy barrier of the epsilon ring relative to the polyene chain around the C6-C7 bond has been found to range from 8.65 to 9.05 kcal/mole whereas the barrier for rotation around the beta-ring varies from 3.06 to 3.40 kcal/mole. Conclusions Conjugation of the beta-ring has no influence on the conformational energy barrier to rotation in carotenoid systems possessing this end-group. The length of the polyene chain (extent of conjugation) and substitution on the ring have little influence on the rotational energy barrier. The most important factor influencing the conformational energy for ring rotation about the C6-C7 bond is the steric interaction between the ring and polyene methyl groups. The high barrier to rotation for the epsilon ring provides a physical basis for the observed differences in the metabolism and properties of carotenoids possessing this end-group in biological systems.

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry with size‐exclusion chromatographic fractionation for structural characterization of synthetic aliphatic copolyesters

We report matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) and off-line coupling of size-exclusion chromatography with MALDI-TOFMS analysis (SEC/MALDI-TOFMS) methods for the detailed characterization of poly[(R,S)-3-hydroxybutyrate-co-L-lactic acid], P[(R,S)-3HB-co-LA], and poly[(R,S)-3-hydroxybutyrate-co-epsilon-caprolactone], P[(R,S)-3HB-co-CL], copolymer samples which are expected to be used in special medical application as scaffolds for cartilage and soft tissue engineering. The novel copolyesters contained randomly distributed (R,S)-3-hydroxybutyrate structural units, were synthesized by transesterification of the corresponding homopolymers, i.e. atactic poly[(R,S)-3-hydroxybutyrate], a-PHB, and poly(L-Lactide) (PLLA) or poly(epsilon-caprolactone) (PCL), respectively. The MS methods used for the characterization of the resulting polydisperse copolyester samples were supported by classical methods (NMR, SEC). The structures of individual copolyester macromolecules, including end-group chemical structures, were established using initially MALDI-TOFMS and then SEC/MALDI-TOFMS. The compositions of the copolyesters were determined by two methods, namely based on 1H NMR and MALDI-TOF spectra. The two sets of values showed good agreement. The sequence distribution was determined using the signal intensities of individual copolyester macromolecules, which appeared in MALDI-TOF mass spectra. Furthermore, sequence analysis gave information about the degree of transesterification. The copolyesters synthesized, with only one exception, were demonstrated to be almost random, which implies that the ester-ester exchange was close to completion.

Molecular spectroscopic characterisation of poly(methyl methacrylate) generated by means of atom transfer radical polymerisation (ATRP)

A combination of nuclear magnetic resonance (NMR) spectroscopy, matrix-assisted laser desorption/ionisation-time of flight (MALDI-TOF) mass spectrometry, matrix-assisted laser desorption/ionisation-collision induced dissociation (MALDI-CID) mass spectrometry and Fourier-transform infra-red (FTIR) spectroscopy has been employed for the characterisation of low molecular weight poly(methyl methacrylate) (PMMA) that was generated by means atom transfer radical polymerisation (ATRP). Evidence for the presence of a variety end group structures is shown. The terminating end groups included a five-membered lactone ring as well as the expected halogen (bromine or chlorine). Oligomers with other terminating end groups are proposed to be present at low abundance in the polymers.

Effects of end group polarity and molecular weight on the lower critical solution temperature of poly(N‐isopropylacrylamide)

AbstractThe lower critical solution temperatures (LCSTs) for mass fractionated samples of poly(N‐isopropylacrylamide) (PNIPAM) were studied to determine the effect of polymer molecular weight on the LCST using a high throughput temperature gradient apparatus. PNIPAM fractions prepared by a conventional radical polymerization using azoisobutyronitrile (AIBN) as the initiator had LCSTs that were largely invariant with molecular weight or dispersity. Only slight deviations were noted with lower molecular weight samples. An 18‐kDa sample had a 0.6 °C higher LCST. A 56‐kDa sample had a 0.2 °C higher LCST. PNIPAM derivatives prepared with a triphenylmethyl (trityl) functionalized azo initiator were also prepared and mass fractionated. These samples' LCSTs were identical to those of PNIPAM samples prepared using AIBN initiation when higher molecular weight samples were compared. The trityl‐containing PNIPAM fractions' LCSTs varied when the molecular weight decreased below 100 kDa. Acidolysis of the trityl end groups provided a third set of PNIPAM derivatives whose LCST differed only with samples with Mw values < 60 kDa. These results show there is no effect of molecular weight on LCST until the degree of polymerization is such that end group structure becomes significant. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1492–1501, 2006

The effect of solvent on the microstructure, nature of hydroxyl end groups and kinetics of polymerization reaction in synthesize of hydroxyl terminated polybutadiene

Hydroxyl terminated polybutadiene was synthesized by solution radical polymerization of 1,3-butadiene in the presence of hydrogen peroxide as initiator and 2-propanol,dioxane, ethylene glycol ethyl ether and also the mixtures of them as solvents. Obtained HTPBs in the presence of various solvents were characterized by NMR spectroscopy. It was observed that the microstructure of the back-bone as well as the chemical structure of hydroxyl end groups of HTPBs depended on the type of solvent used in polymerization reaction. A HTPB with microstructure of 45/22/33 ( trans/vinyl/ cis) was synthesized in the presence of ethylene glycol ethyl ether whereas a microstructure of 56/22/22 and 51/22/27 were obtained for 2-propanol and dioxane, respectively. As molar ratio of 2-propanol to ethylene glycol ethyl ether was changed, the secondary hydroxyl end groups were increased. The kinetic studies showed that the type and amount of solvent affected the rate of reaction seriously. It is noteworthy that the polymerization reaction is the first-order in all solvents but the rate of polymerization reactions in presence of dioxane, ethylene glycol ethyl ether and 2-propanol were obtained different. In 2-propanol, the rate coefficient increased as the amount of initial molar ratio of solvent to monomer decreased.

Structure determination of the amorphous perfluorinated homopolymer: Poly[perfluoro(4-vinyloxyl-1-butene)

The low molecular weight samples were examined to assign the nuclear magnetic resonance (NMR) signals, and to study the cyclic structure and the end group structure for poly[perfluoro(4-vinyloxyl-1-butene)]. While the penta-cyclic structure was dominant, the hexa-cyclic structure also existed. The ratio was penta/hexa = 0.84/0.16. No preference of cyclic structure (penta- or hexa-cyclic) changed from the initial to the final stage of polymerization. The polymerization temperature did not affect to the ratio of cyclic structures, whereas the modification at side chain part could. The end group was observed, (CH 3) 2CHOCOO for initiator end, CF 2Cl for chain transfer end, then only initiator end group changed to CF 3 with fluorination.

Behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives

In this paper, we describe the behavior of ultrathin liquid lubricant films for contact sliders in hard disk drives. In the experiments, the ultrathin liquid lubricant film behavior is investigated using Zdol and cyclotriphosphazene-terminated PFPE lubricant which have different end groups as a function of lubricant film thickness. The disks are examined with a scanning microellipsometer before and after contact slider experiments. It is found that the lubricant film thickness profiles almost do not change, when the lubricant film thickness is less than one monolayer. It can also be observed that lubricant film thickness instability due to dewetting occurs as a result of slider-disk contacts for the tested lubricants and the films undergo spontaneous redistributions, resulting in significantly nonuniform film thickness profiles, when the lubricant film thickness is thicker than one monolayer. In addition, it is found that the observed behavior of ultrathin liquid lubricant films for cyclotriphosphazine-terminated PFPE lubricant contrasts markedly with that for Zdol. The difference between cyclotriphosphazene-terminated PFPE lubricant and Zdol is only the functional end group. Therefore, it may be concluded that their unstable lubricant behavior depends on the chemical structure of functional end groups.

Structural Analyses of Methyl Bicyclobutane-1-carboxylate Oligomers Formed with tert-Butyllithium/Aluminum Bisphenoxide and Mechanistic Aspect of the Polymerization

Anionic polymerization of methyl bicyclobutane-1-carboxylate (MBCB) with tert-butyllithium (t-BuLi)/bis(2,6-di-tert-butylphenoxy)ethylaluminum [EtAl(ODBP)2] in toluene at −78 °C gives transtactic polymers (trans contents > 90%) with high yields, but with low initiator efficiency and broad molecular weight distribution. In order to obtain information on the initiation mechanism, oligomers of MBCB were prepared with t-BuLi/EtAl(ODBP)2 in toluene at −78 °C, and fractionated into dimer to pentamer by size exclusion chromatography. The dimer fraction was further subjected to recycling SEC and found to consist of two types of dimers of different chemical structures. 1H NMR analyses of the dimers revealed that one type of the dimer comprises of an initiating tert-butyl (t-Bu) group and two MBCB units while the other contains one t-Bu carbonyl group within the monomeric units in addition to the initial t-Bu group. Based on the NMR spectroscopic information on the oligomers, 1H NMR spectra of the polymer was investigated with the focus on the end-group structures, that confirms that the carbonyl addition of t-BuLi to MBCB occurred and the ketonic compound thus formed participates in the polymerization.

Tandem Mass Spectrometry Characteristics of Polyester Anions and Cations Formed by Electrospray Ionization

Electrospray ionization of polyesters composed of isophthalic acid and neopentyl glycol produces carboxylate anions in negative mode and mainly sodium ion adducts in positive mode. A tandem mass spectrometry (MS/MS) study of these ions in a quadrupole ion trap shows that the collisionally activated dissociation pathways of the anions are simpler than those of the corresponding cations. Charge-remote fragmentations predominate in both cases, but the spectra obtained in negative mode are devoid of the complicating cation exchange observed in positive mode. MS/MS of the Na(+) adducts gives rise to a greater number of fragments but not necessarily more structural information. In either positive or negative mode, polyester oligomers with different end groups fragment by similar mechanisms. The observed fragments are consistent with rearrangements initiated by the end groups. Single-stage ESI mass spectra also are more complex in positive mode because of extensive H/Na substitutions; this is also true for matrix-assisted laser desorption ionization (MALDI) mass spectra. Hence, formation and analysis of anions might be the method of choice for determining block length, end group structure and copolymer sequence, provided the polyester contains at least one carboxylic acid end group that is ionizable to anions.

Effects of functional end-groups on nano-tribology characteristics of ultra-thin liquid lubricant films in hard disk drives

This paper describes the effects of functional end-groups on lubricant redistribution of ultra-thin liquid lubricant films in hard disk drives. In the experiments, the spreading characteristics of ultra-thin liquid lubricant films have been investigated by using the disks with two kinds of lubricants which have different end-groups. In other words, the tested lubricants were conventional Zdol and novel cyclotriphosphazene-terminated PFPE lubricant. It was found that the mobility of novel lubricant film is lower than that of conventional Zdol and these characteristics depend on the interactions between the end-groups of the lubricants and carbon surfaces. The effect of molecularly thin lubricant films on nano-spacing flying head slider hysteresis behavior for touch-down and take-off process has been studied and the dynamic interactions between the slider and the tested lubricants films in near contact regime have been clarified. In addition, the behavior of two kinds of lubricants films for contact sliders also has been investigated. It could be observed that the lubricant film thickness instability occurs as a result of slider-disk contacts for the tested lubricants. Their unstable lubricant behavior depends on the chemical structure of functional end-group. Therefore, it may be concluded that the functional end-groups play a very important role and affect the tribological performance of lubricants in head-disk interface.

Resistance of Fomblin Z-Type Lubricants to Lewis Acid-Catalyzed Decomposition: Effect of the Chemical and Electronic Structure of End-Groups

In the present paper, the effect of the chemical and electronic structure of the Fomblin Z-type lubricant end-group on the Lewis acid-catalyzed decomposition is studied by both Thermogravimetric Analysis (TGA) and Density Functional Theory (DFT) molecular orbital calculations. TGA results of the mixture of Z-type lubricants and ZrO 2 show that the structure of the end-groups significantly affects the thermal stability of the lubricants in the presence of a Lewis acid. The DFT calculation results suggest that the reactivity of the end-groups and thus the resistance of various lubricants to Lewis acid-catalyzed decomposition are affected by the lubricant molecular orbital structure.

Synthesis and Characterization of Triptych μ-ABC Star Triblock Copolymers

We describe a general procedure for the synthesis of miktoarm star triblock copolymers with a hydrocarbon, a fluoropolymer, and a hydrophilic segment. Several μ-(polyethylethylene)(poly(ethylene oxide))(poly(perfluoropropylene oxide)) [μ-(PEE)(PEO)(PFPO)] star triblock copolymers were prepared using two successive anionic polymerization steps and one polymer−polymer coupling reaction. Initially, living polybutadienyllithium chains were end-capped with 2-methoxymethoxymethyloxirane forming a heterobifunctional 1,2-polybutadiene (PBD) precursor, with a hydroxyl group and a protected hydroxyl group at one chain end. Catalytic hydrogenation of this PBD gave the corresponding polyethylethyene (PEE) while preserving the end group structures. Transformation of the terminal hydroxyl group in the PEE precursor to a potassium alkoxide followed by addition of ethylene oxide and subsequent end-capping with ethyl bromide generated polyethylethylene−poly(ethylene oxide) (PEE−PEO) diblock copolymers with a protected hydroxyl group at the junction. Deprotection of the methoxymethyl group followed by coupling with acid chloride end-capped PFPO yielded well-defined μ-(PEE)(PEO)(PFPO) star triblock copolymers. Detailed molecular characterization of these products and their precursors confirmed the composition and architecture of these new star block copolymers. This modular strategy represents a new, straightforward, and versatile methodology for the preparation of mixed arm star block copolymers.

Structure?property study of waterborne, polyurethane acrylate dispersions based on hyperbranched aliphatic polyester for UV-curable coatings

A series of waterborne, hyperbranched polyurethane acrylates for aqueous dispersions (WHPUDs) based on hydroxy-functionalized hyperbranched aliphatic polyester Boltorn H20 were synthesized and characterized by Fourier transform infrared spectroscopy (FTIR). The aqueous dispersions were electrostatically stabilized with carboxyl groups incorporated into their structures, which were neutralized by triethylamine (TEA). The effects of chemical structures of end groups on various properties of WHPUDs, such as particle size, interfacial tension, and rheological behavior were investigated. The average particle sizes of aqueous dispersions, 43–237 nm, were determined by laser light scattering. Owing to the enlargement of the stabilization site, the particle size decreased as the content of carboxyl group, degree of neutralization, and dielectric constant of the dispersion medium increased. Moreover, the surface tension of aqueous dispersions of WHPUDs decreased as the TEA/COOH mole ratio and degree of neutralization increased. The investigations of the rheological behavior of the WHPUDs suggested that all the dispersions belong to pseudoplastic fluids, and each of them has viscosity much lower compared with the commercial water-based resin EB 2002.

Determination of end-group structures and by-products of synthesis of poly( α,β-malic acid) by direct polycondensation

We studied the synthesis of high molecular weight α,β-PMA derivatives by direct polycondensation to fabricate α,β-PMA for medical use. The terminal group of the polyester is important because it significantly influences hydrolysis of the main polymer chain. The molecular weight of about 4000 compounds at a variety of temperatures showed that the terminal olefinic group ratio changed from 0.23 at 110 °C to 0.30 at 120 °C and 0.60 at 130 °C. The highest α,β-PMA molecular weight, 5300, synthesized by direct polycondensation had a terminal hydroxy group ratio of 0.57. The main by-product of these reactions was unreacted l-malic acid at 110 °C and fumaric acid at 130 °C. The fumaric acid ratio of by-products increased with reaction time. We clearly determined the reaction mechanism for l-malic acid at 110–130 °C.

End‐group effect on physical aging and polymer properties for poly(ether sulfones)

AbstractThe physical aging properties of amorphous thermoplastics having various terminal groups were investigated with creep recovery and linear dilatometry. The structure of the chain end groups affected physical aging at lower molecular weights; however, above the critical molecular weight for entanglements the end‐group effect on aging diminished. Experimental densities and glass‐transition temperatures were also end‐group dependent. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2850–2860, 2003

Enzyme-Catalyzed Polymerization Mechanism of Aliphatic Polyester Investigated by 1H NMR and MALDI-TOF Mass Spectrum

Enzyme-catalyzed polymerization mechanism of adipic acid and hexanediol was investigated by proton nuclear magnetic resonance (1H NMR) and MALDI-TOF mass spectra. When the stoichiometric substrates were used for the reaction, 1H NMR measurements showed that the hydroxyl terminated product was preferentially formed at the early stage, then with reaction time it changed to carboxyl terminated product. This process was also confirmed by MALDI-TOF mass spectra. First the chain propagation follows the step-growth mechanism accompanying the sequential addition of a diacid-diol unit. In this case, an enzyme-substrate complex is attacked by hydroxyl group of substrates, and as a result the hydroxyl terminated product was preferentially formed. And at the later stage of the reaction, the transesterification plays an important role to form the carboxyl terminated product. When an excess diacid was used, the end group structure showed the same behavior as the stoichiometric condition, i.e., the predominant end group is consisted of the hydroxyl group at the initial stage then it rapidly changed to the carboxyl group although the time to change to carboxyl terminated product became shorter. In case of the condition of an excess diol, the reaction was avoided at the early stage because no enzyme forms a complex with diacid.