1993 рік

Ternary neodymium-based catalysts of the form Nd(carboxylate)a/Al(i-butyl)2H/'e'tbutyl chloride for the high-cis polymerization of 1,3-butadiene have been examined. The carboxylates of a range of neo and iso monocarboxylic acids have been used and their effects on catalyst activity and polymer molecular weight examined. Catalysts prepared using neoacids are more active and give lower molecular weight polymer than catalysts prepared using isoacids. Increasing the chain length of carboxylic acid gives more active catalysts; this trend is more prominent within the isoacids than the neoacids. Increased catalyst activity is explained in terms of the changing nature of the Nd-O bond in the Nd(carboxylate)3 and the ease of alkylation in the formative step of catalyst active site creation.

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Blends of bacterial poly(3-hydroxybutyrate) (PHB) with poly(methyl methacrylate) (PMMA) and poly(cyclohexyl methacrylate) (PCHMA) were prepared by melt compounding followed by quenching to room temperature. PHB/PMMA blends containing up to 20 wt% PHB are single-phase amorphous glasses with a composition dependent glass transition temperature (Tg). When the concentration of PHB exceeds 20 wt %, partially crystalline pure PHB coexists with a constant-composition PHB/PMMA (20/80) mixture, which represents the solubility limit of PHB in PMMA. In such blends crystallization of PHB, both isothermally from the melt and with heating from the rubbery state, is retarded by increasing amounts of PMMA in the blend. PHB shows no miscibility at all with PCHMA. In the PHB/PCHMA blends, two phases, consisting of the pure polymer components, are present over the whole composition range and PHB crystallization is unaffected by the presence of the methacrylate polymer

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Halogenated derivatives of polystyrene that are known to perform as single-stage crosslinking electron-beam resists, such as copolymers of styrene or a methylstyrene with a chlorostyrene or chloromethylstyrene, and similar structures formed by chloromethylation of polystyrene or chlorination of a polymethylstyrene, are shown in general to function through parallel non-interacting radiation-induced crosslinking mechanisms. One of these is the same radical mechanism that would operate within the homopolymers of the constituent comonomers. The other mechanism is characteristic of the systems being copolymers and arises from intramolecular excited-state charge-transfer interactions between adjacent chlorine-containing and non-chlorine-containing units of the polymer chains. Pulse radiolysis studies demonstrate the formation of a transient intermediate in poly(p-methylstyrene-stat-p-chlorostyrene), the spectrum of which is consistent with that of a methylstyryl radical-cation. The radiation chemical yields for crosslinking derived from the lithographic sensitivities of a range of such systems, are modelled in terms of these mechanisms and the variation with copolymer composition is shown to be in excellent accord with experimental observation.

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To resolve long-standing uncertainties surrounding the bridge-bond angles of crystalline poly(aryletherketone) s, the structure of an oligomeric ether-ketone, CIArCOArOArCOArC1 (Ar = 1,4-phenylene), has been determined by single crystal X-ray methods. The average torsion angle ~b of the two 'ether-ketone' rings, relative to a plane defined by the bridging oxygen and carbonyl-carbon atoms is 31 °. The bond angles at carbonyl-carbon and ether-oxygen are 121.9 (3) ° and 121.2 (3) °, respectively, values which are substantially lower than those consistently reported for poly(aryletherketone)s on the basis of X-ray powder- and fibre-diffraction studies. [An average bridge-bond angle of 126.5 (6) ° has, for example, been proposed to account for the c-axis length of 10.09(2)• in the prototype poly(etherketone) (PEK) (-OArCOAr-),.] Despite this apparent difference in bridge-bond angles, a PEK c-axis distance of 10.11 (1)A, entirely consistent with that of the polymer, is found in the present oligomer structure. More detailed analysis of the oligomer structure reveals a pattern of sterically induced bond angle distortions at the aromatic carbon atom linked to an ether or ketone bridge, a pattern that is repeated in the X-ray structure of a second oligomer ArOArCOArArCOArOAr (Ar = phenyl or 1,4-phenylene). These findings strongly suggest that bridge-bond angles in poly (aryletherketone)s actually lie in the region of 121-122 °, and that the range 125-127 ° consistently reported in the literature is an artifact deriving from the assumption of ideal geometry elsewhere in the polymer chain.

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Four methods for the nitration of polystyrene are compared: (i) direct nitration with a nitric acid/sulfuric acid mixture; (ii) nitration in carbon tetrachloride with acetyl nitrate; (iii) nitration in N,N'- dimethylformamide with a nitric acid/sulfuric acid mixture; and (iv) nitration in 3-nitrotoluene with a nitric acid/sulfuric acid mixture. It is demonstrated that the first three methods give products with low degrees of substitution and lead to a broadening of the molecular weight distribution, but that method (iv) gives poly(4-nitrostyrene) with minimal effect on the breadth of the molecular weight distribution. The glass transition temperature for poly(4-nitrostyrene) is found to approach 194.6°C at high molecular weights.

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For isotactic polypropylene, the enthalpy and entropy differences between a glass and a crystal, AH g~ and AS g~, and between a glass and a liquid, AH "g and AS as, were discussed via conformational analysis. AH g~ and AS gc are mostly attributed to the difference in bending and torsional vibration modes between a glass and a crystal. AH ag and AS ag are nearly equal to the sum of h c°nf and hint, and that of (s ?°nf- s~ °"f) and s i~t, respectively. Here h e°nf and s ~°~f are the conformational enthalpy and entropy per molar structural unit, s~ °"f (=0.38 J mol-1 K-~) is s ~°"f at 0 K, and h int and s i"t are the intermolecular interaction enthalpy and entropy per molar structural unit. The value of AH ag at 0 K, 5.6 kJ mol- x, is almost equal to the cohesive energy of a CH 3 residue, 5.7 kJ mol- 1. In a glass, the existence of localized ordered regions with enthalpy 6.78 to 9.29kJmol -~ was predicted. The conformational analysis further showed that many rod-like sequences with right- and left-handed helices may be contained in a glass.

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Atomic force scanning microscopy (AFM) was used to investigate the dispersion of low molecular weight compounds in ethylene-propylene copolymers (EPM). Where other microscopical techniques failed to provide morphological details of this type of blend, as a result of the restricted resolution (light microscopy) or the volatility of the low molecular weight component (SEM), the AFM technique provided surface images, which show inclusions in the matrix of the uncrosslinked polymers.

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Electron spin resonance (e.s.r.) spectra of polystyrene (PS) samples labelled at chain ends or at in-chain sites with nitroxide radicals were recorded over a temperature range. Up to 423 K for the end-labelled PS and 453 K for the in-chain labelled PS, the spectra are essentially of the broad-line type and in both cases the spectral changes are reversed exactly on cooling. Both labelled PS samples were blended with polyisoprene (PIP) (1:1, w/w). The e.s.r, spectra of the blend of the in-chain labelled PS are identical to those of the bulk labelled PS. The e.s.r, spectra of the blend of the end-labelled PS reveal, as previously, that PIP plasticizes PS chain ends. Also, for this blend, the spectral changes produced on heating are not reversed exactly on cooling, and even at room temperature a motionally narrowed component persists. It appears that after heating, the original freeze-dried polymer is reorganized so that more nitroxide chain ends are located in a PIP-rich environment. It is concluded that the e.s.r, experiments confirm the theoretical prediction of Helfand that at thermodynamic equilibrium the interphase in a blend of immiscible polymers contains more than the statistical concentration of chain ends.

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The effect was investigated of the addition of different protic compounds, such as water, n-butanol, butanoic acid and methyl ethyl ketone, to triisobutylaluminium used as the cocatalyst for an Hf-Ti bimetallic catalyst supported on magnesium dichloride (Til.0Hfl.3Mg2.oAlo.85C112.45) in the polymerization of ethylene. All the above compounds show a similar behaviour by decreasing catalyst activity, increasing polymer molecular weight and narrowing polymer molecular weight distribution. Minor differences are observed in the end-group unsaturations. An attempt has been made to correlate these effects to the interaction of hafnium and titanium sites with the reaction products from A1R3 and the different protic compounds.

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The effect of pressure on the phase diagram of n-triacontane (C30)/n-docosane (C22) binary mixture and n-triacontane (C30)/n-hexaeosane (C26) binary mixture was studied up to 500MPa by high-pressure differential thermal analysis. The C30/C22 mixture at 0.1 MPa has a eutectic-type phase diagram, with the hexagonal phase appearing just below the melting temperature on the C3o side. The eutectic temperature, Te, is 45.1°C and the eutectic composition lies between 15 and 20 wt% C3o. The hexagonal phase and the low-temperature transition disappear above 100 MPa. The change of the phase diagram with pressure was interpreted by the different pressure dependence of the melting temperature, Tin, of C3o and C22 and that of T~. Pressure dependence of Te was 0.247 K MPa- 1, which is smaller than that of C22 (0.279 K MPa 1) and C3o (0.276 K MPa-1). The C30/C26 mixture has a solid-solution-type phase diagram, showing an almost linear melting curve from the Tm of C3o to the Tm of C22, and the lower critical transition temperature on the hexagonal transition curve at 0.1 MPa. At elevated pressure, the melting curve changes to become downwardly convex and the transition curve moves close to the melting curve.

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