Structure and stability of halogenated polymers: Part 1—Ring-chlorinated polystyrene

Ring-chlorinated polystyrene has been prepared by reaction between polymer and chlorine at −20°C in the presence of iodine, using a 1·2:1 molar ratio of chlorine to styrene units. Although the product has a composition corresponding precisely to 1 Cl atom per styrene unit and the predominant site of chlorination is the para position in the aromatic ring, some ortho chlorination, backbone chlorination and unchlorinated structures have been shown to be present by characterisation spectroscopically and from degradation experiments.The chlorinated polymer loses the backbone chlorine readily as HCl at temperatures from 200°C. The resulting unsaturation in the backbone appears to destabilise the polymer towards chain scission and the main breakdown process, which resembles that of polystyrene in consisting of depolymerisation and transfer reactions, occurs over a wider temperature range and at lower temperatures than the decomposition of polystyrene. Products have been identified and estimated quantitatively.     

Resist polymers: Part IX—Thermolysis of chlorine-containing acrylate polymers

The thermal degradation of poly(chloroethyl methacrylate) (PCEMA), poly(trichloroethyl methacrylate) (PTCEMA), poly(methyl-α-chloroacrylate) (PMCA) and their copolymers with methyl methacrylate (MMA) has been investigated. Both ester decomposition and main chain scission occur for the chloroalkyl methacrylate polymers with the former playing the dominant role. In contradistinction, HCl elimination and aromatization prevail over other processes for PMCA. The thermolysis results are compared with radiolysis results.     

Thermal degradation of keto polymers: Part 1—Poly(vinylacetophenone)

The thermal degradation of poly(vinylacetophenone) (PVAP) was studied at 380°C under high vacuum. The degradation products were separated into gaseous and liquid fractions and identified (gc, nmr, ms). The principal reactions are removal and decomposition of acetyl groups, depolymerization and random chain scission, as evidenced by the pattern of number average molecular weight changes. While the basic mechanism of degradation resembles that of poly(styrene) (PS), i.e. initial random chain scission, the number of transfer reactions is considerably greater due to the presence of methyl radicals which abstract from C-atoms in the polymer chain. The resulting chain radicals undergo β-scission (the principal mode of chain scission) to yield a terminally unsaturated molecule and a macro radical. These two species further decompose to give oligomeric products and monomer. The relative abundance of oligomers to monomer is considerably greater than that observed for PS. This has been attributed to shorter zip lengths for depolymerization and to the occurrence of more transfer reactions in PVAP. Plausible mechanisms for the formation of the various reaction products are given.     

Synthesis and Structure of 2—Acetylthiophene Thiosemicarbazone

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Isolation and Crystal Structure of 2—Bromoaldisin

INTRODUCTION2-Bromoaldisin was first isolated from the marine sponge Hymeniacidon aldis [1,2]. In the course of our systematic search for bioactive substances from marine organisms, we have studied the marine sponge Phacellia fusca Schmidt and obtained the title compound. Up to now, there is no report on the crystal structure of 2-bromoaldisin. This paper reports the isolation and crystal structure of it. EXPERIMENTALIsolation of 2-bromoaldisinThe sponge, Phacellia fusca Schmidt wa…     

Structure and stability of short beta-peptide nanotubes: a non-natural representative of collagen?

Since secondary structure elements are known to play a key role in stabilizing the 3D-fold of proteins for the design of non-natural proteins composed of beta-amino acid residues, the construction of suitable secondary structural elements is mandatory. Folding analogues of alpha-helices and beta-strands of beta-polypeptides were already described (Chem. Biodiversity 2004, 1, 1111 (1)). Here, we present several collagen-like folds composed exclusively of beta-Ala(s). Unlike their natural counterpart, these tubular nanostructures can be composed of more than three polypeptide chains aligned parallel and/or antiparallel. By using ab initio and DFT calculations we have optimized a large number of versatile collagen-like antiparallel nanostructures. In these tubular systems, oligopeptide strands are interconnected by i --> (i) type H-bonds, except for the "closing" set. This latter is called "the H-bond zipper" and is either (i) --> i, ( i + 1) --> i, or ( i + 2) --> i type. Antiparallel, tubular foldamers composed of l number of strands, each of k number of beta-amino acid residues (e.g., apbeta-T(l) i+l ) k , ap(beta-T(l) i+1 ) k , or ap(beta-T(l) i+2 ) k ), are unexpectedly stable supramolecular complexes. Independent of k and l, the local backbone fold of the amino acid residues is usually spiral, abbreviated as "S(P)" or "S*(P)". Nevertheless, in contrast to parallel, in antiparallel nanotubes the backbone fold can occasionally twist out from S(P) or S*(P) type into an alternative local structure. However, the more the local geometry of the strands resembles to S(P) or S*(P), the higher the stability is. Besides the backbone twisting, the overall stability is determined by the type and the geometrical properties of the constituent H-bonds. Interestingly, higher number of total H-bonds can provide a lower overall stability, when H-bond parameters are inferior. In general, the increase of both the number of strands and their length stabilize the supramolecular complex. Now that, for beta-peptides, collagen-like overall folds with their stability were determined, their POG- or PPG-like sequence specificity has to be revealed.     

Photo-oxidation of polymers: Part X—The photo-oxidation of an ethylene-propylene copolymer initiated by benzophenone

An EPM film containing benzophenone quickly becomes insoluble on irradiation in air at 365 nm. The ketone initially acts as a photo-initiator of the oxidation of the polymer but later on, when the local concentration of oxidation products is high enough, the photo-oxidation proceeds via the sensitized decomposition of hydroperoxides. The vacuum photolysis of a preoxidized EPM film indicates that carboxylic acids are produced by decomposition of hydroperoxides hydrogen bonded to ketones.     

The phase separation behaviour and the viscoelastic properties of particles with non-adsorbing polymers:: Part II — Theoretical study

The high frequency shear modulus of a series of dispersions undergoing depletion interactions are modelled using a new depletion potential. This model gives rise to a net attraction between colloidal particles with added non-adsorbing polymer once a critical concentration is reached. This critical concentration is of the order of c*. The curvature of this potential allows the use of liquid state or cell models to predict the high frequency shear modulus. This requires a detailed knowledge of the polymer osmotic pressure as a function of concentration. This pressure has been derived from the elasticity and compared with a model for the pressure based on renormalisation group theory. The second Virial coefficient has been used as an adjustable parameter and the value used found to be in good agreement with experimental values for large values of a_eff/R_g. The approach presented here highlights some of the difficulties in predicting the rheological properties for metastable depletion systems.     

Thermal degradation of bromine-containing polymers: Part 10—Copolymers of 2,3-dibromopropyl methacrylate and 2,3-dibromopropyl acrylate with methyl acrylate

The products of the thermal degradation of copolymers of methyl acrylate (MA) with 2,3-dibromopropyl methacrylate (2,3-DBPM) and 2,3-dibromopropyl acrylate (2,3-DBPA) have been analysed quantitatively using thermal analysis, infra-red spectroscopy, mass spectrometry and gas-liquid chromatography. The dominant products from the first of these systems are 2,3-DBPM monomer and chain fragments reflecting the behaviours of the homopolymers of 2,3-DBPM and MA, respectively. The yield of methyl bromide is at a maximum when the proportion of interunit bonds between the two monomers is at a maximum. From the second system, the dominant products are methyl bromide and the chain fragment or cold ring fraction. The mechanisms by which these and the many other products are formed are discussed with reference to related systems which have been studied previously.     

Structure evolution in the PbO-ZrO2-TiO2 sol-gel system: Part I — Characterization of prehydrolyzed precursors

In this investigation, several spectroscopic and analytical techniques were used to determine the chemical compositions and structures of the lead, zirconium, titanium, and Pb-(Zr, Ti) alkoxides involved in the sol-gel synthesis of PZT thin films. These techniques included ¹H, ¹³C, and ²⁰⁷Pb NMR; FT-IR; gas chromatography; Karl Fischer titration; and number-average molecular weights (M _n ) determined by cryoscopy. It was found that the titanium precursor had a M _n of 548 and a formula of [Ti(OCH₂CH₂OCH₃)₄]_1.6; the zirconium precursor had a M _n of 1015 and a formula of [Zr(OCH₂CH₂OCH₃)₄]_2.6; and the lead precursor had a formula Pb₆(OOCCH₃)₅(OCH₂CH₂OCH₃)₇. 4 H₂O and a molecular weight of 2131 (M _n =2113). It was observed that residual water from the incomplete dehydration of lead acetate trihydrate coupled with released water due to the esterification of acetic acid caused M-O-M (M=Pb, Zr, Ti) bonds in the Pb-(Zr, Ti) alkoxide. Two possible isomeric structures of the Pb-(Zr, Ti) alkoxide have been proposed. They are both cyclic and have a formula of Pb₂MMO₂(OR)₈(ROH)₂, (MM=Zr and/or Ti) and a molecular weight of 1336 (M _n =1386).     

Crystal Structure and Electronic Structure of a Luminescent Compound 2—（2—Pyridyl） Benzimidazole

1 INTRODUCTION Imidazole which is a part of histidine, plays an important role as a ligand in bioinorganic chemistry. Up to now, a variety of coordination compounds with imidazole ligands have been synthesized and characterized[1]. Its complexes have a…     

The mechanism of chemical degradation of polymers: Part III—The anomaly in the hydrolysis of glycolide copolymers

The data published on the chemical and enzymatic degradation of polyesters and some other polymers is analysed. Earlier studies on copolymers have shown that chemical stability changes monotonously with change in composition if the corresponding homopolymers differ in their sensitivity to the action of a degrading agent. The present work shows that anomalous dependence of chemical stability on copolymer composition is observed for a copolymer based on the isomeric monomers, glycolide and ethylene glycol oxalate. In this case the minimum in the chemical stability versus copolymer composition curve corresponds to the minimum in the copolymer melting temperature versus copolymer composition curve. A possible interpretation of the observed phenomenon is given.     

Study on thermal stability of poled non-linear optical polymers

A logarithmic expression is proposed to describe relaxation of the polar order in side chain polymers,together with a new way of plotting temperature dependent relaxation data.This results in a straight line extending even below the glass transition temperature in the case of poled nonlinear optics (NLO) side chain polymers.A simple procedure to determine the rotational diffusion constant Dr is given and Dr values of several polymer systems have been evaluated and compared with each other.It appears that,starting from the conventional and well known poled polymer system currently applied,a further lowering of Dr by about 3 orders of magnitude is necessary in order to reach an acceptable orientational stability or lifetime of poled polymers for practical applications.Attempts have been made to introduce electron push-pull substituents into high thermostable molecular frameworks and results of preliminary measurements are reported.     

Structure and stability of molecular and ionic complexes of AlCl3 with pyrazine and 4,4′-bipyridyl

Structural and thermodynamic characteristics of molecular and ionic complexes of aluminum trichloride with pyrazine (pyz) and 4,4′-bipyridyl (bipy) are calculated at the RI-BP86/def2-SVP level. It is found that for molecular 2AlCl₃·3L and 4AlCl₃·3L complexes an energy difference between isomers does not exceed 4 kJ/mol, and the rotation barrier of the AlCl₃ moiety relative to the N-Al-N bond does not exceed 24 kJ/mol. A comparison of the stability of molecular and ionic complexes of aluminum in the gas phase shows that the maximum energy difference is ～60 kJ/mol. For L = pyz the molecular complex is more stable whereas for L = bipy it is the ionic one.     

Antioxidants and stabilizers: Part CVI—Mechanism of antioxidant action of dithiobisphenols

A disulfidic bridge connecting two alkylated phenolic moieties is considered to be a decisive structural factor in the high antioxidant efficiency of dithiobisphenols in polypropylene (PP) autoxidation and tert-butylhydro-peroxide (TBH) decomposition. Strong retardation of the later phases of the autoxidation of PP doped with two dithiobisphenols (I(d), II(d)) is a consequence of the stepwise accumulation of antioxidant-active S-oxidation products. Thermolabile thiosulfonate and sulfoxide are formed (among other transformation products) and are considered to be sources of peroxidolytic species. The latter are involved, as very efficient agents after depletion of the originally present dithiobisphenols, in the overstoichiometric phase of hydroperoxide decomposition. Calcium (II) salts may partially reduce the overall hydroperoxide decomposition efficiency of dithiobisphenols.