Sequence distribution of cis-1,4- and trans-1,4-units in polyisoprenes

The ¹³C n.m.r. spectra of chicle polyisoprene and cis-trans isomerized 1, 4-polyisoprenes were studied. The splittings of signals were observed in the C₁, C₂, and C₄ carbon signals of the isomerized polyisoprenes. The newly appearing signals were assigned to the carbon atoms in cis-trans linkages. The fractions of the diad sequences (trans-trans, trans-cis, cis-trans, and cis-cis) were determined by using the four signals of C₁ carbon. It was found that the cis-1, 4- and trans-1, 4-units were randomly distributed in the isomerized polyisoprenes and it was confirmed that the chicle polyisoprene was a mixture of cis-1, 4- and trans-1, 4-polyisoprenes.     

Distribution of isomeric structures in polyisoprenes

The ¹³C-n.m.r. spectra of polyisoprenes were investigated. Polyisoprenes were prepared with $\text{n-Buli}\text{Et}{}_2\text{O}$, radical (emulsion polymerization), and Alfin catalysts. Poly(isoprene-4,4-d₂)s were also prepared with $\text{n-BuLi}\text{Et}{}_2\text{O}$ catalysts for the signal assignments. ¹³C-n.m.r. signals were assigned for dyad or triad sequences of cis-1,4-, trans-1,4-, and 3,4- units. Signals due to head-to-head (4,1-1,4) and tail-to-tail (1,4-4,1) linkages were also assigned. On the basis of signal assignments it was revealed that cis-1,4-, trans-1,4-, and 3,4-units were distributed almost randomly in $\text{n-BuLi}\text{Et}{}_2\text{O}$ catalysed polyisoprenes. It was confirmed that radical and Alfin polyisoprenes contained about 15% of each of head-to-head and tail-to-tail linkages. It was found that radical polyisoprene had cis-1,4- and trans-1,4-units distributed randomly along the polymer chain regardless of the head and tail arrangements.     

Effect of conjugated trienes on the oxidation of polyisoprenes

The effect of low concentrations of conjugated triene on the rate of oxidation of polyisoprenes has been investigated. At concentrations similar to those expected to occur in sulfur vulcanizates of NR, conjugated trienes exert a significant accelerative effect on both the uninhibited and the inhibited oxidations of squalene. This accelerative effect is also present, but to a lesser extent, in the inhibited oxidation of carbon–carbon cross-linked NR vulcanizates.     

NMR分析聚苯乙烯序列结构

用高分辨率的核磁共振谱仪分析聚苯乙烯(PS)结构的序列分布,对PS碳谱和氢谱进行归属,归属出聚苯乙烯四单元结构序列,得出聚苯乙烯的序列分布在误差允许的范围内遵循伯努利统计。     

Kinetic treatment for the photodimerization reaction of modified natural polyisoprenes

Summary Natural polyisoprenes modified by the introduction of cinnamoyl groups have been obtained as films. In these polymers, aliphatic-(CH₂)_n- spacers of different lengths separate the main chain from the photosensitive group. Density measurements have been carried out, and the kinetic treatment of the photodimerization reaction is described. The relation between Tg and kobs to the size of the aliphatic spacer suggests that, at least for n=2 to 6, other factors may be affecting the reactivity of the cinnamoyl group.     

按序列分布混沌与SS混沌不等价

构造了一类是按序列分布混沌,但不是SS混沌的极小子转移,从而证明了对于限制在测度中心上的紧系统而言,按序列分布混沌一般地不等价于SS混沌。     

Photosensitivity of modified natural polyisoprenes as function of the alipathic side chain

Summary Modified natural rubber products were prepared by reaction with maleic anhydride followed by ring opening and condensation with different oxyalkylcinnamate esters. The values of Tg obtained for these photocrosslinkable polymers showed a complex relation to the number of C atoms of the aliphatic moiety. The kinetics of the dimerization reaction for thin films was followed by UV spectrometry. The film thickness plays an important role on conversion up to a threshold value, characteristic of each product. The expected relation between Tg and kobs was observed for products when Tg values were markedly different.     

A comparative study of stress-induced crystallization of guayule,hevea, and synthetic polyisoprenes

A comparative study of the stress induced crystallization of natural (Hevea, Guayule) and synthetic high cis-1,4-polyisoprenes at room temperature is presented. Hevea and Guayule are both found using WAXS to crystallize into the unit cell of Morss and Bunn previously described for Hevea. DSC studies of polyisoprenes held at ?25°C indicate that Hevea and Guayule crystallize more rapidly than synthetics. Studies of relative birefringence and stress decay following uniaxial extension confirm this. Under conditions of comparison at the same stress, raw Hevea crystallizes more rapidly than extracted Hevea, Guayule, and the other polyisoprenes.     

Preparation and characterization of polyisoprenes and polybutadienes having 1,2- and 3,4-linkages preferentially

Polyisoprenes (PI) and polybutadienes (PB) both having vinyl-type side chains preferentially were prepared and characterized. Both polymers were anionically polymerized with cumyl potassium or pottassium naphthalenide as initiators in a polar solvent, tetrahydrofuran, at low temperature. From the ¹H NMR measurement, the PI contains 3,4- and 1,2-microstructures and PB does 1,2-microstructure preferentially. All the samples covering the molecular weight range of 37 k ≤ M_w ≤ 724 k for PI and 35 k ≤ M_w ≤ 197 k for PB were confirmed to have narrow molecular weight distribution. Measured glass transition temperatures, i.e., 11.0 °C for PI and ?0.7 °C for PB are both considerably high compared with those of 1,4-microstructure-rich analogues. Intrinsic viscosity measurements in 1,3-dioxane for PI and 2-octanol for PB were carried out, and the segment lengths of the 3,4-/1,2-rich PI and 1,2-rich PB were estimated to be 0.60 nm and 0.59 nm, which are both considerably shorter than the values for two polydienes having 1,4-microstructures preferentially, i.e., 0.66 nm for both 1,4-rich PI and PB. Furthermore plateau moduli of both polymers are determined by dynamic viscoelastic measurements.     

Sequence distribution and thermal properties of poly(butylene naphthalate terephthalate)

Poly(butylene naphthalate terephthalate) (PBNT) copolyesters were synthesized from bis(4-hydroxybutyl) terephthalate (BHBT) and bis(4-hydroxybutyl) naphthalate (BHBN) as starting materials. BHBT and BHBN were either homopolymerized or copolymerized at 260∼270 °C in the presence of titanium tetrabutoxide (TBT) as a catalyst to provide PBNT with various compositions. The copolyesters were characterized using inherent viscosity, X-ray, d.s.c., t.g.a. and ¹H NMR. The composition and sequence distribution of the copolyesters was determined from ¹H NMR spectra. The copolyesters exhibited a degree of randomness of about 1, indicating that the reactivity of BHBT and BHBN was almost the same. X-ray and d.s.c. showed PBNT copolyesters to be crystalline polymers. T.g.a. kinetics showed PBNT copolyesters to exhibit higher degradation activation energy, that is, better thermal stability, than PBTs.     

序列分布导向的CGC催化乙烯与1-辛烯共聚过程建模

限制几何构型催化剂(constrained geometry catalyst,CGC)特别适用于乙烯与α-烯烃溶液聚合法制备高性能聚烯烃弹性体POE(polyolefin elastomer)。基于CGC催化乙烯与1-辛烯共聚反应机理,建立了乙烯与1-辛烯共聚反应动力学模型并确定了动力学参数。采用前期实验获得的乙烯消耗速率曲线与催化剂平均活性验证了动力学模型的准确性。基于动力学模型和面向序列结构的共聚机理,建立了乙烯与1-辛烯共聚过程序列分布模型。模型可准确预测序列分布与短支链含量及其随反应条件的变化趋势。结果表明,随着1-辛烯浓度的增加,乙烯序列长度逐渐减小,其平均序列长度线性降低,而1-辛烯平均序列平均长度呈线性增加的趋势。所建模型可为从聚合过程角度调控POE链结构提供理论参考。     

Sequence distribution and crystal structure of poly(ethylene/trimethylene terephthalate) copolyesters

The sequence distribution and the crystal structure of copolyesters synthesized from ethylene glycol, 1,3-propanediol, and dimethyl terephthalate with different molar volume ratios were investigated in this study. The triad sequence probabilities of ethylene/trimethylene terephthalate were characterized from the aromatic quaternary carbons by ¹³C NMR. The composition of the copolyesters was determined from the aromatic quaternary carbons by ¹³C NMR, and the methylene protons by ¹H NMR. Results show that 1,3-propanediol reacted faster with terephthalic acid in copolyester polymerization than ethylene glycol. The difference in monomer reactivity is significant in the polymerization. Although the constitutional units revealed a random distribution in the molecular chain by ¹³C NMR, crystallites formed across the full range of ethylene glycol/1,3-propanediol composition by use of differential scanning calorimetry, a hot stage polarizing microscope, and a wide angle X-ray diffraction method. The WAXD deconvolution results show that the major constitutional repeating unit in the molecular chain dominates the crystal structure as a host crystal. The crystal structure was examined by a scanning electron microscope after a solvent etching. Photomicrographs show that the random distribution of the third constitutional unit in the molecular chain of copolyester significantly disturbs the host crystal formation and lamellar orientation.     

线性SSBR胶料的结构、序列分布与性能的研究

研究了几种不同结构的线性溶聚丁苯橡胶的物理及动态力学性能,探讨苯乙烯含量、苯乙烯均匀无规分布、乙烯基含量对胶料性能的影响。结果表明,随苯乙烯和乙烯基含量的增加,胶料的玻璃化转变温度、压缩疲劳温升、抗湿滑性增高,耐老化性有所改善,抗张强度、撕裂强度、磨耗值降低。苯乙烯均匀无规分布可明显提高SSBR的柔顺性,胶料的回弹值和抗湿滑性提高,硬度和内摩擦生热降低,炭黑分散获得改善。苯乙烯均匀无规分布可明显提高SSBR胶料的回弹值和抗湿滑性,降低胶料的硬度和内摩擦生热。综上所述,高乙烯基且St均匀分布的SSBR胶料具有高的柔顺性,高抗湿滑性和低滚动阻力特性。     

Sequence distribution of styrene units in poly(styrene-co-tetramethylene disulfide) obtained from chain transfer copolymerization

Synopsis Copolymers obtained from radical styrene polymerization in the presence of poly(tetramethylene disulfide) were subjected to sulfur-sulfur bond cleavage by butyllithium. The resulting oligo-and poly (styrene) molecular weight distribution is identical with the sequence distribution of styrene sequences in the copolymer which proved to be a multiblock. From this, the apparent chain transfer constant of poly(tetramethylene disulfide) was calculated. The author is indebted to Deutsche Forschungsgemeinschaft and to Fonds Der Chemischen Industrie for financial support.     

Differential scanning calorimetry study on intermolecular interactions in polymer–diluent blends involving polyisoprenes and dimethyl terephthalate

The classical expression for estimating the polymer-solvent interaction parameter χ from melting point data of the solvent is refined by considering χ to be concentration-dependent. Differential scanning calorimetry (DSC) measurements are performed on blends of cis-polyiso-prene-dimethyl terephthalate (DMT) and trans-polyisoprene–DMT. It is found that the presence of DMT would not affect the DSC events of the polyisoprenes appreciably. However, in both cases, the melting point of DMT decreases, and the rate of depression increases with increasing proportions of polymer. These DSC data are analyzed by the proposed procedure. The results on the interaction energy density parameter are presented as a function of the blend concentration, and rationalized by the specific interactions between different moieties on the component molecules. The limitations of the present technique are discussed.     

Sequence distribution, crystallization and melting behavior of poly(ethylene terephthalate-co-trimethylene terephthalate) copolyesters

The compositions of a series of copolyesters of ethylene- and trimethylene- terephthalate were determined by NMR. The values of randomness parameter revealed these copolyesters were random copolymers with average-number sequence lengths varied from 1 to 10. These copolyesters were investigated using temperature-modulated differential scanning calorimeter (TMDSC) and wide angle X-ray diffractometer (WAXD). The crystallization behavior was evidenced from the non-isothermal DSC thermograms and WAXD patterns. The crystallization rate and the intensity of diffraction peaks decrease with increasing the minor co-monomer content. The copolyester with equal amounts of monomers is still crystallizable even though the average sequence length is only 2.0, and its WAXD pattern is quite different from those of homopolymers. In the melting study, three distinct peaks of reversing melting were observed. Two peaks in the high temperature side are due to remelting of the recrystallized crystals formed during the heating scan, whereas the highest temperature peak is attributed to the melting of the crystals that are formed in relatively high temperature. The results of WAXD and TMDSC support the mechanism of melting-recrystallization-remelting for the multiple melting behaviors.     

Unsaturated polyesters based on terephthalic acid: 4. Sequence distribution of crosslinks in styrene cured resins

The sequence distribution of the styrene crosslinks in a cured polyester formed from propylene glycol, terephthalic acid and maleic anhydride has been studied by ¹³C nuclear magnetic resonance and infrared spectroscopy. The n.m.r. method involved the degradation of the crosslinked polyester into a copolymer of styrene and fumaric acid and the interpretation of the peaks in the ¹³C spectrum, which arise from the quaternary phenyl carbon atoms in sequences of styrene units, by considering the ¹³C spectra of copolymer standards of styrene and diethyl fumarate. The crosslinking process was monitored directly by infra-red spectroscopy, and the distribution of sequence lengths of styrene units was then estimated by considering that crosslinking may be interpreted by a statistical treatment of copolymerization. Values of mol fractions of styrene units in sequences of monads, diads and triads determined by ¹³C n.m.r. and infra-red spectroscopy were in fair agreement.     

Hierarchical Structure of Protein Sequence

Most non-communicable diseases are associated with dysfunction of proteins or protein complexes. The relationship between sequence and structure has been analyzed for a long time, and the analysis of the sequences organization in domains and motifs remains an actual research area. Here, we propose a mathematical method for revealing the hierarchical organization of protein sequences. The method is based on the pentapeptide as a unit of protein sequences. Employing the frequency of occurrence of pentapeptides in sequences of natural proteins and a special mathematical approach, this method revealed a hierarchical structure in the protein sequence. The method was applied to 24,647 non-homologous protein sequences with sizes ranging from 50 to 400 residues from the NRDB90 database. Statistical analysis of the branching points of the graphs revealed 11 characteristic values of y (the width of the inscribed function), showing the relationship of these multiple fragments of the sequences. Several examples illustrate how fragments of the protein spatial structure correspond to the elements of the hierarchical structure of the protein sequence. This methodology provides a promising basis for a mathematically-based classification of the elements of the spatial organization of proteins. Elements of the hierarchical structure of different levels of the hierarchy can be used to solve biotechnological and medical problems.     

层序地层形成中的构造作用

层序地层的形成主要受控于沉积基准面变化,沉积基准面是全球海平面变化、构造沉降、沉积物供给、古气候四个主要因素综合作用的结果。通过对构造运动与其它三个因素关系的探讨,指出构造运动对海平面变化的影响可以是全球性的;在陆相盆地中,构造运动控制着湖平面变化、沉积物供给,进而控制着层序地层的形成,同’时对古气候的影响尤为重要。     

表面活性物质形成的某些特殊有序结构

介绍了目前关于蠕虫状胶束和囊泡一类有序结构的形成体系、表征方法和在应用等方面的研究进展情况。