异戊橡胶微观序列结构与结晶性能研究

天然橡胶特有的应变诱导结晶能力赋予其优异的力学性能.异戊橡胶作为唯一能够替代天然橡胶的合成橡胶品种,其应变诱导结晶能力受到关注.本文对3种异戊橡胶的微观序列结构进行了分析,并研究了其结晶性能.核磁分析结果表明:3种异戊橡胶的顺-1,4-结构含量差别不大;从1,4-结构单元的键接方式(序列结构)看,SKI-5和YS-IR分子链中顺-1,4-结构单元均以头-尾相接的方式沿分子链排列,不存在头-头键和尾-尾键接方式;SKI-3中约有0.4%~0.5%的1,4-单元采取头-头键接方式,约有0.3%~0.6%的1,4-单元采取尾-尾键接方式;根据定量计算结果,从分子链上1,4-结构单元的序列分布来看,SKI-3的规整性与SKI-5、YS-IR相近或略高.XRD研究结果表明:炭黑填充的天然橡胶硫化胶拉伸至400%以上时发生取向结晶;而炭黑填充的异戊橡胶硫化胶需拉伸至500%以上时才发生取向结晶.基本物理机械性能研究表明:3种异戊橡胶的性能相当,拉伸强度和撕裂强度明显低于天然橡胶;由于结构和组成上的差异,异戊橡胶的结晶能力较天然橡胶差.     

Biomimetic carbocationic polymerizations III: Investigation of isoprene polymerization initiated by dimethyl allyl bromide

Natural rubber (poly(1,4‐cis‐isoprene), NR) is a polymer of considerable industrial importance due to its exceptional properties. It is mostly produced from the cultivation of Hevea brasiliensis, and to a limited extent from Parthenium argentatum. Till date none of the synthetic equivalent of NR exists. Recently we suggested that the mechanism of NR biosynthesis is based on carbocationic polymerization, similarly to that of natural oligoisoprenoids forming by enzyme‐catalyzed prenylation. In this article we present proof of concept of a new bio‐inspired synthetic route towards the synthesis of polyisoprenes based on carbocationic polymerization initiated by dimethyl allyl bromide (DMABr)/TiCl₄. It is shown that using this strategy, 1,4‐oligoisoprene carrying a dimethyl allyl head group is produced in both cis and trans configurations, together with cyclized sequences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2172–2180, 2009     

Crystallinity and crystalline phase orientation of poly(1,4‐cis‐isoprene) from Hevea brasiliensis and Taraxacum kok‐saghyz

Crystallization is studied for poly(isoprene‐1,4‐cis) from Hevea brasiliensis (natural rubber [NR]) and from taraxacum kok‐saghyz, mainly by collecting wide‐angle X‐ray diffraction patterns after processing and stretching. Although rubber samples before stretching are generally fully amorphous, crystallization can be induced in NR samples by processing at room temperature under moderate pressure. This phenomenon is possibly associated with nucleation by saturated fatty acid components. For rubber samples being fully amorphous in the undeformed state, strain‐induced crystallization occurs only at high strain ratios (α > 4), leading to high degrees of crystalline phase orientation (f_c > 0.9 for α = 5). Rubber samples presenting some crystallinity already in the unstretched state, on the contrary, reach much lower degrees of axial orientation, even for high strain ratios (f_c < 0.7 for α = 5). These differences in crystallinity and in crystalline phase orientations produce large differences in stress–strain behavior of the rubber. By room temperature processing, the considered NR samples can also develop an unreported disordered crystalline modification, with low intensity of 120 and 121 reflections. This disordered crystalline modification, which is also maintained after axial stretching procedures, can rationalized by a structural disorder along the b axis, possibly associated with statistical sequences of A⁺TA^? or A^?T A⁺ conformations for poly(isoprene‐1,4‐cis) chains. Copyright © 2016 John Wiley & Sons, Ltd.     

Butadiene–isoprene copolymerization with V(acac)3‐MAO. Crystalline and amorphous trans‐1,4 copolymers

Butadiene‐isoprene copolymerization with the system V(acac)₃‐MAO was examined. Crystalline or amorphous copolymers were obtained depending on isoprene content. Both butadiene and isoprene units exhibit a trans‐1,4 structure and are statistically distributed along the polymer chain. Polymer microstructure, comonomer composition, and distribution along the polymer chain were determined by ¹³C and ¹H NMR analysis. The thermal and X‐ray behaviors of the copolymers were also investigated and compared with results from solid‐state ¹³C NMR experiments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4635–4646, 2007     

Isoprene polymerization on titanium-magnesium catalysts

The high activity of a titanium-magnesium catalyst in the polymerization of isoprene with formation of a unique thermoplastic material, synthetic gutta percha, was shown. It is established that a change in polymerization conditions over a wide range has no effect on the content of trans-1,4 units in the polymer. Unlike natural gutta percha with the crystalline phase containing a mixture of α-and β-crystalline modifications, the synthetic trans-1,4-polyisoprene crystallizes only in an α-monoclinic form, the melting temperature of which is close to 70°C. The melting followed by crystallization results in formation of a stable β-crystalline modification with a melting temperature approximating 50°C.     

Functional polyisobutylenes via electrophilic cleavage/alkylation

Lewis‐acid catalyzed degradation of poly(isobutylene‐co‐isoprene) (butyl rubber) in the presence of an alkoxybenzene compound was studied as a new route toward low molecular weight multifunctional polyisobutylenes. Simultaneous cleavage and functionalization of butyl rubber was conducted at ?70 °C and ?40 °C under TiCl₄ or AlCl₃ catalysis in 60/40 hexane/methylene chloride cosolvents in the presence of (3‐bromopropoxy)benzene (BPB) for various times up to 24 h. The butyl rubber (EXXON? Butyl 365) possessed M _n = 1.91 × 10⁵ g/mol, PDI = 1.66 (GPC/MALLS), and 2.30 mol % isoprene units (nearly exclusively trans ?1,4). At ?70 °C with TiCl₄, molecular weight was reduced to various values within the range 7 to 11 × 10³ g/mol depending on conditions; lower BPB concentration produced lower molecular weight. However, the ratio of isobutylene repeat units to BPB units (IB/Q ) remained constant at about 43:1, which is approximately the same as the ratio of isobutylene to isoprene repeat units (IB/IP) in the starting butyl rubber (42.5:1). At ?40 °C with TiCl₄, molecular weight was reduced to about 5 × 10³ g/mol, and IB/Q was reduced below IB/IP, indicating nearly a difunctional telechelic structure. AlCl₃ was a more active catalyst and produced results similar to TiCl₄ at ?40 °C, even when used at seven times lower concentration. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 1991–1997     

Biomimetic processes. IV. Carbocationic polymerization of isoprene initiated by dimethyl allyl alcohol

In this article, we investigate a new bio‐inspired synthetic route towards NR homologs based on the carbocationic polymerization of isoprene initiated by dimethyl allyl alcohol (DMAOH)/TiCl₄ or BF₃.Et₂O as the catalytic system. This study is the continuation of our studies related to the proof of principle that NR biosynthesis is based on a carbocationic mechanism. It is shown that using the biomimetic strategy of initiation by allylic carbocations, polyisoprene carrying a dimethyl allyl head group is produced almost exclusively via 1,4 addition, yielding repeating units with cis and trans configurations. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2181–2189, 2009     

Natural rubber/clay nanocomposites: Influence of poly(ethylene glycol) on the silicate dispersion and local chain order of rubber network

A novel preparation of natural rubber (NR)/Na⁺-montmorillonite (MMT) nanocomposites in only one step by using poly(ethylene glycol) (PEG) has been investigated. PEG behaves as dispersing agent favouring the intercalation of rubber chains into the silicate galleries and providing substantially improved clay dispersion. Intercalated/exfoliated miscible hybrids were observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The influence of PEG on the network structure has also been evaluated by static proton double-quantum nuclear magnetic resonance spectroscopy (¹H DQ NMR) at low-field. Silicate nanoparticles with a high aspect ratio (clay tactoids) and a more crosslinked rubber network have been obtained for an optimum PEG/MMT ratio. Both effects were responsible of the enhancement on mechanical properties.     

Anionic Polymerization and Copolymerization of Hydrocarbon Monomers Catalyzed by Organobarium Initiators

The barium salt of the dimeric dianion of 1,1-diphenylethylene (Ba-DPhE) initiates polymerization and copolymerization of monomers capable of anionic polymerization (butadiene, isoprene, styrene) in ethereal and hydrocarbon solvents. Ba-DPhE is more stereospecific in butadiene polymerization (up to 70% of cis-1, 4-units in hydrocarbon medium) than initiators based on other metals of Groups I and II. The relative reactivity of monomers in copolymerization processes in THF decreases in an order typical for anionic polymerization: styrene > butadiene > isoprene. The most interesting feature of organobarium initiators is their ability to form random butadiene-styrene copolymers with high cis-1,4-butadiene unit content when copolymerization proceeds in a hydrocarbon medium.

A new phenomenon in anionic polymerization, the dependence of diene units structure on copolymer composition, was observed. Thus an increase of styrene content in butadienestyrene copolymer leads to conversion of cis-1,4-butadiene units into trans-1,4-units (in benzene) or to conversion of 1,4-units to 1,2-units (in THF). Similarly, an increase of butadiene content in its copolymer with isoprene (in benzene) leads to conversion of cis-1,4-isoprene units into trans-1,4-units.

Spectrophotometric, conductometric, and viscometric methods were used to study organobarium active centers. Certain anomalies connected with the formation of specific aggregates due to coupling of bifunctional hydrocarbon chains with bivalent counterions were observed.     

Study of chain branching in natural rubber using size-exclusion chromatography coupled with a multi-angle light scattering detector (SEC-MALS)

The different rheological behaviour of natural rubber (NR) compared to industrial synthetic poly(cis-1,4-isoprene) (SR) has been attributed to the gel phase and long-chain branching. Previous studies on branching in NR were carried out using the fractionation technique by precipitation to obtain narrow molar mass distribution. In this study, chain branching of poly(cis-1,4-isoprene) in NR was characterised by size-exclusion chromatography coupled with an online multi-angle light scattering detector (SEC-MALS). The nanoaggregates adsorbed on the column packing interfered with branching characterisation for short and medium chains (M_w < 1000 kg/mol). Using a master curve of linear standard poly(cis-1,4-isoprenes), SEC-MALS revealed no or very little branching in the higher chains (1000 < M_w < 10,000 kg/mol) of natural rubber contrary to previous studies. This study showed that the soluble portion of NR samples was composed of almost linear poly(cis-1,4-isoprene) and nanoaggregates with rather compact structures.     

Effects of Epoxidation Content of ENR on Morphology and Mechanical Properties of Natural Rubber Blended PVC

This research work has concerned a study on toughness of PVC/natural rubber (NR) blends compatibilized with epoxidized natural rubber (ENR). The aim of this work was to investigate the effect of degree of epoxidation on morphology and mechanical properties of the blends. Epoxidized natural rubber with a variety of epoxidation contents were prepared by reacting the NR latex with formic acid and hydrogen peroxide at various chemical contents. Chemical structure and epoxidation content of epoxidized natural rubber were evaluated by FTIR and ¹H-NMR techniques. After that, three grades of ENR with epoxidation contents of 15, 25 and 42 % (by mole) were further used for blending with PVC and NR in an internal mixer at 60 rpm and at 170 °C. From tensile and impact tests, it was found that tensile elongation and impact strength of the materials remarkably increased with degree of epoxidation. On the other hand, tensile strength and modulus of the materials rarely changed with the epoxidation content. An increase in toughness of the blends with epoxidation content was related to a better molecular interaction between PVC and ENR as suggested by torque-time curves of the materials.     

The characteristics of lanthanide coordination catalysts and the cis-polydienes prepared therewith

The stereoregularity of polydienes is almost the same in regard to the individual elements of the lanthanide series, whereas the activity of the Ln catalysts in diene polymerization varies from one to the other within the series. The latter may be attributed to the difference in the number of electrons that occupy the 4f orbitals. It has been proved that the polymerization of dienes with Ln catalysts under certain conditions proceeds by a “living polymer” mechanism. With regard to the polymerization of butadiene, the most active catalyst is a Nd³⁺species a new binary system of NdCl₃-3ROH + AlR₃ has been discovered. The cis- 1,4 content in polybutadiene is about 97% and the 1,2 content, less than 1%. For the polymerization of isoprene with a Nd³⁺ catalyst system, the effects of ligand and alkyl groups in AIR₃ on cis-1,4 content (ca. 95%) in polyisoprene can be neglected. For the copolymerization of butadiene and isoprene, the cis-1,4 contents of these two monomeric units in the copolymer are greater than 95% the reactivity ratios r₁ and r₂ are determined. and the T_g's of the copolymers of various compositions deviate slightly from the calculated values for random copolymers. A linear relationship exists between the yield strength from the stress-strain curve of Ln-polvbutadiene and its [n] This relationship is verified by Ln-polyisoprene and natural rubber but different slopes are obtained     

Synthesis and chemical structure of natural rubber adduct with SO2 and study of the thermal stability

Natural rubber (NR, cis-1,4-polyisoprene) was reacted in solution with SO₂ at room temperature in different solvents (tetrahydrofuran, CHCl₃, CH₂Cl₂ and tetralin). The reaction product consisted of an adduct between NR and SO₂. The chemical structure of the adduct was studied by FT-IR spectroscopy, by thermogravimetry coupled with FT-IR (TGA-FT-IR), by differential thermal analysis (DTA) and by derivative thermogravimetry (DTG). The maximum SO₂ uptake from NR was 21% as determined by TGA. The TGA of the NR-SO₂ shows that the adduct decomposes in two steps initially releasing SO₂ at lower temperature and then decomposing at higher temperature like common NR. It is proposed that the chemical structure of the NR-SO₂ adduct consists of SO₂ molecules added to the double bonds of the isoprene units forming a sulfone derivative with three-membered rings known as thiirane-1,1-dioxide. On heating the thiirane ring decomposes releasing SO₂ in an extrusion reaction giving back the NR macromolecule.The possible potential applications of the NR-SO₂ adduct which is cheap and easy to prepare are outlined.     

Effects of cellulose nanocrystals on the vulcanization of natural rubber/cellulose nanocrystals nanocomposite and corresponding regulating strategies

Vulcanization is a vital process in rubber processing, it endows rubber with valuable physical and mechanical properties, making rubber a widely used engineering material. In addition to vulcanization agent, reinforcing fillers play a non-ignorable influence on the vulcanization of rubber nanocomposites. Herein, the effects of cellulose nanocrystals (CNCs) on the vulcanization of natural rubber (NR)/CNCs nanocomposite was studied. It was found that even though the addition of CNCs can effectively improve the dispersion of ZnO in NR matrix, the vulcanization of NR was inhibited. This may be attributed to the CNCs' adsorption of vulcanizing agents (DM, ZnO) and the acidic chemical environment on the surface of CNCs. In order to improve the vulcanization properties of NR/CNCs nanocomposite, tetramethyldithiochloram (TMTD) and triethanolamine (TEOA) were used as a combination accelerator and curing activator, respectively, and polyethylene glycol (PEG) was introduced to screen hydroxyl groups on the surface of CNCs to prohibit the CNCs' adsorption of vulcanizing agents. The results indicate that TMTD and TEOA effectively improved the vulcanization rate of NR/CNCs nanocomposite and increased the crosslink density by an order of magnitude. Subsequently, the tensile strength, tear strength, and so forth. of NR/CNCs nanocomposite were significantly improved. However, PEG hardly help to improve the vulcanization properties of NR/CNCs nanocomposite. In addition, the control samples without CNCs were prepared and characterized, the comparation between NR and NR/CNCs nanocomposite shows that the synergistic effect of crosslink density and CNCs' reinforcement more effectively improve mechanical properties of NR. This work not only elucidates the inhibiting mechanisms of CNCs on the vulcanization of NR, but also provides practical strategies for improving the vulcanization and properties of NR/CNCs nanocomposite. It may accelerate the application of CNCs as rubber reinforcing filler.     

Copolymerization of isoprene with ethylene catalyzed by cationic half‐sandwich fluorenyl scandium catalysts

Isoprene polymerization and copolymerization with ethylene can be carried out by using cationic half‐sandwich fluorenyl scandium catalysts in situ generated from half‐sandwich fluorenyl scandium dialkyl complexes Flu'Sc(CH₂SiMe₃)₂(THF)_n, activator, and AlⁱBu₃ under mild conditions. In the isoprene polymerization, all of these cationic half‐sandwich fluorenyl scandium catalysts exhibit high activities (up to 1.89 × 10⁷ g/mol_Sc h) and mainly cis?1,4 selectivities (up to 93%) under similar conditions. In contrast, these catalysts showed different activities and regio‐/stereoselectivities being significantly dependent on the substituents of the fluorenyl ligands in the copolymerization of isoprene with ethylene under an atmosphere of ethylene (1 atm) at room temperature, affording the random copolymers with a wide range of cis?1,4‐isoprene contents (IP content: 64 ? 97%, cis?1,4‐IP units: 65 ? 79%) or almost alternating copolymers containing mainly 3,4‐IP‐alt‐E or/and cis?1,4‐IP‐alt‐E sequences. Moreover, novel high performance polymers have been prepared via selective epoxidation of the vinyl groups of the 1,4‐isoprene units in the IP‐E copolymers. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2898–2907     

Modified titanium-magnesium nanocatalysts in the polymerization of isoprene

The polymerization of isoprene on titanium-magnesium nanocatalysts modified with electron-donor compounds based on organic phosphines and sulfides has been studied. It was shown that the catalyst modification makes it possible to increase the content of trans-1,4 units in polyisoprene up to 97% (according to the data of ¹H NMR spectra). In the case of a catalyst modified with tributylphosphine, the effects of the phosphorus-to-titanium ratio on the polymerization kinetics, on the microstructure of trans-1,4-polyisoprene, and on the molecular mass of the polymer have been estimated.     

Preparation of phenyl‐modified natural rubber in latex stage

Phenyl‐modified natural rubber was prepared in latex stage by bromination of deproteinized natural rubber followed by Suzuki‐Miyaura cross‐coupling reaction. First, the bromination of natural rubber was carried out using N‐bromosuccinimide in latex stage. The bromine atom content increased as amount of N‐bromosuccinimide increased. Second, the allylic bromine atom was replaced with a phenyl group using phenyl boronic acid in the presence of a palladium catalyst, according to the Suzuki‐Miyaura cross‐coupling reaction in latex stage. The resulting products were characterized by nuclear magnetic resonance (NMR) spectroscopy. Signal at 7.13 ppm was assigned to the phenyl group of the product, while signals at 3.98, 4.14, and 4.44 ppm were assigned to the remaining allylic brominated cis‐1,4‐isoprene units. The estimated phenyl group content and the conversion of the Suzuki‐Miyaura cross‐coupling reaction were 1.32 and 23.7 mol%, respectively. Glass transition temperature (T_g) of deproteinized natural rubber increased from ?62°C to ?46.7°C, when the phenyl group was introduced into the rubber.     

Metathesis transformations of natural products: cross-metathesis of natural rubber and mandarin oil by Ru-alkylidene catalysts

This study reports on the degradation of natural rubber (NR) via crossmetathesis with mandarin oil and d-limonene, an abundant compound in essential oils; that were used as chain transfer agents (CTAs) and green solvents. Reactions were performed in the presence of the ruthenium-alkylidene catalysts (PCy?)?(Cl)?Ru=CHPh (I) and (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) (PCy?)Cl?Ru=CHPh (II), respectively. Catalyst II bears an N-heterocyclic carbene ligand (NHC) bounded to the ruthenium atom, which has a strong basic character; therefore it is more active toward trisubstituted olefins in comparison with catalyst I. In both cases, isolated monoterpene-terminated isoprene oligomers were obtained as products of the cross-metathesis degradation of NR. In the presence of catalyst II molecular weight values around M(n) × 102 and yields of 80% were obtained; whereas with catalyst I, the molecular weights of products were about M(n) × 10? with yields ranging 70 to 74%. The composition and yield of NR degradation products were determined by GC/MS (EI) analysis and it was found that the oligomers obtained have primarily one vinyl group and one terpene-monocyclic group at the chain end, with isoprene units A(m) = 2, 3 y 4.     

Copolymerization of 1,3‐butadiene and isoprene with cobalt dichloride/methylaluminoxane in the presence of triphenylphosphine

The homopolymerization and copolymerization of 1,3‐butadiene and isoprene were achieved at 0 °C with cobalt dichloride in combination with methylaluminoxane and triphenylphosphine (Ph₃P). For 1,3‐butadiene, highly cis‐specific and 1,2‐syndiospecific polymerization proceeded in the absence or presence of Ph₃P, respectively, although the activity with Ph₃P was much higher than that without Ph₃P. Only a trace of the polymer was, however, obtained in isoprene polymerization when Ph₃P had been added. For copolymerization, the polymer yield in the presence of Ph₃P was about three times higher than that in its absence. Copolymerization in the presence of Ph₃P was, therefore, investigated in more detail. Unimodal gel permeation chromatography elution curves with narrower polydispersity (weight‐average molecular weight/number‐average molecular weight ≈ 1.5) indicated that the propagation reaction proceeded by single‐site active species. Both the yield and molecular weight of the copolymer decreased with an increasing amount of isoprene in the feed, and this was followed by an increase in the isoprene content in the copolymer. The monomer reactivity ratios, r₁ (1,3‐butadiene) and r₂ (isoprene), were estimated to be 2.8 and 0.15, respectively. Although the 1,3‐butadiene content in the copolymer was strongly dependent on the comonomer composition in the feed, the ratio of 1,2‐inserted units to 1,4‐inserted units of 1,3‐butadiene was constant. Concerning the isoprene unit, the percentage of 1,2‐ and 3,4‐inserted units was increased at the expense of 1,4‐inserted units with an increasing isoprene content in the feed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3086–3092, 2002     

Synthesis of N,N-diethyldithiocarbamate functionalized 1,4-polyisoprene, from natural rubber and synthetic 1,4-polyisoprene

N,N-Diethyldithiocarbamate functionnalized 1,4-polyisoprenes were prepared from 1,4-polyisoprenes (natural or synthetic). The syntheses were performed by nucleophilic addition of N,N-diethyldithiocarbamate salts upon oxirane rings of epoxidized units according to a SN2 mechanism with ring opening. Studies on model molecules of epoxidized 1,4-polyisoprene units (1,2-epoxy-1-methylcyclohexane and 4,5-epoxy-4-methyloctane) were previously achieved to develop the procedure. The best yields were obtained at low temperature in polar medium, and more especially in water with sodium N,N-diethyldithiocarbamate (DEDT-Na) as reagent. A diastereospecific addition was noted when reaction was performed in water with DEDT-Na. Afterwards, the developed procedure was successfully generalized to epoxidized synthetic polyisoprenes and epoxidized natural rubber (in THF, then in latex medium). Excellent results were obtained in latex medium with epoxidized natural rubber (ENR) latices. As with the models, a diastereospecific addition of sodium N,N-diethyldithiocarbamate trihydrate onto epoxidized 1,4-polyisoprene units of ENR was observed at the condition to bring the latex medium to pH 8 before introduction of DEDT-Na. Influence of temperature, drc, and DEDT-Na concentration were successively examined to determine the best conditions of the addition on ENR latices.