Microstructur polyisoprene

Alfin Catalysts. Alfin catalysts (44,45) give polyisoprenes of high trans-1,4 microstructure (46). For example, a typical Alfin catalyst gives polyisoprene of 52% trans-1,4, 27% cis-1,4, 16% 3,4, and 5% 1,2 content (ir analysis) (46). One type of Alfin catalyst consists of allylsodium, sodium isopropoxide, and sodium chloride (47,48). Because of the mixed microstructure polyisoprene produced, Alfin catalysts are not used commercially. 

Results for the structurally more complex, mixed microstructure polydienes, which have substantial in-chain and side-chain unsaturation, are more difficult to interpret. Clearly, mixed microstructure polybutadiene (PBD-57) exhibits a value (6.0) intermediate to that found for high 1,4 (C = 5.1) and high 1,2 (C = 7.0) materials. Conversely, a mixed microstructure polyisoprene (PI-51) of similar microstructure to PBD-57 yields a C o of 5.3, a result virtually identical to that found for the high 1,4 material. Both mixed microstructure polymyrcene (PMYRC-64) and poly(2,3-dimethyl butadiene) (PDMB-55) yield C values (7.6 and 8.4, respectively) that are considerably larger than those for their high 1,4 counterparts. Interpretation of these results at the present level of theoretical development would be speculative. 

Early work on the microstructurc of the diene polymers has been reviewed.1 While polymerizations of a large number of 2-substituted and 2,3-disubstituted dienes have been reported,88 little is known about the microstructure of diene polymers other than PB,89 polyisoprene,90 and polychloroprene.91... 

Fig. 11. Effect of different solvents on the microstructure of polyisoprene. (S. L. Aggarwal et al., Ref. 75 >)...

Fig. 18. Microstructure of polyisoprene obtained in cyclohexane at 30 °C with sec. BuLi as initiator. Conversion 10% (D. J. Worsfold, S. Bywater, Ref. 12S )...

Polybutadiene (PB) and polyisoprene with cis-trans controlled microstructure (synthesis of "natural rubber"). 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

The microstructure of polyisoprene prepared by lithium initiation in hydrocarbons is 95% 1,4 under all conditions. The trans 1,4 content however falls from about 20% to zero as the monomer/initiator ratio increases leading finally to a 95% cis 1,4 polymer. This variation can be explained with the following scheme. 

Figure 2. Microstructure of polyisoprene initiated with alkyllithium in cyclohexane...

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

The commerical polybutadiene (a highly 1,4 polymer with about equal amounts of cis and trans content) produced by anionic polymerization of 1,3-butadiene (lithium or organolithium initiation in a hydrocarbon solvent) offers some advantages compared to those manufactured by other polymerization methods (e.g., it is free from metal impurities). In addition, molecular weight distributions and microstructure can easily be modifed by applying appropriate experimental conditions. In contrast with polyisoprene, where high cis content is necessary for suitable mechanical properties, these nonstereoselective but dominantly 1,4-polybutadienes are suitable for practical applications.184,482... 

The microstructure of polyisoprene prepared in a variety of solvents and solvent mixtures (113) has been determined. Various ethers and sulphides vary in their ability to reduce the 1,4 content of the polymer. The most effective ether was tetrahydrofuran. The presence of only two molecules per active chain was reported to reduce the 1,4 content to that observed in the pure ether. More recent investigations have failed to confirm that the requirement is as low as this 74,126) but relatively small amounts of tetrahydrofuran do markedly decrease the cis-1,4 content and increase the 3,4 content. Similar results have been obtained for butadiene 60) with respect to 1,4 and 1,2 structures. 

The microstructure of the polymer varies little with changing reaction conditions 68,104). The effect of temperature is generally small and the alkali metals or their alkyls normally give the same product. Significant differences in microstructure have been noted between potassium and its alkyls (104) and between two different cesium compounds 88) but these effects are not general and their cause is obscure. A more difficult problem exists in that there is poor agreement between the microstructures reported by different authors for a particular initiator and solvent. Tables 3 and 4 include some of the data given for polyisoprene and polybutadiene. Standard infra-red methods were used for the analysis except... 

The physical properties of any polyisoprene depend not only on the microstructural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the structure is completely stereoregular. These stereoregular sequences must be linear structures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

The microstructures of several synthetic ar-l,4-polyisoprenes and natural mbber as determined by 13C-nmr (8,13) are shown in Table 1. 1H-nmr measurements may also be used for microstructure characterization (8,13—15). 

Amorphous (most likely atactic) 3,4-polyisoprene of 94—100% 3,4-microstructure was prepared with a (C2H )3A1—Ti(0— -C3H7)4 catalyst (11). Crystalline 3,4-polyisoprene containing about 70% 3,4-units and about 30% cis- 1,4-microstructure was prepared using a catalyst derived from iron acetyl acetonate, trialkylaluminum, and an amine in benzene (37). However, this polyisoprene contained gel and was obtained in poor yield. Essentially gel-free crystallizable 3,4-polyisoprene of 70—85% 3,4-microstructure with the remainder being cis-1,4 microstructure was prepared in conversions of greater than 95% with a water-modified trialkylaluminum, ferric acetyl acetonate, and 1,10-phenanthroline catalyst (38). The 3,4-polyisoprene is stereoregular and believed to be syndiotactic or isotactic. 

Alkali Metal Catalysts. The polymerization of isoprene with sodium metal was reported in 1911 (49,50). In hydrocarbon solvent or bulk, the polymerization of isoprene with alkali metals occurs heterogeneously, whereas in highly polar solvents the polymerization is homogeneous (51—53). Of the alkali metals, only lithium in bulk or hydrocarbon solvent gives over 90% cis-1,4 microstructure. Sodium or potassium metals in -heptane give no cis-1,4 microstructure, and 48—58 mol % trans-1,4, 35—42% 3,4, and 7—10% 1,2 microstructure (46). Alkali metals in benzene or tetrahydrofuran with crown ethers form solutions that readily polymerize isoprene however, the 1,4 content of the polyisoprene is low (54). For example, the polyisoprene formed with sodium metal and dicyclohexyl-18-crown-6 (crown ether) in benzene at 10°C contains 32% 1,4-, 44% 3,4-, and 24% 1,2-isoprene units (54). 

A change from an aliphatic or aromatic hydrocarbon solvent (cyclohexane, benzene) to a polar solvent (THF) leads to a large increase in trans-1,4 and 3,4 microstructure (58). Organolithium compounds are highly associated sec-butyllithium in benzene or cyclohexane exists as a tetramer, and -butyllithium as a hexamer (64,65). This association in hydrocarbon solvents results pardy in the slow initiation observed between some organolitbiirms and isoprene (66). At low initiator concentrations, the polymerization rate of isoprene in alkyUithium polymerization is proportional to monomer and alkyUithium concentrations (67). 3,4-Polyisoprenes are obtained by modification of the lithium polymerization with ethers, such as the dialky] ethers of ethylene glycol or tertiary amines (68,69). 

Since emulsion polyisoprene has low or-1,4 microstructure, it has poorer physical properties than the high cis- 1,4-polyisoprene and has not been commercialized. 

Morton and Rupert 209) have presented microstructure results for polybutadiene and polyisoprene as a function of conversion and temperature. (Tables 18 and 19). 

The addition of small amounts of a polar solvent can markedly alter the copolymerization behavior of, for example, the diene-styrene pair. The solvation of the active centers manifests itself in two ways the incorporation of styrene is enhanced and the modes of diene addition other than 1,4 are increased 264,273). Even a relatively weak Lewis base such as diphenyl ether will bring about these dual changes in anionic copolymerizations, as the work of Aggarwal and co-workers has shown 260>. Alterations in polyisoprene microstructure and the extent of styrene incorporation were found for ether concentrations as low as 6 vol. % (diphenyl ether has been shown52) to cause partial dissociation of the poly(styryl)lithium dimers. The findings of Aggarwal and co-workers 260) are a clear demonstration that even at relatively low concentrations diphenyl ether does interact with these anionic centers and further serve to invalidate the repetitive claim 78,158-i60,i6i) tjjat diphenyl ether — at an ether/active center ratio of 150 — does not interact with carbon-lithium active centers. 

Microstructure variation in both polybutadiene and polyisoprene polymers has been realized by using alkyllithium initiators in the presence of polar modifiers. 

The chemical microstructures of cis-polyisoprene (HR) vulcanised with sulfur and N-t-butyl-2-benzothiazole sulfenamide (TBBS) accelerator were studied as a function of extent of cure and accelerator to sulfur ratio in the formulations by solid-state 13C NMR spectroscopy at 75.5 MHz [29]. Conventional (TBBS/Sulfur=0.75/2.38), semi-efficient (SEV=1.50/1.50) and efficient (EV=3.00/1.08) vulcanisation formulations were prepared, which were cured to different cure states according to the magnitude of increase in rheometer torque. The order and types of the sulfurisation products formed are constant in all the formulation systems with different accelerator to sulfur ratios. However, the amount of sulfurisation has been found to vary directly with the concentration of elemental sulfur. 

Rossi has synthetized block copolymers polyisoprene-poly(vinyl-2-pyridine) and polyisoprene-poly(vinyl-4-pyridine) of various composition and molecular weight by anionic polymerization under high vacuum205, 208. The polymerization in THF dilute solutions with Cumylpotassium as initiator yielded a 1,2 + 3,4-microstructure of the polyisoprene block. The polymerization in toluene solutions with sec-butyl-lithium as initiator yielded a 1,4-c/s-microstructure of the polyisoprene block. 

Once again one finds the striking stereospecificity withlithium dependent initiators and a rather conglomerate microstructure when the other alkali metals are involved as shown in Fig. 3. However, the use of nonhydrocarbon solvents drastically affects the structure of the polyisoprene derived from lithium-dependent initiators. This effect is lessened in the case of sodium and potassium as can be seen from the data in Table 2 (81). 

Fig. 3. Microstructure of Polyisoprene Prepared with organo alkali metals in bulk or in hydrocarbon solvent...

A few other pertinent observations have been made. Although the effect of temperature on structure in the case of sodium or potassium metal polymerized butadiene was shown to lead to the gradual approach to a nearly random mixture between 0 and 45° (41,32), in the case of phenyllithium in tetrahydrofuran there is observed only a few percent difference between — 78° and + 100° (76). Furthermore, the use of lithium, n-butyllithium, n-amyllithium or isoamyllithium produces polyisoprene of the same microstructure in tetrahydrofuran (77). Kuntz (34) found that polybutadiene prepared with n-butyllithium in... 

The system with which we have begun our investigations is the styrene-dimethylsiloxane system. The dimethylsiloxane blocks should be considerably less compatible with polystyrene blocks than either polybutadiene or polyisoprene since the solubility parameter of dimethylsiloxane is much farther from that of polystyrene than are the solubility parameters of polybutadienes or of polyisoprenes (17), no matter what their microstructure. Furthermore, even hexamers of polystyrene and of polydimethylsiloxane are immiscible at room temperature and have an upper critical-solution temperature above 35°C (18). In addition, the microphases in this system can be observed without staining and with no ambiguity about the identity of the phases in the transmission electron microscope (TEM) silicon has a much higher atomic number than carbon or oxygen, making the polydimethylsiloxane microphases the dark phases in TEM (19,20). 

Three diblock copolymers of cis-1,4 polyisoprene (IR) and 1,4-polybutadiene (BR) have been studied in dynamic mechanical experiments, transmission electron microscopy, and thermomechanical analysis. The block copolymers had molar ratios of 1/2, 1/1, and 2/1 for the isoprene and butadiene blocks. Homopolymers of polybutadiene and polyisoprene with various diene microstructures also were examined using similar experimental methods. Results indicate that in all three copolymers, the polybutadiene and polyisoprene blocks are essentially compatible whereas blends of homopolymers of similar molecular weights and microstructures were incompatible. 

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