Polyisoprene polymer microstructure

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

The procedures used in polymer product isolation and evaluation were the same as presented earlier (13). Basically, the dried polymers were extracted with a 50 50 by volume mixture of hexane and isopropyl alcohol to remove low molecular-weight material, herein called extractables. Physical properties such as inherent viscosity, % gel, and polymer microstructure were determined on the solid polyisoprene residue. 

Use of Ziegler-Natta catalysts, as seen from Table 5.8, can yield an almost all-cw-l,4-polyiso-prene or an almost all-franj- 1,4-polyisoprene. The microstructure depends upon the ratio of titanium to aluminum. Ratios of Ti A1 between 0.5 1 and 1.5 1 yield the cis isomer. A1 1 ratio is the optimum. At the same time, ratios of fi Al between 1.5 1 to 3 1 yield the trans structures. The titanium-to-aluminum ratios affect the yields of the polymers as well as the microstructures. There also is some influence on the molecular weight of the product. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only cis and trans arrangements are affected. In addition, the molecular weights of the polymers and the microstructures are relatively insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the rates, the molecular weights, and the microstructures. 

Figure 11.1 Polymer microstructures of c/s-1,4 polybutadiene, trom-1,4 polybutadiene, 3,4 polyisoprene, 1,2-polybutadiene (syndio- and isotactic).

In spite of the high activities shown for BD polymerization by homogeneous catalyst systems, amongst these only a few have been found to be active for isoprene polymerization. MAO-activated titanium complexes produce 1,4-polyisoprene polymer with a prevalently cis microstructure (>94%). Recently, Miyazawa et al. have shown that monocyclopentadienyl titanium complexes activated by MAO can promote isoprene polymerization, giving polymers with narrow molecular weight distributions (Mw/Afn < 2) and cis contents of up to 92% with small amounts of 1,2- and 3,4-enchained structures also present. ... 

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... 

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. 

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). 

Recent studies of blends of polyisoprene (PIP) with polybutadiene (PBD) have revealed a number of remarkable features [1-5]. Non-polar hydrocarbon polymers such as PIP and PBD are not expected to exhibit miscibility given the absence of specific interactions. When the polybutadiene is high in 1,2 microstructure, however, it has a remarkable degree of miscibility with PIP. This miscibility is the consequence of a close similarity in both the polarizability and the expansivity of the two polymers [3,4]. Their mixtures represent a very unusual instance of miscibility between chemically distinct, non-reacting homopolymers. As its 1,4- content increases, both the polarizability and the thermal expansivity of the PBD diverge from that of PIP, resulting in a reduced degree of miscibility. This effect of PBD microstructure on miscibility with PIP can be seen in the data in Table I [3]. ... 

The hydrogenation of polyisoprene [55] provides the equivalent of poly (ethylene-a//-propylene) or PEP, typically with low levels of a 3-methyl-1-butene repeat unit due to 3,4 incorporation of isoprene during the anionic polymerization (Scheme 23.5). Hydrogenated polyisoprene is amorphous regardless of the microstructure of the polymer prior to hydrogenation. 

The microstructure of the polyisoprene is affected by temperature, a higher Al/Ti ratio being required at —30° C. to produce a structure comparable to that obtained with lower ratios at room temperature. Thus, a 1 to 1 ratio yields a polymer with 40% trans-, A- structure at —30° C. and 0% at room temperature. A 0% trans-, A- structure is obtained at —30° C. with a 1.2 to 1.4 1 Al/Ti mole ratio. 

Polymer Preparation. Two bifunctional (telechelic) polymers were used in this study. Carboxy-telechelic polybutadiene (PB) is commercially available from B. F. Goodrich (Hycar CTB 2000X156) with molecular characteristics of Mn=4,600, Mw/Mn= 1.8, functionality 2.00 and cis/trans/vinyl ratio of 20/65/15. Carboxy-telechelic polyisoprene (PIP) was prepared by anionic polymerization in THF at -78°C with a-methylstyrene tetramer as a difunctional initiator. The living macrodianions were deactivated by anhydrous carbon dioxide. Five polymers werejjrepared with Mn=6,000 10,000, 24,000, 30,000 and 37,000 having Mw/Mn=sl.l5 a microstructure ratio of 3, 4/1, 2 of 65/35, respectively, and a functionality >1.95. 

On the other hand, changes in the recipes of alkali metal polymerizations frequently make appreciable changes in the microstructures of the resultant polymers (2, 10, 12). Thus, sodium polybutadiene, or sodium polyisoprene, has a microstructure different from that of the corresponding potassium-catalyzed polymer. It also has been established that promoters or modifiers like dioxane or dimethoxytetraglycol affect the microstructure in these alkali metal catalyzed systems. One further example is afforded by the Alfin catalyst, which is apparently related to alkali metal catalysts but which gives a polybutadiene or polyisoprene with a microstructure very different from that of the corresponding alkali metal polymers. 

The microstructures of the polybutadienes, butadiene-styrene copolymers, and polyisoprenes were determined by infrared spectroscopic methods (1,3). The spectra of alkali metal-catalyzed polybutadienes and polyisoprenes show that other reactions occur during polymerization in addition to those involving cis- and trans 1,4, 1,2, and 3,4 additions. For sodium and potassium polybutadienes and polyisoprenes, the absorbances of the bands arising from these additional structures could be taken into account satisfactorily by the methods described. No foreign structures are found in lithium-catalyzed polyisoprenes and the additional band foimd near 14.2 microns in polybutadiene spectra does not appear to affect the cis-1,4 band at 14.7 microns. (Cesium and rubidium, as well as additives such as dimethoxy-tetraglycol, affect the polymerization of butadiene so markedly that it was not possible to obtain satisfactory analyses of such polymers. The effect of these catalysts in isoprene polymerizations does not appear to be so marked and satisfactory analyses were obtained by the method described. 

However, the excellent cold properties of the lithium polymer can be explained on the basis of microstructure in Table II. It seems reasonable to assume that of the three possible microstructures the 1,2 structure is the least desirable for low temperature flexibility followed by the frans-1,4 structure, with the cis-1,4 structure the most desirable. A comparison of the low temperature flexibility of balata (or gutta-percha) vs. Hevea rubber would indicate a preference for the cis-1,4 structure over the trans-1,4 structure, although these natural products are polyisoprenes rather than polybutadienes. In the case of the 1,2 structure, it is generally assumed that the prevalence of this structure in sodium-catalyzed polybutadiene, or butadiene copolymers, accounts for its poor cold properties however, the occurrence of a natural or synthetic product with an entirely 1,2 structure would help to confirm this more definitely. The relative predominance of any single structure is another important consideration in the performance of a rubber at low temperatures because a polymer with a large percentage of one structure would be more likely to crystallize at a low temperature. 

Lithium metal-catalyzed polyisoprene and polybutadiene have unusual microstructures compared to the analogous polymers made with the other alkali metals. The lithium metal-catalyzed polyisoprene, named Coral rubber, has a microstructure almost identical to that of Hevea rubber. The unusual microstructure of the lithium metal-catalyzed polybutadiene, or butadiene-styrene copolymer, probably accounts for its superior rubberlike properties at low temperatures. 

It may be enquired why it is that natural rubber is highly stereoregular, and in this respect is ery different from butadiene polymers obtained by free-radical polymerization which contain a mixture of microstructures. The reason is that the polyisoprene produced by the Hevea brasiliensis tree is formed not by polymerization of isoprene, free-radical or otherwise, but by an enzyme-catalysed condensation of isopentenyl pyrophosphate (see Section 23.5.2). Natural rubber usually contains some crosslinked polyma- gel, at least after it has left the tree and become exposed to the atmospha-e. Crosslinking does not, however, occur by polymerization through the olefinic double bonds of the polyisoprene chain, but by reactions involving minor concentrations of other functional groups which are attached to the polyisoprene chain. 

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