Polyisoprene Anionic polymerization

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

Synthetic cA-l,4-polyisoprene (structure 5.42) is produced at an annual rate of about 100,000 t by the anionic polymerization of isoprene when a low dielectric solvent, such as hexane, and K-butyllithium are used. But, when a stronger dielectric solvent, such as diethy-lether, is used along with w-butyllithium, equal molar amount of tra i -l,4-polyisoprene and cA-3,4-polyisoprene units is produced. It is believed that an intermediate cisoid conformation assures the formation of a cis product. An outline describing the formation of cA-1,4-polyisoprene is given in structure 5.42. 

Over 5.5 billion pounds of synthetic rubber is produced annually in the United States. The principle elastomer is the copolymer of butadiene (75%) and styrene (25) (SBR) produced at an annual rate of over 1 million tons by the emulsion polymerization of butadiene and styrene. The copolymer of butadiene and acrylonitrile (Buna-H, NBR) is also produced by the emulsion process at an annual rate of about 200 million pounds. Likewise, neoprene is produced by the emulsion polymerization of chloroprene at an annual rate of over 125,000 t. Butyl rubber is produced by the low-temperature cationic copolymerization of isobutylene (90%) and isoprene (10%) at an annual rate of about 150,000 t. Polybutadiene, polyisoprene, and EPDM are produced by the anionic polymerization of about 600,000, 100,000, and 350,000 t, respectively. Many other elastomers are also produced. 

These efforts coupled with the much earlier work on sodium and lithium initiated polymerizations led to an appreciation of the stereospecificity of the alkyllithium initiators for diene polymerization both industrially and academically. Polymerization of isoprene to a high cis polyisoprene with butyllithium is well known and the details have been well documented 2 Control over polybutadiene structure has also been demonstrated. This report attempts to survey the unique features of anionic polymerization with an emphasis on the chemistry and its commercial applications and is not intended as a comprehensive review. 

The first report on anionic polymerization appeared in the patent literature in 1910-1911. Matthews and Strange (l) in 1910 and later Harries (2) in 1911, described the preparation of polyisoprene using sodium and potassium as initiators. They mentioned the use of lithium as a possible initiator for this polymerization, but there seems to be no description of the polymer... 

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... 

These results show that the 1,2-polymerization of butadiene requires a less anionic catalyst than the anionic polymerization of styrene. Tobolsky and Rogers (58) studied the same effects of catalyst anionicity on the copolymerization of styrene and isoprene. They found that the increased anionic character of the lithium-THF combination relative to butyllithium catalysts increased the styrene content of the polymer as well as decreased the 1.4-structure of the polyisoprene. 

ABC copolymers polystyrene-polyisoprene-poly(vinyl-2-pyridine)(S.I.V2P) with number-molecular average weight of 23000,102000, and 23000 were prepared by stepwise anionic polymerization. Films obtained by solvent casting from methylcyclohexane and benzene were observed by electron microscopy after staining the polyisoprene block with osmium tetroxide or the poly(vinyl-2-pyridine)... 

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. 

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. 

Polybutadiene, CAS 9003-17-2, is a common synthetic polymer with the formula (-CH2CH=CHCH2-)n- The cis form (CAS 40022-03-5) of the polymer can be obtained by coordination or anionic polymerization. It is used mainly in tires blended with natural rubber and synthetic copolymers. The trans form is less common. 1,4-Polyisoprene in cis form, CAS 9003-31-0, is commonly found in large quantities as natural rubber, but also can be obtained synthetically, for example, using the coordination or anionic polymerization of 2-methyl-1,3-butadiene. Stereoregular synthetic cis-polyisoprene has properties practically identical to natural rubber, but this material is not highly competitive in price with natural rubber, and its industrial production is lower than that of other unsaturated polyhydrocarbons. Synthetic frans-polyisoprene, CAS 104389-31-3, also is known. Pyrolysis and the thermal decomposition of these polymers has been studied frequently [1-18]. Some reports on thermal decomposition products of polybutadiene and polyisoprene reported in literature are summarized in Table 7.1.1 [19]. 

The standard molecular structural parameters that one would like to control in block copolymer structures, especially in the context of polymeric nanostructures, are the relative size and nature of the blocks. The relative size implies the length of the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more elaborate description that includes its solubility characteristics, glass transition temperature (Tg), relative chain stiffness, etc. Using standard living polymerization methods, the size of the blocks is readily controlled by the ratio of the monomer concentration to that of the initiator. The relative sizes of the blocks can thus be easily fine-tuned very precisely to date the best control of these parameters in block copolymers is achieved using anionic polymerization. The nature of each block, on the other hand, is controlled by the selection of the monomer for instance, styrene would provide a relatively stiff (hard) block while isoprene would provide a soft one. This is a consequence of the very low Tg of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain. 

Anionic Vinyl Polymerization. The carbanionic terminals in living anionic polymerization can be transformed into carbon—halogen bonds suited for radical generation. The backbones utilized thus far for this approach include polystyrene (B-58 to B-62)215,374 and polyisoprene (B-63 and B-64),374,375 although the former segment can also be prepared by the living radical polymerization (Figure 19). 

A transformation method can introduce some functional groups at the junction as in B-63, which bear a fluorescent dye between the polyisoprene and polystyrene segments.375 The preparation is based on quenching the living anionic polymerization of isoprene with l-(9-phenanthryl)-l-phenylethylene followed by addition of excess a,a -dibromo-/>xylene, which affords a C—Br terminal effective for the copper-catalyzed radical polymerization of styrene. 

A new star—block copolymer architecture, the inverse star—block copolymer, was recently reported.87 These polymers are stars having four polystyrene-/risoprene) copolymers as arms. Two of these arms are connected to the star center by the polystyrene block, whereas the other two are connected through the polyisoprene block. The synthetic procedure is given in Scheme 32. The living diblocks (I) were prepared by anionic polymerization and sequential addition of monomers. A small quantity of THF was used to accelerate the initiation of the polymerization of styrene. The living diblock copolymer (I) was slowly added to a solution of SiCL. The reaction was monitored by SEC on samples with-... 

The sulfonation of low molecular weight model olefins was undertaken to determine the feasibility of this approach. Competitive sulfonations using acetyl sulfate were carried out on the model compounds below, representing the repeat structures of cis-l,4-polyisoprene (PIP), cw-l,4-polybutadiene (c-PBD), and trans-l,4-polybutadiene (Z-PBD), respectively. It was necessary to model both the cis and trans isomeric forms of 1,4-polybutadiene, since ttey have a nearly equal probability of occurrence when the anionic polymerization (Ii counterion) is conducted in a nonpolar hydrocarbon medium... 

Since rate constants for.PVL In acetonitrile or DMSO are the same or higher than those of ormethyl-or-propylproplolactone, the polymerization rate of PVL In THF should be Just as high as that of the methyl, propyl derivative. If this Is so, the above data suggest anionic polymerization of PVL proceeds as fast as anionic polymerization of Isoprene In hydrocarbons to cls-1,4-polyIsoprene. This Is Indeed quite Impressive. 

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. 

When lithium alkyls are used in anionic polymerizations, they tend to give cis products (m-polybutadiene and m-polyisoprene). There is also no termination step with these polymerizations. The rate of polymerization depends on the amounts of initiator and monomer present [13]. 

---