Polyisoprene, living polymer

The observations discussed above suggest that the kinetic order of lithium poly-isoprene propagation should vary with the living polymer concentration. The effect is imperceptible in aliphatic hydrocarbons, but is observed in benzene solutions. The apparent propagation constants of lithium polyisoprene (MW 2 2 10 ) were determined in benzene and the results are displayed in Fig. 16 in the form of a plot of log kapp vs log c, c denoting the total living polymer concentration. 

Brody, H., M. Ladacki, R. Milkovich and M. Szwarc Molecular weight of living polymers. Polybutadiene and polyisoprene. J. Polymer Sci. 25,221 (1957). 

Amino-terminated telechelic polybutadiene was prepared by LiAlH4 reduction of amidino end-group in polybutadiene, which was polymerised by a water-soluble initiator, 2,2 -azobis(amidinopropane)dihydrochloride. The structure was analysed by 1H- and 13C-NMR, but functionality of 2.0 was obtained by a titration method [70]. Synthesis of co-epoxy-functionalised polyisoprene was carried out by the reaction of 2-bromoethyloxirane with living polymer that was initiated with sec-butyl lithium. The functionality of the resulting polyisoprene was 1.04 by 1H-NMR and 1.00 by thin layer chromatography detected with flame ionisation detection [71]. 

The ability of living polymers to resume growth with the addition of fresh monomer provides an excellent opportunity for the preparation of block copolymers. For example, if a living polymer with one active end from monomer A can initiate the polymerization of monomer B, then an A-AB-B type copolymer can be obtained (e.g., styrene-isoprene copolymer). If, however, both ends of polymer A are active, a copolymer of the type B-BA-AB-B results. Examples are the thermoplastic rubbers polysty-rene-polyisoprene-polystyrene and poly(ethylene oxide)-polystyrene-poly(ethylene oxide). In principle, for fixed amounts of two monomers that are capable of mutual formation of living polymers, a series of polymers with constant composition and molecular weight but of desired structural pattern can be produced by varying the fraction and order of addition of each monomer. 

The reaction scheme requires that the end block be polymerized first in cyclohexane and then transfered as living polymer to a isopentane solution to make the second block of pure cis 1,4 polyisoprene. To complete the systhesis the di-block polymer is coupled with a di-ester or di-functional chlorosilane coupling agent to obtain the final linear tri-block copolymer. The series of reactions are as follows ... 

With the exception of a few commercial polymers such as polyisobutylene, polybutadiene and styrene-butadiene block copolymers, living polymers are prepared in small quantities under stringent conditions. Larger amounts can only be prepared by repeating the synthesis many times, and this is a costly and time-consuming process. In the case of hydrogenated polybutadiene, to prepare samples that resemble polyethylene, the need for a secondary reaction step renders the preparation even more costly. This has so far limited the extent to which it has been possible to use these materials to test models. Cell et al. [ 18] prepared asymmetric stars with structures similar to ethylene-propylene copolymers by hydrogenation of star-branched polyisoprene. The reactions to produce these materials took up to three weeks, and... 

Although there are several mechanisms by which living polymers can be prepared, anionic polymerization to date represents the most successful commercial application. The particular usefulness of lithium alkyls has been with dienes to give cw-polyisoprene and c -polybutadiene, although they can yield isotactic or atactic polymers of styrene and MMA. It is a peculiarity of polymerizations with lithium alkyl that there is no termination step. The rate of polymerization depends on the amount of initiator and monomer present [19]. 

Materials that are constructed from organic polymers such as polyethylene, polystyrene, polyisoprene (natural rubber and a synthetic elastomer) and poly(vinyl chloride) are common features of our daily lives. Most of these and related organic polymers are generated from acyclic precursors by free radical, anionic, cationic or organometallic polymerisation processes or by condensation reactions. Cyclic precursors are rarely used for the production of organic polymers. 

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. 

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-... 

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. 

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