Polyisoprenes preparation

Fig. 24a and b. Differential molecular weight distribution curves of polyisoprene prepared, a) with oligoisoprenyl-Li b) with oligoisoprenyl-Li in the presence of NNN N tetramethyl ethylenediamine [TMDA]/[OLi] = 0.01) (A. Davidjan, N. N. Lolaev, V. Sgonnik, B. Belenkii, V. Nesterov, B. Erus-salimsky, Ref. 1531... 

Unlike polybutadiene, polyisoprene prepared at low temperatures shows little or no inclination to crystallize either on stretching or cooling. This may seem surprising in view of the even greater preponderance of trans-1 4 units in polyisoprene than in poly butadiene. The explanation for the contrasting behavior in this respect between low temperature synthetic polyisoprene, on the one hand, and guttapercha and low temperature polybutadiene, on the other, probably is to be found in the appreciable occurrence of head-to-head and tail-to-tail sequences of 1,4 units of the former. 

The spectra may first be evaluated qualitatively.The polyisoprene prepared in solution shows a pronounced band at 888 cm which indicates a high proportion of 3,4-link-ages. For the product of bulk polymerization this band is much reduced in favor of absorptions at 1127 and 1315 cm indicating predominantly c/s-1,4-linkage of the monomeric units in this case.The polymer made by radical polymerization in emulsion (see Example 3-12) shows the presence of all possible structural units,although the proportion of c/s-1,4-linkages is low. 

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. 

Meanwhile, development of coordination catalyst was proceeding full scale. The polyisoprene prepared using this coordination catalyst (TiClj, AIR ) proved to be more suitable in physical properties than the one made by lithium metal or organolithium compounds in hydrocarbon media. The Ziegler polyisoprene, as it was called, has greater stereoregularity and stress-induced crystallization properties than polyisoprene made by the alkyl lithium catalyst. How-... 

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. 

Fig. 12a c. Chromatographs of polyisoprenes prepared in the presence of low concentrations of TMEDA. (Reprinted with permission from Ref. la9>, Copyright 1979, Huthig and Wepf Ver-lag)... 

The functionality of hydroxy-telechelic polybutadiene and polyisoprene was determined by -NMR after modification with phenyl isocyanate or naphthyl isocyanate. Liquid telechelic polybutadiene and polyisoprene, prepared by choosing appropriate reaction conditions, showed Fn value of about 1.9, which agreed with the values determined by titration method [69]. 

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

Polyisoprene prepared with /3-TiCh catalyst activated with AlEta or, in particular, AlEt2Cl has a high structural purity. Optimum cis-contents of up... 

The termination reactions appear to be quantitative and specific for the reaction of saturated polymer molecules attached to aluminium and titanium [116], but applied to diene polymerizations the method is less satisfactory mainly because of the greater stability of allylic carbon-transition metal bonds. Polybutadiene has been labelled by terminating with tritiated methanol with the Cr(acac)3/AlEt3 catalyst [55], and similarly polyisoprene prepared with VCl3/AlEt3 [107]. Polybutadiene prepared with Til4/Al(i-Bu)3 has been labelled using C02 [115]. 

Most of the data referred to above were obtained in earlier work, and were based on infrared spectroscopy. In recent years, more reliable data were obtained by means of NMR spectroscopy, using both and resonances (9-14). Some of these investigations suggested that, aside from the dramatic effects of polar solvents on the chain structure in organolithium systems, there were some subtle effects even in non-polar media, e.g., caused by initiator concentration and type of non-polar solvent. Sinn and coworkers ( ), for example, used infrared spectroscopy to show an effect of butyl lithium concentration on the chain structure of polyisoprene and polybutadiene. Hence an extensive study was carried out recently (, M) on the influence of reaction parameters on the chain structure of polybutadiene and polyisoprene prepared in non-polar media. 

The raicrostructure of a polyisoprene prepared by this initiator was reported to contain 92% 1,4-units and 8% 3,4-units. 

Synthetic polyisoprene, prepared by free-radical polymerization of isoprene monomer, is a copolymer of six structurally distinct kinds of isoprene chain units. Unlike natural rubber, which is a regularly repeating Class I structure (cis-1,4), such synthetic polyisoprene does not crystallize. On the other hand, by the use of the appropriate stereospecific catalyst, isoprene monomer can be converted to a regular Class I polymer with the same structure as natural rubber (. ... 

Isomeric Composition of Polyisoprenes Prepared by Alkali Metal Catalysts... 

The polyisoprenes prepared in strongly polar solvents, such as ethers, have a mixed isomeric composition. The composition depends on solvent as well as on initiator. Tobolsky and Rogers (1959) found that the predominate isomers are those from 3,4 and tmns-1,4 additions. If weakly coordinating solvents are used. 

The influence of the reaction conditions upon the stereochemistry of polymerization has been examined for a number of systems. The presence of a small amount of water increased the isotacticity of the polymerization of ethyl methacrylate in toluene at -78 °C. A new type of cyclic structure for polybutadiene-a substituted cyclopentane - was identified in polymer prepared using an organo-lithium initiator in the presence of tetramethylethylene diamine. The polymerization of 1-phenylbutadiene initiated by alkyl-lithium in hydrocarbon solvents results in 50—60% trans-, A, some 25% c/r-1,4, and 10— 25% 3,4 enchainment. When THF is employed as solvent, the corresponding values are about 80, 10, and 10%, respectively. The anionically-prepared polymers have structures very different from those formed cationically, which comprise lately 3,4 addition accompanied by extensive cyclization. The distribution of cis-and trans-1,4 structures in polyisoprene prepared using Bu Li in non-polar media has been shown by C n.ra.r. spectroscopy to be nearly random. The polymerization of fumaronitrile is initiated in benzene solution by the addition of butyl-lithium across one C N group in THF solution initiation is by transfer of an electron from BuLi. ... 

In Table IV are shewn the results of our NMR analyses of 20 sanples of cyclized polyisoprene, prepared as noted in Table I. Because of the relevance bo the block copolymersl0 12 mentioned above, the bulk of the data are for cycliz lithivm polyisoprene. Ihere are, however, sene results for cyclized natural rubber, both hevea and gu yule. 

Polyisoprene-Z)/oc -polyferrocenyldimethylsilane (PI-i-PFDMS) was synthesized by the anionic ring-opening polymerization of 1 initiated by living polyisoprene prepared inTHF (Scheme 1). The products were recovered as amber gumlike materials. 

Polyisoprenes prepared by alkyl lithium initiated polymerizations conducted in hydrocarbon solvents contain predominantly cis and trans units, but these may be enchained in a 1,4- or 4,1- manner. Cmr studies indicate that the cis and trans units are distributed randomly and that negligible amounts of 4,1-1,4 and... 

Polyisoprene prepared with a Ti(OR)4 -Et3Al catalyst system contains 3,4- and 1,4-cis-isoprene units almost exclusively and these are enchained mostly in a head-tail fashion [13]. The 3,4-3,4 dyads have a predominantly meso configuration. The cmr spectrum of this polymer has been interpreted in terms of a structure which obeys first order Markoffian statistics [108]. This is an interesting result because the structures of all other polymers prepared from dienes seem to have Bernoullian structiires, even when 1,4-cis, 1,4-trans, 1,2- or 3,4- units are present simultaneously [11,13,23]. 

General Characteristics of Polyisoprene. As a homopolymer with a high 1,4-cis units content, IR is not very different from natural rubber (NR) and it is thus produced only in the countries that are not supplied in NR. Polyisoprenes prepared by anionic polymerization exhibit low dispersities. 

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