Polyisoprenes stereoregular

The physical properties of any polyisoprene depend not only on the microstmctural 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 stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures 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). 

Amorphous (most likely atactic) 3,4-polyisoprene of 94—100% 3,4-microstmcture was prepared with a (C2H 3A1—Ti(0—/ -C Hy) catalyst (11). Crystalline 3,4-polyisoprene containing about 70% 3,4-units and about 30% i7j -l,4-microstmcture was prepared using a catalyst derived from iron acetyl acetonate, trialkyl aluminum, 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-microstmcture with the remainder being cis-1,4 microstmcture was prepared in conversions of greater than 95% with a water-modified tri alkyl aluminum, ferric acetyl acetonate, and 1,10-phenanthroline catalyst (38). The 3,4-polyisoprene is stereoregular and beheved to be syndiotactic or isotactic. 

Related to stereoregularity is the possibility of cis, trans isomerism. The molecule of natural rubber is a c/s-1,4-polyisoprene whilst that of gutta percha is the trans isomer. 

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or anionic initiators are used. The most important coordination catalyst is a-TiCls cocatalyzed with aluminum alkyls. The polymerization rate and cis... 

During the last two decades, a number of diene homopolymers and copolymers have been developed to fill the diverse elastomer needs in the production of tires. The earliest developments were mainly concerned with the preparation of stereoregular cis-1,4-polyisoprene, as a substitute for natural rubber, using... 

The general correlations of structure and properties of homopolymers are summarized in Table 2.13. Some experiments which demonstrate the influence of the molecular weight or the structure on selected properties of polymers are described in Examples 3-6 (degree of polymerization of polystyrene and solution viscosity), 3-15, 3-21, 3-31 (stereoregularity of polyisoprene resp. polystyrene), 4-7 and 5-11 (influence of crosslinking) or Sects. 4.1.1 and 4.1.2 (stiffness of the main chain of aliphatic and aromatic polyesters and polyamides). 

Tacticity and geometric isomerism affect the tendency toward crystallization the tendency increases as the tacticity (stereoregularity) is increased and when the geometric isomers are predominantly trans. Thus isotactic PS is crystalline, whereas atactic PS is largely amorphous and c/s-polyisoprene is amorphous, whereas the more easily packed trans isomer is crystalline. 

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 Ziegler-Natta catalysts have acquired practical importance particularly as heterogeneous systems, mostly owing to the commercial production of linear high- and low-density polyethylenes and isotactic polypropylene. Elastomers based on ethylene-propylene copolymers (with the use of vanadium-based catalysts) as well as 1,4-cz s-and 1,4-tran.y-poly(l, 3-butadiene) and polyisoprene are also produced. These catalysts are extremely versatile and can be used in many other polymerisations of various hydrocarbon monomers, leading very often to polymers of different stereoregularity. In 1963, both Ziegler and Natta were awarded the Nobel Prize in chemistry. 

Some of the polybutadienes obtained with transition metal-based coordination catalysts have practical significance the most important is cA-1,4-polybutadiene, which exhibits excellent elastomeric properties. As regards isoprene polymers, two highly stereoregular polyisoprenes, a cA-1,4 polymer (very similar to natural rubber) and a trans- 1,4-polymer (of equal structure to that of gutta percha or balata) have been obtained with coordination catalysts. Various polymers of mixed 3,4 structure, amorphous by X-ray, were also obtained [7]. 

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 catalysis of the stereospecific polymerization of conjugated dienes is of considerable interest from both the scientific and the industrial points of view [1,2]. From butadiene and isoprene, as the industrially most important 1,3-dienes, in comparison with the polymerization of olefins many more structurally different stereoregular polymers can be derived cf the structures of the stereoregular polybutadienes and polyisoprenes given in Scheme 1 [106]. 

Scheme 1. The structurally different stereoregular polybutadienes and polyisoprenes obtainable from the monomers by 1,4-, 1,2- or 3,4-C-C bond linking, respectively.

Synthetic natural rubber, cw-polyisoprene, is an example of a stereospecific polymer made possible by this means. There are five types of stereo specific (or stereoregular) structures cis, trans, isotactic, syndi-otactic, and tritactic. 

The most important factor governing the properties of polyisoprene is the stereoregularity of the polymer chain. The very unique characteristic of NR is the ability to crystallize under strain, the phenomenon known as strain-induced crystallization. Stretching of vulcanizates of polyisoprene having at least 90% cis content leads to crystallization, which in turn leads to strengthening of mbber [10]. 

Regularity per se is not sufficient to ensure crystallizability in polymers. The spatial regularity and packing are important. To illustrate this, let us consider two examples of stereoregular polymers. Table 3.8 shows the properties of two isomers cis- and /raw5-polyisoprene. It is obvious from the table that the stereoregular trans form is more readily packed and cry stallizable and has properties of crystalline polymers. 

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. 

Thus, macromolecules stereoregularity is provided by chemical nature of used catalytic system and its formation conditions (components ratio, time ageing, temperature, catalytic system modification by electro-donors). At that by polymerization conditions varying one can influence on resulting polyisoprene MM and MMD. At the same time it is obvious that mechanical effect... 

Formation of the TiCl4-Al(/-C4H9)3 catalyst in the turbulent mode (excluding Method 3) does not change the polyisoprene microstructure (Table 3.8). A highly stereoregular polymer is formed in all cases. Isoprene polymerisation in the tubular turbulent prereactor, in the presence of the Ti-Al catalyst, leads to the formation of the polymer with an increased content of a s-l,4-links, (up to 96.8%) and a lower content of 1, -trans and 3,4-links (1.7% and 1.5%, respectively). 

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