Isoprene Ziegler—Natta polymerization

Discuss the use of homogeneous versus heterogeneous reaction conditions for the coordination and traditional Ziegler-Natta polymerizations of propene, isoprene, styrene, methyl methacrylate, and n-butyl vinyl ether. 

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cycliza-tion to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create in-termolecular cross-links that render the exposed areas insoluble. 

This review article is concerned with chemical behavior of organo-lithium, -aluminum and -zinc compounds in initiation reactions of diolefins, polar vinyls and oxirane compounds. Discussions are given with respect to the following five topics 1) lithium alkylamide as initiator for polymerizations of isoprene and 1,4-divinylbenzene 2) initiation of N-carboxy-a-aminoacid anhydride(NCA) by a primary amino group 3) activated aluminum alkyl and zinc alkyl 4) initiation of stereospecific polymerization of methyloxirane and 5) comparison of stereospecific polymerization of methyloxirane with Ziegler-Natta polymerization. A comprehensive interpretation is proposed for chemistry of reactivity and/or stereospecificity of organometallic compounds in ionic polymerizations. 

Isoprene Ziegler-Natta-catalyzed chain polymerization Car tires (55%), mechanical goods, sporting goods, footwear, sealants, and caulking compounds... 

Cyclized rubber or cyclized 1,4-poly-(cis-isoprene) which serves as polymer mtrix can be made from Ziegler - Natta polymerization of isoprene. It is soluble in non-polar organic solvents like toluene, xylene, halogenated aliphatic hydrocarbons. 

Alkenes. —Reviews on Ziegler-Natta catalysis and the stereoregular and sequence-regular polymerization of butadiene have been published and the stereoselective oligomerizations of isoprene by lithium and palladium catalysts have been compared. Semi-empirical MO calculations suggest that Ziegler-Natta polymerization proceeds via a bis-alkene complex and a metallacyclo-pentane intermediate. ... 

Coordination polymerization of isoprene using Ziegler-Natta catalyst systems (Section 6 21) gives a material similar in properties to natural rubber as does polymerization of 1 3 butadiene Poly(1 3 buta diene) is produced in about two thirds the quantity of SBR each year It too finds its principal use in tires... 

Conjugated dienes are among the most significant building blocks both in laboratories and in the chemical industry [1], Especially, 1,3-butadiene and isoprene are key feedstocks for the manufacture of polymers and fine chemicals. Since the discovery of the Ziegler-Natta catalyst for the polymerizations of ethylene and propylene, the powerful features of transition metal catalysis has been widely recognized, and studies in this field have been pursued very actively [2-7]. 

The all-cis structure of natural rubber is vita) to its elasticity. The all-trans compound is known and it is hard and brittle. Though dienes such as isoprene can easily be polymerized by cationic methods, the resulting rubber is not all-cis and has poor elasticity and durability. However, polymerization of isoprene in the Ziegler-Natta way gives an all-cis (90-95% at least) polyisoprene very similar to natural rubber. 

For instance, in the field of elastomers, alkyllithium catalyst systems are used commercially for producing butadiene homopolymers and copolymers and, to a somewhat lesser extent, polyisoprene. Another class of important, industrial polymerization systems consists of those catalyzed by alkylaluminum compounds and various compounds of transition metals used as cocatalysts. The symposium papers reported several variations of these polymerization systems in which cocatalysts are titanium halides for isoprene or propylene and cobalt salts for butadiene. The stereospecificity and mechanism of polymerization with these monomers were compared using the above cocatalysts as well as vanadium trichloride. Also included is the application of Ziegler-Natta catalysts to the rather novel polymerization of 1,3-pentadiene to polymeric cis-1,4 stereoisomers which have potential interest as elastomers. 

Only single insertions into an Al-C bond occur for propene and higher alkenes and this is utilized for catalytic dimerization of propene as illustrated in Scheme 3. Insertion of propene into an Al-C bond of "PrsAl followed by )3-hydride elimination yields an aluminum hydride and 2-methylpent-l-ene. Insertion of propene into the Al-H bond regenerates "PrsAl. Thermal cracking of 2-methylpent-l-ene gives isoprene, which is subsequently polymerized with a Ziegler-Natta catalyst to form the synthetic rubber, cA-1,4-polyisoprene. 

It is possible to control the type of polymer produced. This is best illustrated with polymerization of the monomer isoprene 2-methyl-1,3-butadiene. The Ziegler-Natta catalyst, consisting of a titanium tetrachloride... 

The structure of the polymer obtained in the polymerization of butadiene and isoprene with heterogeneous Ziegler-Natta catalysts depends on the nature of the monomer, catalyst system, and reaction conditions. Previously reported results are reviewed and a mechanism is proposed for the stereoregulated polymerization of conjugated dienes. The polymerization of cyclopentadiene with LiAlH -TiCl4 or LiAlR4-TiCl4 catalyst system yields a readily oxidized polymer for which a 1,2-structure is proposed. 

The polymerization of butadiene and isoprene with an organometallic compound-transition metal compound Ziegler-Natta type catalyst can lead to polymers wdth a microstructure which is all cis-1,4-, trans l,4-, isotatic 1,2- (or 3,4-), syndiotactic 1,2- (or 3,4-), or a combination of two or more of these structural imits. The polymer microstructure may be dependent upon the ratio of the catalyst components, the temperature, and the reaction medium. 

Table IV. Isoprene Polymerization with Ziegler-Natta Catalysts...

Figure 1. Scheme for polymerization of isoprene to cis-1,4-polyisoprene with Ziegler-Natta type catalyst... 

Additionally, if the initiation reaction is more rapid an the chain propagation, a very narrow molecular weight distribution, MJM = 1 (Poisson distribution), is obtained. Typically living character is shown by the anionic polymerization of butadiene and isoprene with the lithium alkyls [77, 78], but it has been found also in butadiene polymerization with allylneodymium compounds [49] and Ziegler-Natta catalysts containing titanium iodide [77]. On the other hand, the chain growth can be terminated by a chain transfer reaction with the monomer via /0-hydride elimination, as has already been mentioned above for the allylcobalt complex-catalyzed 1,2-polymerization of butadiene. 

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