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the first commercial catalyst resulted from the industrial extension of Ziegler & Natta's pioneering work on the relationship between the crystal strustires of titanium chlorides and the overall activity and selectivity of the catalyst . Natta and his collaborators discovered that Ticl3 was more stereoselective and that only three structural modification , out of the four possible ones for Ticl3 , were highly stereoselective . the active modification ( namely alpha , beta , gamma )had a deep purple colour and a layer lattice structure , whereas beta-Ticl3 was brown and had a chain like structure .

since 1964 , a llewis base was added to the catalyst , essentially in order to improve the stereoregularity of polymers . probably montecatini took the first patent for the catalytic system composed of Ticl3/AlEt3 and pyridine . the adition of a lewis base was a typical 'chemical move' , based on knowledge from coordination chemostry . however , the higher stereospecificity did not corrwespond to a similar increase in the activity .

a real progress was obtained only later , when solway introduced the second generation catalyst . in this case , the crucial move towards increasing activity was made following the solid state chemistry procedure : the transformation of the brown Ticl3 into the stereoselective delta-Ticl3 at a low temperature ( < 100 c) in the presence of Ticl4 , which acts as a catalyst (!) for the change of phase . the lowering of temperature from 160-200 to 65 c prevented the catalyst particles from growing . this type of inovation increased the activity of catalysts by a factor of 5 , and is stereospecificity to , so that the removal of the atactic fraction from the fnal product was spared . it is iportanmt to stress that that the success of the catalyst of the second generation was due to the new morphology of the catalyst particles . they were smaller , and beccause of a pre - treatment with ether ( for extracting Alcl3 ) , they had a porous and weakly bonded matrix .

the pre-history of the third generation catalyst began in 1960 , when shell patented a catalyst for propeylene polymerizatin that used Ticl4 supported on Mgcl2 .the third generation of catalyst brought about a 50-fold increase of activity , so that the removal of the residual catalyst from the final product was no more necessary .

the fourth generation catalysts , based on metallocene compounds , is now evolving towards an industrial success . Kaminsky ans Sinn suspected the formation of methyl aluminoxane (MAO) , and, on folowing their conjecture , they discovered that MAO - activated homogenous metallocene catalyst were capable of polymerising propene and higher olefins . the discovery by Kaminsky and Sinn offered a uniqe occasion in the site of a single-site catalyst . infact " practical application of metallocene catalyst requires their preadsorption on solid supports such as alumina and silica gels " .

it is this control of the architecture or the three dimensional structure of the catalyst that has enabled the polymerisation not only to reproduce the shape of the catalyst , but also to generate a solid particle with a controlled reproducible prorsity . the metaphor of the reactor granule is reveling a new understanding and will to control of the polymeriisation reaction, whose practical result is " opening the way to the creation of new and completely revolutionary families of materials . "

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