In early 1954 Montecatini and the Polytechnic learned that Karl Ziegler had managed to obtain high molecular weight ethylene polymers with a low-pressure reaction based on using a mixture of triethylaluminium and titanium tetrachloride as a catalyst.
Giulio Natta and all of us always acknowledged that our research stemmed from this finding, which was made available by the licensing agreements reached with Ziegler himself.
Giulio Natta’s revolutionary idea was to attempt to polymerize propylene using the same catalyst.
At the time, in fact, high molecular weight ethylene polymers could be produced by operating at ultra-high pressures and using free radical initiators. It was also well known that ethylene did not polymerize with cationic catalysts. Instead, propylene did not polymerize with the use of free radicals, while it easily polymerized with cationic catalysts, providing low molecular weight polymers consisting of oily products having limited industrial applications (additives for lubricants).
Essentially, all the available knowledge showed that ethylene and propylene acted entirely differently in polymerization reactions.
At this point, a fact known only to those who witnessed it at the time bears noting. At this point, a fact known only to those who witnessed it at the time bears noting. A couple of months before the first propylene polymerization we did, and before the first patent was filed, Montecatini’s patent experts deemed it appropriate to formally ask Karl Ziegler whether he thought it possible to obtain propylene polymers using the same types of catalysts with which he obtained new ethylene polymers. Ziegler’s reply was a resounding no.
On 06 June 1954 Montecatini filed a patent, naming Giulio Natta as the inventor, for new propylene polymers, in particular those with high crystallinity (extraction residue), and the procedure for producing these polymers.
In conjunction with the filing of this first patent, a lack of reproducibility in the propylene polymerization tests was found, and, for several consecutive weeks, all attempts at polymerization failed.
It was during a polymerization test conducted by me and Paolo Longi that we decided, as a last-ditch attempt, to inject an additional quantity of titanium tetrachloride into the autoclave: immediately, a sharp increase in temperature indicated that polymerization was in progress and, when we opened the autoclave, we found a large quantity of polypropyle.
On 27 July 1954 Montecatini filed a second patent (no. 537425), naming Giulio Natta, Piero Pino, and Giorgio Mazzanti as inventors, for the process based on the preparation of the catalyst in the presence of the monomer and solid crystalline polymers of propylene.When extended abroad, the two patents were merged together, indicating Giulio Natta, Piero Pino, and Giorgio Mazzanti as inventors.
The results obtained in propylene polymerization were certainly of great scientific interest, but industrial production was still unthinkable so long as the yield from the high crystallinity fraction usable as a plastic remained at 30%.
I thought that to selectively obtain the crystallizable portion – one that therefore had a highly regular structure – it was necessary for a solid crystalline surface to become part of the catalyst system. Thanks to its regular surface structure, this could be in a condition of “imposing” the same steric configuration on the monomer units when they joined the polymer chain. Consequently, I had the idea of replacing the titanium tetrachloride, which is a liquid, with titanium trichloride, a crystalline solid. From the very first polymerization test, the yield of the highly crystalline fraction reached 90%.
Reasoning in reverse, by using highly dispersed catalysts, almost completely amorphous propylene polymers were obtained, which were particularly suited for the production of completely amorphous ethylene-propylene copolymers usable as synthetic rubbers.
Thus “stereospecific polymerization” began, opening the way for the industrial production of isotactic polypropylene and of the new ethylene-propylene rubbers.
These results became the object of patents no. 526101 (3 dicembre 1954) and no. 545332 (16 December 1954).
In late 1957, only three years after the first propylene polymerizations done in the Polytechnic’s labs, thanks to the courageous act of faith and to the consequent, enormous efforts made by Montecatini, Ferrara plant XXIII began industrial production of isotactic polypropylene.
A year later, in 1958, the Ferrara plants were carrying out the first production of ethylene-propylene rubbers.
At present, on the world market, polypropylene is the number-two plastic in terms of production volume; ethylene-propylene copolymers are the number-two synthetic rubber.