Initial Oligomerisation Trials

Initial catalytic trials with the 2,6-iPr complex were performed using 1-hexyne as

monomer. The reason for this was that the supply of acetylene was uncontrollably delayed upon commencement at Imperial College, thus the use of 1-hexyne served as a probe for general reactivity of this catalyst toward alkynes. Owing to the high reported activity of this complex with ethylene, only 5 µmol was used per trial.

Standard conditions for the system were 100 equivalents of MAO and 500 of diethylzinc, in 50 mL of toluene. The addition of 2000 equivalents of 1-hexyne led to an initial darkening of the solution, however this did not develop further over 30 minutes. GC-FID and GC-MS analysis of the quenched solution showed evidence of several oligomeric products. Most prominent was a large peak for 3-octene, which could form via a single hexyne insertion into an ethyl group derived from diethylzinc. There were three minor unidentified products: two of molecular weight 194 and one of 276, which were not definitively identified, but are the correct masses for two and three hexyne insertions into an ethyl group. Some cyclotrimerisation was evident based on the presence of small amounts of 1,2,4- and 1,3,5-tributylbenzene, and overall around half of the unreacted monomer remained. An increase in temperature to 60 ºC had little effect on product output, or overall conversion of

1-hexyne. The use of neat ZnEt2 failed to generate any oligomeric products, even

after stirring for 20 hours, highlighting the role of the iron catalyst. The cobalt and manganese analogues of the iron catalyst were trialled under these conditions (100 eq. MAO, 500 eq. ZnEt2), but did not produce the oligomers seen for the iron

catalyst. This result is compatible with the previously reported greater reactivity of the iron catalyst.118 So, initial screenings showed the iron catalyst to be reactive toward alkynes, although the major product suggests just a single linear insertion, and total monomer conversion was not achieved. The presence of cyclotrimers is interesting, as these structures cannot form via linear insertions into an ethyl group, and suggests that another parallel growth process may be occurring.

Before screening the iron catalyst with acetylene, the reactivity of this monomer with neat diethylzinc was tested. Given the results when triethylaluminium was used as an activator (see Chapter 3), it seemed sensible to test the metal alkyl alone at the onset. The reactivity observed for triethylaluminium was not, however, seen for diethylzinc. At room temperature the reaction solution developed a blue colour over 30 minutes, while at 60 ºC a darker purple hue evolved. GC analysis revealed the presence of only a trace of 1-butene (less than 1 mol% of the diethylzinc added), even at the higher temperature, and no higher oligomers. This was only a minute output compared to that of triethylaluminium with acetylene, so was not considered to be an issue.

The mild reactivity of the iron catalyst toward 1-hexyne was not the best indication of things to come. When the Fe/MAO/ZnEt2 system was exposed to 1 barg of

acetylene, a rapid reaction occurred and a bright red colour quickly formed (ca. seconds). This was accompanied by an initial exotherm to almost 50 ºC, which

was controlled using an external ice bath and the temperature kept around 20 ºC from then on. Interestingly, the initial flurry of activity did not continue throughout a 30 minute run, although the red solution did thicken noticeably after around 10 minutes. A slower acetylene uptake and a milder exotherm were noted, but these seemed to cease after around 15 minutes. Work-up yielded almost 2 g of bright red polymer, which darkened to black over time when left in air. GC analysis of the yellow organic phase confirmed the presence of 1-butene, 1,3-hexadiene and several isomers of octatriene, as well as showing evidence for some higher oligomers. These polyenes are consistent with linear acetylene insertion into an ethyl group; a small amount of benzene above the solvent background was also detected. Thus, there was evidence that this system was producing oligomeric products, and although the bulk of the product at this stage was solid polymer, this catalyst was certainly worth investigating further. During these initial experiments, the cobalt analogue tested with 1-hexyne was trialled, but showed only a mild reactivity toward acetylene. Given this and the similar results of the cobalt and manganese complexes with 1-hexyne, the manganese analogue was not trialled with acetylene.

It was useful to benchmark the reactivity of the iron catalyst without diethylzinc present, simply activated by MAO, to see if the metal alkyl was having an obvious effect. The difference between the two reactions was striking. Exposure of the activated iron catalyst to acetylene effected an almost instantaneous evolution of a dark purple colour in solution, after which no further activity was observed. The purple solid that resulted was jelly-like and weighed over 16 g when wet, however after sitting in a beaker in the fume hood overnight, only 250 mg of solid remained. It appears that polymer production resulted in the formation of a polymer/solvent gel, holding the majority of the reaction solvent, but which later evaporated. Attempts to

swell the polymer again by soaking it in toluene were not successful. It did seem curious that such a seemingly active catalyst would deactivate so quickly – the catalyst in the presence of diethylzinc also seemed to slow after its initial flurry of activity, but not in such a drastic way. This catalyst deactivation is discussed later, as is the nature of the polymer (see Sections 6.2.4 and 6.2.5).