181 TABLA 3.RESULTADOS DE LA EVALUACIÓN A LA DIMENSIÓN INSUMOS

While oligomer chain growth at aluminium is evidently occurring, in the process discussed thus far the production of oligomers is stoichiometric in aluminium; the oligomers are only released upon quenching the solution. Thus, it was of interest to

investigate methods that promote chain termination or chain transfer, rendering the process catalytic. Hydrogenative chain termination (Reaction 3-5) is used widely for molecular weight control in ethylene polymerisation with transition metals, although the reactivity of aluminium alkyls towards hydrogen was unknown at the time of this investigation. A further reason for considering hydrogen is that it is a co-product of the pyrolysis of natural gas to produce acetylene, and as such mixed acetylene/hydrogen streams would be available from such a process.

M R + H2 M H + RH (3-5)

Trials were performed using a standard amount of AlEt3 in toluene (0.3 M), with

mixtures of acetylene and hydrogen (Table 3-2). The acetylene partial pressure was kept constant at 2 bar, in line with earlier experiments. Using 2 bar of hydrogen in the mix, the production of solid polymer was greatly suppressed, particularly in longer runs; the organic phase appeared much clearer than runs without H2, with

none of the waxy green residue or black polyacetylene characteristically seen. An increase to 9.5 bar of hydrogen completely stopped the production of polymer.

Table 3-2. Oligomer and Polymer Yields for H2/acetylene mixturesa H2 Pressure (bar) Time (h) Polymer Mass (g) Oligomer Mass (g) Oligomer (mmol) 2 1 Trace 1.15 15.5 2 4 0.125 1.41 13.6 9.5 4 None 0.98 9.7

a_{All runs at 100 °C, 2 bar absolute acetylene pressure, [Al] = 0.3M}

Inspection of the GC chromatograms for the runs using 2 bar of hydrogen showed a normal spread of oligomers, when compared to experiments in the absence of hydrogen. The quantified oligomer output was comparable on a molar basis, although the oligomer mass was reduced. This decrease in productivity seems to

suggest that some inhibition of oligomerisation results from the presence of hydrogen – this observation is further discussed below. A run quenched with D2O revealed

some deuteration of the known peaks observed by GC-MS. However, there was a significant proportion of [D0]-oligomer present. The proportions of [D0]- and

[D1]-oligomer (Table 3-3) were calculated by inspection of the mass spectra of

known compounds, allowing for both natural isotopic abundances and fragmentations involving loss of protons. These results all suggest that some degree of chain termination is occurring in the presence of hydrogen; the reduction in polymer production for these runs would support this notion.

Table 3-3. [D0]-oligomer percentages in D2O quenched C2H2/H2 runa

Oligomer Percentage [D0] 1-butene 27% 1,3-hexadiene 42% C10 46% a _{2 bar H} 2, 2 bar C2H2, 4 h, 100ºC, [Al] = 0.3M

The run using 9.5 bars of hydrogen was more drastically affected in terms of product output. The C4 and C6 regions showed a fairly normal yield, however the actual

compounds present showed a change from normal growth patterns. There appeared to be a large increase in the amount of n-butane in this run, with the ratio of butane to

butene 12 times higher than a comparable run with no hydrogen (recalling from Chapter 2 that some n-butane is detected in a standard run, resulting from Al-nBu

impurities in the AlEt3). It was not considered that direct hydrogenation across the

double bond of Al-CH=CHCH2CH3 was very likely, as this kind of process is

typically catalysed by transition metal compounds (eg Pd, Pt, Ni, Rh).82 The most likely explanation seems to be that 1-butene, formed through chain termination with hydrogen, inserts into the Al-hydride formed in the process to yield an Al-Bu species

(Scheme 3-9(a)). Such a sequence is expected to be more favourable under elevated hydrogen pressure.

Scheme 3-9. Possibleformation of Branched C6 oligomers

The C6 fraction featured no 1-hexene or 1,3-hexadiene, but a single peak which was

identified as 3-methylenepentane by GC-MS and co-elution. This can again be explained by the presence of 1-butene; 1,2-insertion in AlEt3 followed by β-hydride

elimination (or direct β-hydrogen transfer to more 1-butene)88 to produce

3-methylenepentane seems possible (Scheme 3-9(b)). β-Hydride elimination from a β-branched Al-alkyl is known to be more facile.88 None of the oligomers expected

from C8 and above were observed, and an approximate quantification of the minor

products formed showed the yield in this area to be greatly reduced. Taken together, the above observations suggest that at high partial pressures of hydrogen, chain termination occurs more rapidly than further insertion into Al-CH=CHCH2CH3,

generating 1-butene as the primary product. Insertion of this into an Al-hydride bond can lead to butane, whereas insertion into Al-Et leads to 3-methylenepentane. All of this suggests a controlling effect of dihydrogen toward oligomer growth in this

Al Et Al Al H + Al H₂ Al-Et (a) (b) Al H₂O -AlH

system, especially under higher pressure. At the same time, unfortunately, the presence of hydrogen seems to greatly inhibit the productivity; as such the reaction does not appear to become catalytic. At this stage it is unclear why the presence of hydrogen hinders further acetylene insertion. One possibility is that the formation of Al-hydrides results in strongly bound µ-H dimers,88 which do not readily take up

acetylene (Reaction 3-6). The nature of such likely species has been investigated computationally, and is discussed in Chapter 4.

2 Et2Al H Al H H Et Et Al Et Et 2 Et2AlH (3-6)