Isomerisation-polymerisation

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

In the 1960s, after Kennedy and Thomas [25] had established the isomerisation polymerisation of 3-methylbutene-l, this became a popular subject. From Krentsel s group in the USSR and Aso s in Japan there came several claims to have obtained polymers of unconventional structure from various substituted styrenes by CP. They all had in common that an alleged hydride ion shift in the carbenium ion produced a propagating ion different from that which would result from the cationation of the C C of the monomer and therefore a polymer of unconventional structure the full references are in our papers. The monomers concerned are the 2-methyl-, 2-isopropyl-, 4-methyl-, 4-isopropyl-styrenes. The alleged evidence consisted of IR and proton magnetic resonance (PMR) spectra, and the hypothetical reaction scheme which the spectra were claimed to support can be exemplified thus ... 

The short conclusion from the long quest was that all these alleged isomerisation-polymerisations proved to be entirely mythical, and that therefore the relevant Russian and Japanese claims are invalid. 

Criticisms of Claims in the Field of Isomerisation Polymerisation, Part I. Polymerisation of p-Methylstyrene, J.P. Kennedy, P.L. Magagnini, and P.H. 

The polymers of /i-olefins with 1,2-linked monomeric units have been obtained by polymerisation in the presence of Ziegler Natta catalysts, such as TiCl3—AIR3 (R=Et, z -Bu), used preferably in a combination with Ni(II) compounds, especially NiCb. For instance, monomer isomerisation-polymerisation of m-2-butene with the TiCh/NiCh AIEf (1 1 3) catalyst, carried out in rt-hcptane at 80 °C, produced poly(l-butene) in a yield of ca 72% within 24 h. Polymers obtained under such conditions are characterised by relatively high molecular weight (40 x 103—85 x 103) and contain a significant amount (up to ca 72%) of the isotactic fraction [444], They also contain head-to-head (tail-to-tail) as well as 2,3-linked monomeric units to some extent [193]. 

The /l-olefin isomerisation-polymerisations are composed of two distinct reactions, isomerisation to the respective a-olefin and a-olefin polymerisation via a 1,2-insertion mechanism, which involve different active sites ... 

Readily polymerisable a-olefins consumed by the polymerisation are replenished by the isomerisation of /i-olefins [447], Kinetic studies of /1-olefm isomerisation-polymerisation indicate that the polymerisation, and not the isomerisation, is the rate-determining step under the conditions mentioned. The advantage of the /1-olefin polymerisation procedure, apart from the properties of the polymers obtained, is the one-pot synthesis of a-olefin polymers using / -olefin monomers [444],... 

However, cz s-isotactic and cz s-syndiotactic polymers have been obtained predominantly from common cycloolefins such as cyclopentene the 1,3-insertion isomerisation-polymerisation of cycloalkenes such as cyclopentene farely yields polymers with a trans structure (usually in an amount not exceeding 3%) [15,19,20], Figure 6.2 shows both cw-isomers of poly(l,3-cyclopentylene). 

The 1,3-insertion isomerisation-polymerisation characteristic of cyclopentene is not possible in the case of norbornene polymerisation, since no [1-hydrogen atoms are available in norbornene to stabilise the product of its initial 1,2-insertion owing to the rigid structure of the monomeric unit [15]. 

The molar heat of formation of this endothermic compound (+230-250 kJ, 4.5 kJ/g) is comparable with that of buten-3-yne (vinylacety lene). While no explosive decomposition of the isocyanide has been reported, the possibility should be borne in mind [1], It is stable at — 15°C, but isomerises to acrylonitrile and polymerises at ambient temperature [2],... 

In this paper the author presents some of his contributions to the theory and practice of the cationic polymerisation (CP) of alkenes since 1944. The first phase of his work at the University of Manchester comprises the discovery of co-catalysis by water with TiCl4, the invention of the pseudo-Dewar reaction vessel, the use of trichloroacetic acid as co-catalyst, and the disproof of the alleged cationic isomerisation of cis-stilbene. 

The oligomerisation of isobutene, with and without isomerisation or fragmentation, and its polymerisation and co-polymerisation are industrial processes of considerable... 

The energetic aspects of the peculiarity of isobutene, and of the individual steps in cationic polymerisations have been discussed in detail by Plesch [4a]. A discussion of the energetics of the closely related ionic isomerisation and cracking reactions has been given by Greensfelder [4b] see also Chapter 1. 

A ubiquitous co-catalyst is water. This can be effective in extremely small quantities, as was first shown by Evans and Meadows [18] for the polymerisation of isobutene by boron fluoride at low temperatures, although they could give no quantitative estimate of the amount of water required to co-catalyse this reaction. Later [11, 13] it was shown that in methylene dichloride solution at temperatures below about -60° a few micromoles of water are sufficient to polymerise completely some decimoles of isobutene in the presence of millimolar quantities of titanium tetrachloride. With stannic chloride at -78° the maximum reaction rate is obtained with quantities of water equivalent to that of stannic chloride [31]. As far as aluminium chloride is concerned, there is no rigorous proof that it does require a co-catalyst in order to polymerise isobutene. However, the need for a co-catalyst in isomerisations and alkylations catalysed by aluminium bromide (which is more active than the chloride) has been proved [34-37], so that there is little doubt that even the polymerisations carried out by Kennedy and Thomas with aluminium chloride (see Section 5, iii, (a)) under fairly rigorous conditions depended critically on the presence of a co-catalyst - though whether this was water, or hydrogen chloride, or some other substance, cannot be decided at present. 

This discussion is certainly an over-simplification. Unfortunately there are no detailed experimental results for this reaction under strictly homogeneous conditions, but even with heterogeneous catalysts (e.g., AlCl3 and Ni [13]) only mixtures of branched paraffins, naphthenes and polyenes of low molecular weight are obtained. If isomerisation is slower than propagation, as indicated, e.g., by the experiments of Meier [5] on the polymerisation of 3,3-dimethyl butene-1, this would modify in detail but would not invalidate the above general conclusions. 

The ether-catalyst complex (II) splits into a complex anion (III) and a carbonium ion (IV), which rearranges to the configuration of maximum stability (V). This carbonium ion (V) could itself initiate polymerisation, but it is more likely that it attacks the double bond of the closely associated anion (III), giving the double ion (VI) in equilibrium with the aldehyde (VII). Rearrangements of the type (I)-(VII) have been observed for vinyl ethers [7], and a closely parallel isomerisation is that of isobutyl phenyl ether into para-tertiary butyl phenol under the influence of A1C13 [8]. It is unlikely that the steps from (II) to (VI) take place in a well defined succession. The process probably proceeds by a single intramolecular transformation. 

Subsequently it was found that in solvents of low dielectric constant (hydrocarbons and carbon tetrachloride) neither polymerisations [2] nor alkylations [3] and isomerisations [4] could be effected by a metal halide alone, but that a third substance, called the co-catalyst, is required. The most common, and indeed ubiquitous co-catalyst is water other substances proved to act in this capacity are alcohols [5], organic acids [6] and nitro-compounds [7]. In each case the catalysis can be ascribed to a complex protonic acid PXnA H+ ... 

The present paper is an attempt to unravel a rather confused aspect of cationoid polymerisations. This concerns the phenomenon comprised in the term monomer complexation of the growing cation . The idea seems to have occurred for the first time in the work of Fontana and Kidder on the polymerisation of propene by AlBr3 and HBr in w-butane [3]. The kinetics indicated a reaction of zero order with respect to monomer, M to explain this, it was assumed that the growing end of the chain, written as a carbenium ion, Pn+, is complexed with M and that the rate-determining growth step is an isomerisation of this complex ... 

The ionic conductivity at the end of a polymerisation is due to whatever cations Pn+ are formed or left when the monomer is exhausted and the anions A- of the initiating salt, plus a very minor contribution from the ions formed from impurities, which will be ignored. In order to analyse the relation between the observed iq, c0 and the ionic conductivity A of the electrolyte, it is necessary to clarify the electrochemistry of the solutions. We note first that the polymeric cations, whatever their structure, (i.e., as they were when propagating or subsequently isomerised), are much larger than the anions, SbF6, so that these carry virtually all the current so that A A, (SbF6), and therefore A, can be calculated-see below. Next, we note that all the iq- c0 plots, including that reported earlier [2], are rectilinear. This means ... 

The ternary complex consisting of the carbenium ion with an anion and a monomer molecule can isomerise with incorporation of the previously complexed monomer molecule into the chain and a shift of the positive charge to the new chain end. This is a unimolecular propagation reaction of zero order with respect to the monomer concentration. It occurs in polymerisations of bulk monomer and in nonpolar solvents, and at relatively high monomer concentrations in polar solvents. 

Furthermore, it follows that all other polymerisations in which isomerisation accompanies polymerisation are also ionic the polymerisation of P-pinene is perhaps the most familiar example, but there are many others. 

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