Initiators, polymerisation

Polymerisation Initiators, Product information BuUetiu, Wako Pure Chemical Industries, LTD. 

T)u Pont Wasp Polymerisation Initiators—Properties, Uses, Storage and Handling, Product Information BuUetin, Du Pont Chemicals, Wilmington, De., July 1984. 

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

The use of monomer-polymer doughs has been largely confined to the production of dentures. A plaster of Paris mould is first prepared from a supplied impression of the mouth. Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough. A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water. After polymerisation, which usually takes less than half an hour, the mould is cooled and opened. This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited. 

More frequently either methyl ethyl ketone peroxide or cyclohexanone peroxide is used for room temperature curing in conjunction with a cobalt compound such as a naphthenate, octoate or other organic solvent-soluble soap. The peroxides (strictly speaking polymerisation initiators) are referred to as catalysts and the cobalt compound as an accelerator . Other curing systems have been devised but are seldom used. 

A road tanker was loaded with l-chloro-2,3-epoxypropane and then driven 250 miles overnight to the delivery point. On arrival, the contents were found to have self heated (undoubtedly from polymerisation initiated by some unknown contaminant) to the boiling point (115°C at ambient pressure) and soon afterwards the relief valve lifted and discharged large volumes of vapour. Cooling with water sprays eventually restored thermal control over the remaining tanker contents [1]. The material is incompatible with strong acids, caustic alkalies, zinc, aluminium, aluminium chloride or iron(III) chloride, all of which catalyse exothermic polymerisation [2],... 

The explosive decomposition during distillation may well have been caused by polymerisation initiated by acidic decomposition products. 

When aqueous solutions of the polymerisation initiators 2,2/-azobis(2-amidinio-propane) chloride and sodium peroxodisulfate are mixed, the title compound separates as a water insoluble shock-sensitive salt. The shock-sensitivity increases as the moisture level decreases, and is comparable with that of lead azide. Stringent measures should be used to prevent contact of the solutions outside the polymerisation environment. (The instability derives from the high nitrogen (21.4%) and oxygen (31.6%) contents, and substantial oxygen balance, as well as the structural factors present in the salt.)... 

Though regarded as one of the more stable peroxides, it becomes shock-sensitive on heating, and self-accelerating decomposition sets in at 49° C [1]. One of the recently calculated values of 46 and 42°C for induction periods of 7 and 60 days, respectively, for critical ignition temperatures is closely similar to that (4577 days) previously recorded. Autocatalytic combustion of the polymerisation initiator is exhibited. Although not ordinarily shock sensitive, it responds to a detonator [2],... 

Unlike radical chain polymerisation, initiation in cationic polymerisation uses a true catalyst that is recovered at the end of the polymerisation and is not incorporated at one end of the growing chain. Catalysts for cationic chain polymerisation are molecules able to withdraw electrons, mainly Bronsted (H2SC>4, H3PO4) and Lewis acids (BF3, A1C13, SnCh). The choice of solvent for cationic polymerisation is also important because it plays a major role in the association between cation and counter ion. A too tight association will prevent monomer insertion during the propagation step. However, the use of "stabilized"... 

P. H. Plesch, New Views on the Cationic Polymerisations initiated by Ionising Radiations, (Developments in the Theory of Cationic Polymerisation, Part XI), Phil. Trans. R. Soc. Lond., 1993, A342, 469. - Part I of this series is P. H. Plesch, Developments in the Theory of Cationic Polymerisation, J. Applied Chem., 1951, 1, 269. 

A scheme equivalent to our supposition 2 was put forward by Chmelir, Marek and Wichterle to explain the polymerisations initiated by aluminium bromide in heptane [11]. The fact that these reactions were of second order with respect to the initiator demanded an explanation in terms of a pre-initiation reaction between two molecules of initiator. 

A quite different situation prevails when the monomer is run into a solution of the aluminium halide. The polymerisation initiated by the AlX+2 ions in solution is very fast so that there is never sufficient monomer to complex an appreciable fraction of the aluminium halide thus there is no obstacle to the continuous formation of AlX+2 ions by reaction (iii), and the polymerisation therefore goes to completion. Such polymerisations are always accompanied by a rapid and large increase in conductivity. 

The fundamental problem, why such a complexation can occur instead of the monomer reacting with the cation when it meets it in solution, is merely reformulated here in terms of potential energy minima, but not addressed clearly. The solution of that mystery did not come to this author until he started grappling with the problems presented by the polymerisations initiated by ionising radiations (Section 4.9). 

In the mid-1960s the first measurements of propagation rate-constants for unsaturated monomers became available, from polymerisations initiated by y-radiation [5]. The circumstances of these experiments were such that it was immediately clear that these very high rate constants (106 to 108 1 mol"1 s 1) were those of unpaired cations, kp. All these reactions were carried out with bulk monomer, i.e., the polymerisations occurred in a medium of very low polarity (e c. 2 for hydrocarbons and 5 to 6 for alkylvinylethers). Unfortunately, the y-radiation method is not applicable to polymerisations in solution, especially in polar (usually alkyl halide) solvents. The methods which have been used to... 

Regrettably, the insight and new understanding provided by Section 5.6 invalidate the calculation and conclusion in the last paragraph of Section 5.5. This is because we know too little about the propagating species in polymerisations initiated by SnCl4 in nitrobenzene. 

Finally, the availability of the value of kp+ for styrene in PhN02 enables us to solve an old mystery, namely, what is the concentration of growing ions in polymerisations initiated by SnCl4 with a co-initiator Dainton and Colclough [22] reported on the polymerisation of styrene by equimolar concentrations of SnCl4 and t-BuCl in PhN02 at 25 °C. Their Figure 3 shows that for m = 0.8, the rate R = 1.28 x 10"3 mol dm s"1. If we take kp+ = 200, we get... 

The concept of the chain-carriers in the cationic polymerisations has become increasingly sophisticated over the last five decades. The view that the chain-carrier in the polymerisations initiated by, e.g., A1C13 is a carbenium ion (then known as a carbonium ion) evolved around 1940, and by the end of the decade the need to consider the pairing of ions, especially in non-polar solvents, had been generally accepted. The idea that the cations can form donor-acceptor bonds with the monomers originated in 1947, but its full implications were not appreciated until very much later. Because of these developments and for other reasons, the propagation rate-constants of such cationoid polymerisations... 

The common finding, discussed in the previous section, that when cationic polymerisations are initiated with protonic acids, HA, as in the stopped-flow experiments, the [Pn ] [HA]0 can be explained along the same lines, as will be shown in Section 4.3.2. We have here a useful discriminatory test, because in the pseudocationic polymerisations initiated by HA (with or without a modifier, such as an organic sulphide), the concentration of growing ester is equal to [HA]0 (Plesch, 1992). 

In this section we deal with the cationic polymerisations initiated by gamma rays, e.g., from a 60Co source, or by high energy electrons. Both types are treated together because... 

Table 2 Values of kp+ from polymerisations initiated by ionising radiations ...

One obvious test of the F-Cat theory was to try to synthesise the perchlorate ester of styrene and to find out whether it will act as a polymerisation initiator however, the ester is stable only in the presence of an excess of styrene. This proved to be the first instance of the stabilisation of a hyperactive ester by an electron donor to give a species which is sufficiently long-lived to be an effective propagator (C). 

With regard to (iv), the whole pattern of the polymerisations initiated by HA changes drastically with temperature, a feature which has been explored and explained very lucidly by Pepper [14],... 

The character of a polymerisation initiated by HC104 or AcC104 can be changed from pseudo-cationic to cationic by the addition of AgSbF6 to the reaction mixture [32],... 

It is the nature of the activator that holds the answer to the inherent question If the polymerisations initiated by protonic acids, e.g., CF3S03H, and by syncatalytic systems, such as that consisting of the same acid + thiolan [25] or Me2S [26] are both pseudo-cationic, why does the one yield a product whose DP is determined by proton transfer to monomer, whilst the other produces a living system A review of the facts shows a simple phenomenological distinction, from which a mechanistic explanation can be deduced. The distinction is this ... 

The parasitic formation of polymers of high DP and/or broad DPD in the same reaction mixture as the living polymers is due to cationic polymerisations initiated by adventitious impurities it can be prevented by cation scavengers such as halide ions and other bases. 

Further, because there had been so little work on the physical chemistry of the oxonium ions involved, it seemed appropriate to also study other aspects of their chemistry. These episodic and mainly practical researches are not presented here, but they included a direct demonstration by conductivity measurements of the solvation of the EtsO+ ion by Et20 and DCA and the effect of this on the dissociation constant of Et30+ salts 77. Also, because of the frequent use of theses salts as polymerisation initiators by others, we made a detailed study of their spontaneous decomposition in solution which had been known since their discovery by Meerwein 74, 110. ... 

Since it is now evident that extremely rigorous drying of all reagents is essential to prevent the formation of tert-mm in these systems, all previous work done under less stringent conditions, which purported to prove that tert-oxonium ions are essential features of the polymerisation initiated by anhydrous perchloric acid, must be discounted. In other words, we see no reason to deny the existence of tert-oxon um ions in those systems in which Jaacks and his collaborators alleged that they found them indeed, considering their experimental conditions in the light of what we have found out since then, it is entirely credible that in some of their studies the tertiary ions were dominant. 

A number of amines have been investigated for their suitability as polymerisation initiators, including aliphatic amines (such as butylamine [17] and 1,3-diaminopropane [18]), polymer supported amines (such as cross-linked aminomethyl polystyrene [CLAMPS], Fig. 1, giving rise to immobilised polyamino acids [19]) and resin-bound amines. 

The PP is produced by melt-kneading PP, an isoprene monomer and a radical polymerisation initiator, which has a high melt viscosity and a high melt tensile strength and is difficult to cause drawdown. Foamed articles made therefrom have a low density, high closed cell content, good appearance and excellent heat resistance. 

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