The Polymerisation Process

The most common way of creating polymers is through addition polymerisation - a process which involves three steps, namely polymer initiation, addition and termination. 

In initiation, a chemical creates an active free radical. This free radical is quite unstable but very reactive because of unpaired electrons in the molecule. This is a monomer with an unpaired electron. Once this is formed, the addition begins as the free radical reacts with another monomer radical. This reaction results in the formation of another monomer and the chain reaction is started as the addition continues with subsequent monomers. Within a fraction of a second, addition of tens of millions of monomers takes place. Finally, when two of the free radical ends encounter each other and bond together to form a large molecule, the termination occurs as the unpaired electrons are joined. 

The properties of elastomeric materials are greatly influenced by the strong inter-chain, i.e., intermolecular forces which can result in the formation of crystalline domain. Thus the elastomeric properties are those of an amorphous material having weak inter-chain interaction and hence no crystallisation. At the other extreme of polymer properties are fibre-forming polymers, such as Nylon, which when properly oriented lead to the formation of permanent crystalline fibres. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonate, etc. 

---

The full ab-initio molecular dynamics simulation revealed the insertion of ethylene into the Zr-C bond, leading to propyl formation. The dynamics simulations showed that this first step in ethylene polymerisation is extremely fast. Figure 2 shows the distance between the carbon atoms in ethylene and between an ethylene carbon and the methyl carbon, from which it follows that the insertion time is only about 170 fs. This observation suggests the absence of any significant barrier of activation at this stage of the polymerisation process, and for this catalyst. The absence or very small value of a barrier for insertion of ethylene into a bis-cyclopentadienyl titanocene or zirconocene has also been confirmed by static quantum simulations reported independently... 

A weU-known feature of olefin polymerisation with Ziegler-Natta catalysts is the repHcation phenomenon ia which the growing polymer particle mimics the shape of the catalyst (101). This phenomenon allows morphological control of the polymer particle, particularly sise, shape, sise distribution, and compactness, which greatiy influences the polymerisation processes (102). In one example, the polymer particle has the same spherical shape as the catalyst particle, but with a diameter approximately 40 times larger (96). 

The polymerisation process proceeds in a manner similar to that of other type AABB polyamides, such as nylon-6,6. The final resin had found apphcation in automotive and other high performance end uses but was withdrawn from the market in 1994. 

Studies have shown that, in marked contrast to carbanionic polymerisation, the reactivity of the free oxonium ion is of the same order of magnitude as that of its ion pair with the counterion (6). On the other hand, in the case of those counterions that can undergo an equiUbrium with the corresponding covalent ester species, the reactivity of the ionic species is so much greater than that of the ester that chain growth by external attack of monomer on covalent ester makes a negligible contribution to the polymerisation process. The relative concentration of the two species depends on the dielectric constant of the polymerisation medium, ie, on the choice of solvent. 

In addition, buffer salts such as disodium hydrogen phosphate may be used to prevent the pH of the aqueous phase falling during polymerisation. Small amounts of an anti-foam agent may be employed to reduce frothing when discharging from the vessel at the end of the polymerisation process. 

Polymers can also be produced by combining two or more different monomers in the polymerisation process. If two monomers are used the product is called a copolymer and the second monomer is usually included in the reaction to enhance the properties of the polymer produced by the first monomer alone. It is possible to control the way in which the monomers (A and B) link up and there are four main configurations which are considered useful. These are ... 

It is commonly found that polymers are less stable particularly to molecular breakdown at elevated temperatures than low molecular weight materials containing similar groupings. In part this may be due to the constant repetition of groups along a chain as discussed above, but more frequently it is due to the presence of weak links along the chain. These may be at the end of the chain (terminal) arising from specific mechanisms of chain initiation and/or termination, or non-terminal and due to such factors as impurities which becomes built into the chain, a momentary aberration in the modus operandi of the polymerisation process, or perhaps, to branch points. 

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

Other chain transfer processes may occur. For example, the radical may abstract an atom from along the backbone of a previously formed polymer molecule, and thus initiate the growth of a branch to the main chain. There can also be chain transfer to monomer, which in the nature of the polymerisation process must be a relatively rare phenomenon. However, it can occur infrequently and give rise to a restriction in the size of the polymer molecules without ceasing the overall radical chain reaction. 

Additionally, our experimental regime now includes extensive use of computer modelling of the polymerisation process and we need to extract chemical, thermal and engineering data for model assembly, verification and for final process improvement. In ICI at Slough we have developed our own approach to the control and data acquisition process used on our semi-technical reactors. 

We calculated the molecular weight of the crosslinking molecules from the ratio of monofunctional (Si(CH3)3Cl) and bifunctional (Si(CH3)2Cl2 or SiH(CH3)Cl2) units present during the hydrolysis of chlorosiloxanes, which forms the polymerisation process. It was checked by viscosity, measured on the undiluted material at 25°C, according to the equation... 

The polydiacetylene crystals (1-4) most strikingly corroborate these conjectures. Along this line of thought is also shown that this electron-phonon interaction is intimately interwoven with the polymerisation process in these materials and plays a profound role there. We make the conjecture that this occurs through the motion of an unpaired electron in a non-bonding p-orbital dressed with a bending mode and guided by a classical intermolecular mode. Such a polaron type diffusion combined with the theory of non radiative transitions explains the essentials of the spectral characteristics of the materials as well as their polymerisation dynamics. ... 

The solid state polymerisation of diacetylenes (2) with U.V. radiation, heating or shear force is most indicative of the predominant influence of electron-lattice coupling. The details of the chemical changes that occur during th polymerisation process are crucial (2,40) but the overall description only needs part of this chemical information. The kinetics and thermodynamics of the polymerisation process using an elastic strain approach have been worked out in (41). 

The above picture points to the very interesting possibility of selectively inducing or enhancing the polymerisation process, at a temperature where this is unlikely, by resonantly driving with an intense laser beam in the infrared the vibrational modes and wc that are involved in the polymerisation. As a consequence of their anharmonicity (45) these modes, when driven near resonance by an electromagnetic field, beyond a certain critical value of the later, can reach amplitudes comparable to the critical ones required for the polymerisation to be initiated or proceed the anharmonicity in the presence of the intense laser beam acts as a defect and localizes the phonons creating thus a critical distorsion. 

The linear and nonlinear optical properties of one-dimensional conjugated polymers contain a wealth of information closely related to the structure and dynamics of the ir-electron distribution and to their interaction with the lattice distorsions. The existing values of the nonlinear susceptibilities indicate that these materials are strong candidates for nonlinear optical devices in different applications. However their time response may be limited by the diffusion time of intrinsic conjugation defects and the electron-phonon coupling. Since these defects arise from competition of resonant chemical structures the possible remedy is to control this competition without affecting the delocalization. The understanding of the polymerisation process is consequently essential. 

So far we have been discussing the processes that are carried out in liquid phase and are very popular and widely used for industrial preparation of polymers. However, the polymerisation process can also be carried out in solid and gaseous phases. 

As stated earlier, LDPE consists of molecules which are branched. The branching occurs during the polymerisation process, either by intermolecular chain transfer reactions or by intermolecular chain transfer as under ... 

The three phases that are present in the Ziegler-Natta polymerisation are (i) the monomer (ii) the solvent and (iii) the catalyst. Reactions take place at certain points on the surface of catalyst particles. The polymer molecule grows as the monomer units join the chain where earlier monomer is attached to the catalyst particle. The precise nature of the action of catalyst is not yet known. However, the first step in the polymerisation process proposed is the formation of a monomer-catalyst complex between the organometallic compound and the monomer. 

The polymerisation process repeats itself, and the resulting chain with eight benzene rings, three of which are quinonoid, is dehydrogenated exactly as before to a chain of four quinonoid rings. (Write out the formulae.)... 

Berlin [69] also confirmed the importance of the presence of OH radicals in his investigation of the polymerisation of polystyrene in the presence of styrene monomer when he found the addition of water to the reaction solvent (benzene) greatly enhanced the yield of polymer. However, latterly it has been argued for these systems that the appearance ofwater decomposition products (e. g. H2O2) led to oxidation of the various impurities, which previously, may have acted as inhibitors in the polymerisation process. 

Until the work by Kruus [76], few workers had systematically investigated the effects of varying such parameters as intensity, frequency, temperature and the nature of the gas on the polymerisation process. 

Returning to ion-pair zirconocene catalysts, the initiation of the polymerisation process requires the displacement of the anion so that the alkene can be coordinated. The mobility of the anion is therefore an important factor and has become the focus of a number of detailed investigations. The original mechanistic scheme of alkene insertion and polymer chain growth (Scheme 8.4) implied dissociation of the anion and formation of a 14-electron cationic intermediate, which then reacted... 

The monomer type and ratio change the adsorption properties of the polymer on the calcium carbonate particles. The type of end group (which depends on the polymerisation process) can also influence adsorption characteristics. 

It seems that increasing the surfactant concentration causes thinning of the films between adjacent droplets of dispersed phase. Above a certain level, the films become so thin that on polymerisation, holes appear in the material at the points of closest droplet contact. A satisfactory explanation for this phenomenon has not yet been postulated [132], It is evident, however, that the films must be intact until polymerisation has occurred to such an extent as to lend some structural stability to the monomer phase if not, large-scale coalescence of emulsion droplets would occur yielding a poor quality foam. In general, vinyl monomers undergo a volume contraction on polymerisation (i.e. the bulk density increases) and in the limits of a thin film, this effect may play a role in hole formation, especially at higher conversions in the polymerisation process. 

---