Free radical chain polymerisation

Free radical chain polymerisation is the method used to prepare the most common polymers. A free radical is generated and reacts with one molecule of monomer (initiation). Then monomer molecules react with this first species, leading to formation of a long chain by successive additions of monomer (propagation). Finally, chains are terminated by reaction of two chains bearing radicals (termination). As radicals are very reactive species, side reactions are likely to occur and modify the simple process (transfer). 

Chain polymerisation can be initiated by free radicals generated through different mechanisms. Free radicals are generally very unstable and reactive species thus, they can easily react with the tt electrons of carbon=carbon double bonds, leading to the bonds opening and starting chain polymerisation. 

Chemical initiation by thermal decomposition of fragile bonds 

The most common process used to generate free radicals is the homolytic thermal decomposition of a molecule containing a fragile symmetrical bond, such as peroxides (R-O-O-R), hydroperoxides (R-O-O-H), azonitriles (R-N=N-R) or peroxodisulphates (see Equations 3.4-3.6). 

Benzoyl peroxide (BPO) and 2,2 -azo-bis-isobutyronitrile (AIBN) are soluble in organic medium, whereas peroxodisulphates are water-soluble. The rate of decomposition is significant for AIBN over 60 °C and for peroxides over 80 °C. 

---

The major limitations of the feed forward control strategy presented here are that (i) it is only as good as the fundamental data which are used in the models and (ii) it can only be used for systems which conform to the conventionally accepted mode of behaviour of free radical chain polymerisation in solution. However, the same approach can be used with the appropriate models for any copolymerisation process. The range of application can be increased by making an arbitary assessment of the parameters necessary for the control models and/or by introducing a feedback loop which incorporates some state measurement device, e.g., an in-line gas chromatograph for measurement of residual monomers concentrations. Such a scheme is shown in Figure 21. 

In a classical free radical chain polymerisation, the slowest step is usually the initiation, for instance in the case of thermal decomposition of a peroxide. In the reaction medium, new radicals are continuously generated, initiating new chains. Growth and termination of chains are very fast, and the active centres are rapidly inactivated, as the termination rate is proportional to the square of radical concentration (Rf = fef[M ] ). Such a reaction is not controlled, resulting in a large distribution of molecular weight of polymers synthesised by classical free radical chain polymerisation. 

Interesting research on the dynamic mechanical and thermal properties of fire-retardant high-impact polystyrene (HIPS) is published by Chang and co-workers [19]. HIPS may be produced by the free-radical chain polymerisation of styrene in the presence of an unsaturated elastomer. The authors showed that the melting point of the additive in relation to the processing temperature of the thermoplastics and the compatibility of the additive with the polymer phases are the two important variables governing the interaction of additive with polymer matrix. 

More often than not, molecular imprinting in polymers has involved the synthesis of a crosslinked polymeric network around a template molecule [27,28]. Generally an appropriate template molecule, T, is identified or synthesised in order to complex with suitable polymerisable binding sites in a solvent (Fig. 8). A crosslinking comonomer and a free radical initiator are added, and a radical chain polymerisation initiated thermally. 

The different conversion-dependence of rj is related to the molecular weight evolution and network development. For addition step-growth polymerisation systems, the molecular weight of the polymer chains gradually increases, while for (linear) free radical chain-growth polymerisations the... 

Vinyl ester resins are similar to unsaturated polyester resins in that they are cured by a free radical initiated polymerisation. However, they differ from the polyesters in that the unsaturation is at the ends of the molecule rather than along the polymer chain. Unlike polyesters, vinyl esters show a greater resistance to hydrolysis as well as lower peak exotherm temperatures and less shrinkage upon cure. Cured vinyl ester resins exhibit excellent resistance to acids, bases and solvents. They also show improved strain to failure, toughness and glass transition temperatures over polyesters. They can be used in filament winding, pultrusion, resin injection, vacuum moulding and conventional hand lay-up. 

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

Functional oligomers with a terminal alpha-substituted acrylate group can be synthesised by catalytic free-radical chain transfer polymerisation based on cobalt II or II chelates. The apphcations of such oligomers in the design of low molec.wt., graft and block copolymer emulsions and dispersions for waterborne, two-component PU paints are reviewed. The emulsions and dispersions are shown to have composition and molec.wt. control and to exhibit... 

The photoinduced addition of a thiol (RSH) to an olefinic double bond has been used to produce polymer networks by taking multi-functional monomers [37-44]. The thiol-ene polymerisation proceeds by a step growth addition mechanism which is propagated by a free radical, chain transfer reaction involving the thiyl radical (RS ). The initial thiyl radicals can be readily generated by UV-irradiation of a thiol in the presence of a radical-type photoinitiator. The overall reaction process can be schematically represented as follows ... 

ESBR and SSBR are made from two different addition polymerisation techniques one radical and one ionic. ESBR polymerisation is based on free radicals that attack the unsaturation of the monomers, causing addition of monomer units to the end of the polymer chain, whereas the basis for SSBR is by use of ionic initiators (qv). 

In the case of mechanism (6) there are materials available which completely prevent chain growth by reacting preferentially with free radicals formed to produce a stable product. These materials are known as inhibitors and include quinone, hydroquinone and tertiary butylcatechol. These materials are of particular value in preventing the premature polymerisation of monomer whilst in storage, or even during manufacture. 

Monomer molecules, which have a low but finite solubility in water, diffuse through the water and drift into the soap micelles and swell them. The initiator decomposes into free radicals which also find their way into the micelles and activate polymerisation of a chain within the micelle. Chain growth proceeds until a second radical enters the micelle and starts the growth of a second chain. From kinetic considerations it can be shown that two growing radicals can survive in the same micelle for a few thousandths of a second only before mutual termination occurs. The micelles then remain inactive until a third radical enters the micelle, initiating growth of another chain which continues until a fourth radical comes into the micelle. It is thus seen that statistically the micelle is active for half the time, and as a corollary, at any one time half the micelles contain growing chains. 

A further feature of anionic polymerisation is that, under very carefully controlled eonditions, it may be possible to produee a polymer sample which is virtually monodisperse, i.e. the molecules are all of the same size. This is in contrast to free-radical polymerisations which, because of the randomness of both chain initiation and termination, yield polymers with a wide molecular size distribution, i.e. they are said to be polydisperse. In order to produce monodisperse polymers it is necessary that the following requirements be met ... 

A mass of polymer will contain a large number of individual molecules which will vary in their molecular size. This will occur in the case, for example, of free-radically polymerised polymers because of the somewhat random occurrence of ehain termination reactions and in the case of condensation polymers because of the random nature of the chain growth. There will thus be a distribution of molecular weights the system is said to be poly disperse. 

Mention may finally be made of graft polymers derived from natural rubber which have been the subject of intensive investigation but which have not achieved commercial significance. It has been found that natural rubber is an efficient chain transfer agent for free-radical polymerisation and that grafting appears to occur by the mechanism shown in Figure 30.8. 

Chain reactions do not continue indefinitely, but in the nature of the reactivity of the free radical or ionic centre they are likely to react readily in ways that will destroy the reactivity. For example, in radical polymerisations two growing molecules may combine to extinguish both radical centres with formation of a chemical bond. Alternatively they may react in a disproportionation reaction to generate end groups in two molecules, one of which is unsaturated. Lastly, active centres may find other molecules to react with, such as solvent or impurity, and in this way the active centre is destroyed and the polymer molecule ceases to grow. 

Chain polymerisation typically consists of these three phases, namely initiation, propagation, and termination. Because the free-radical route to chain polymerisation is the most important, both in terms of versatility and in terms of tonnage of commercial polymer produced annually, this is the mechanism that will be considered first and in the most detail. 

The monomers used in chain polymerisations are unsaturated, sometimes referred to as vinyl monomers. In order to carry out such polymerisations a small trace of an initiator material is required. These substances readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation from around or just beyond the blue end of the spectrum. The two most commonly used free radical initiators for these reactions are benzoyl peroxide and azobisisobutyronitrile (usually abbreviated to AIBN). They react as indicated in Reactions 2.1 and 2.2. 

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

Chain polymerisation necessarily involves the three steps of initiation, propagation, and termination, but the reactivity of the free radicals is such that other processes can also occur during polymerisation. The major one is known as chain transfer and occurs when the reactivity of the free radical is transferred to another species which in principle is capable of continuing the chain reaction. This chain transfer reaction thus stops the polymer molecule from growing further without at the same time quenching the radical centre. 

However, other molecules exist which form free radicals of such high stability that they effectively stop the chain process. These molecules are called retarders or inhibitors the difference is one of degree, retarders merely slowing down the polymerisation reaction while inhibitors stop it completely. In practice vinyl monomers such as styrene and methyl methacrylate are stored with a trace of inhibitor in them to prevent any uncontrolled polymerisation before use. Prior to polymerisation these liquids must be freed from this inhibitor, often by aqueous extraction and/or distillation. 

Emulsion polymerisation is initiated using a water-soluble initiator, such as potassium persulfate. This forms free radicals in solution which may initiate some growing chains in solution. These radicals or growing chains pass to the micelles and diffuse into them, which causes the bulk of the polymerisation to occur in these stabilised droplets. 

The linkage between two chains can also be ionic. Thus the copolymer between ethylene and methacrylic acid (MA) (up to 15% MA), made by free radical polymerisation, yields a polymer with pendant carboxyl groups. Neutralisation with zinc ions gives a crosslinked, thermo-reversible polymer (Surlyn ). The resulting polymer (ionomer) has limited properties, although it is the favoured material for the outer covering of golf balls. 

---