Free radical chain polymerisation initiation

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. 

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). 

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. 

The polymerisation of DVF was first investigated by two independent research groups (2-7) who reported that soluble polymers could readily be obtained using free radical and cationic initiators. These authors proposed that cyclopolymerisation had occurred with the formation of linear polymers having three-carbon bridged ferrocene units (I), and some acyclic units (II), in the chain. Evidence for structure (I) as the predominant unit in the polymer chain was provided by the low level of vinyl unsaturation detectable by NMR or infrared spectroscopy and the observation that bands attributable to a bridged ferrocene (8) were to be found in the infrared spectra of these polymers. 

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 ... 

The photosensitive resin consists either of a liquid functionalised oligomer, usually with acrylate groups, or a reactive polymer which will be applied as hot melt. There are two main classes of reactive polymers acrylic hot melts which may contain a tackifying monomer, and thermoplastic rubbers which predominantly block copolymers with polybutadiene or polyisoprene chains [3]. A radical-type photoinitiator is always needed to produce, upon UV-exposure, the free radicals that will initiate the polymerisation or cross-linking process. 

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). 

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 ... 

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. 

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. 

Figure 1 Reaction scheme for the free-radical polymerisation (I is the initiator, R the fragment of initiator, M the monomer and AH the chain transfer agent).

In this section, we review the properties of a series of PNIPAM-b-PEO copolymers with PEO blocks of varying length, with respect to the PNIPAM block. Key features of their solutions will be compared with those of PNIPAM-g-PEO solutions. PNIPAM-b-PEO copolymers were prepared by free-radical polymerisation of NIPAM initiated by macroazoinitiators having PEO chains linked symmetrically at each end of a 2,2/-azobis(isobutyronitrile) derivative [169,170]. The polydispersities of PEOs were low, enabling calculations of the number-average molar mass for each PNIPAM block from analysis of their H-NMR spectra (Table 2). 

Chain polymerisation involves three major steps (i.e., initiation, propagation and termination). This process of chain polymerisation can be brought about by a free radical, ionic or coordination mechanism. 

In solution polymerisation, the reaction is carried out in presence of a solvent. The monomer is dissolved in a suitable inert solvent along with the chain transfer agent. A large number of initiators can be used in this process. The free radical initiator is also dissolved in the solvent. The ionic and coordination catalysts can either be dissolved or suspended in the medium. The solvent facilitates the contact of monomer and initiator and helps the process of dissipation of exothermic heat of reaction. It also helps to control viscosity increase. 

When free radical initiation is because of the addition of the radical to one end of the double bond, then we may expect that the radical would be attached to the end of the Polymer chain. The presence of such end fragments of the initiator has been confirmed for radical polymerisations and with several monomers by endgroup analysis. 

This reaction has been shown to be very rapid77. Sulphuric and acetic acids sup press the polymerisation. Evidently their anions are ineffective as initiators, and the enhanced proton concentration provided by them must reduce the chain lifetime. The slight retarding effect of oxygen could be due to electron scavenging. However, the authors suggest that there may be a small free radical component of the chain reaction, which is inhibited in the presence of oxygen. 

Chain polymerisation is characterised by three steps namely initiation (Eq. 5.1), propagation (Eq. 5.2) and termination (Eq. 5.3) where I, R and M refer to the initiator molecule, free radical and monomer respectively and kj, kp and Iq are the respective rate constants for the processes. 

In the presence of an organic peroxide Initiator, the alkenes and their derivatives undergo addition polymerisation or chain growth polymerisation through a free radical mechanism. Polythene, teflon, orlon, etc. are formed by addition polymerisation of an appropriate alkene or Its derivative. Condensation poiymerisation reactions are... 

Free radical polymerisation can also be promoted by a mixture of ferrous sulphate and hydrogen peroxide (FeS04 + H2O2). These two compounds react to produce hydroxyl radicals. (OH ) which act as chain initiators. They are used as catalyst in the manufacture of Orion and Teflon. 

In the hrst step, a redox reaction occurs between Ce(IV) and the -CH2OH end group of PEO, generating a free radical in a-position of the -OH group of PEO. In a consequent step, the radical is transferred from the PEO chain to the vinyl monomer. The radicals formed initiate the actual polymerisation reaction (propagation) ... 

Oxidative degradation of polyethylene (PE) and polypropylene (PP) can occur at all stages of their lifecycle (polymerisation, storage, processing, fabrication and in-service). The auto-oxidation process of polyolefins is best described by the classical free-radical-initiated chain reaction outlined in Scheme 1 [1]. Impurities initially present in the polymers during polymerisation or melt processing, exert profound effects on the behaviour of the final polymer article in service. 

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