Free-radical polymerisation transfer

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 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. 

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).

If free-radical polymerisation is carried out in an ideal back-mixed flow reactor, the concentrations of the reactant species become constant and the molecular weight distributions can be obtained from eqns. (83) and (84). Figure 8 shows how changes in P /Pn with conversion compare for the two reactor types. These plots represent idealised behaviour, in practice, Pw/Pn will be influenced by changes in at high conversion and by the occurrence of chain transfer reactions. 

The utility of a novel intra-ion-pair electron transfer cleavage reaction has been reported by Schuster et al. [128] for free radical polymerisation initiation... 

The chain length in free radical polymerisations is usually lower than would be expected from the mechanism of termination. The reason for this discrepancy is that the growing polymer chain can transfer the radical to other species, leading to termination of one chain, and thus generating a new radical that will react further. The following transfer mechanisms may occur ... 

The first element of this definition requires that the substance consist of a minimum of three repeat units of monomer(s) in reacted form bonded to another monomer or other reactant [9]. Other reactants, in this case, may not necessarily be monomers, but are incorporated into the polymer to provide a particular function. Other reactants may include a free-radical initiator used to initiate a vinyl-free radical polymerisation (e.g., peroxide used to make polyethylene or polyacrylic acid), a chain transfer agent used to control molecular weight, e.g., mercaptan used in free-radical polymerisations, or crosslinkers used to build molecular weight, e.g., divinyl benzene. 

Figure 5.4 A tentative reaction mechanism for formation of carboxyl-terminated poly(2-ethyl hexyl acrylate) (CTPEHA) from free radical polymerisation of EHA in the presence of 4, 4 -azobis (4-cyanovaleric acid) (ABCVA) as a free radical initiator and dithiodiglycolic acid (DTDGA) as a chain transfer agent. (P = nM ) Reprinted with permission from D. Ratna, A.K. Banthia and P.C. Dtb, Journal of Applied Polymer Science, 2000, 78, 716. 2000, John Wiley and Sons Publishers...

The use of a phase transfer catalyst can allow the use of water soluble radical initiators, such as potassium peroxomonosulfate, to promote the free radical polymerisation of acrylonitrile (Figure 5.24) [55]. The rate of polymerisation was found to be proportional to the concentration of the monomer, the initiator, and the phase transfer catalyst. 

Although free radical polymerisation is most common, other types of polymerisations have been carried out in emulsion polymerisation, including reversible addition-fragmentation transfer (RAFT) (131), atom transfer radical polymerisation (ATRP) (76, 222), and stable free radical polymerisation (SFRP) (77). 

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 ... 

Paczkowski J, Neckers DC (2001) Photoinduced electron transfer initiating systems for free-radical polymerisation. In Balzani (ed) Electron transfer in chemistry molecular level electronics, imaging, energy and environment, vol V. Wiley-VCH, Weinheim, pp 516-585... 

Present During Free Radical Polymerisations in the Presence of Added Transfer Agents.135... 

However, it must be pointed out that values of os calculated using Eq. (10) will be overestimates of the weight fraction of polymer molecules produced by radical-radical termination reactions and which have undesired functionality (to,). This is because for certain polymerisations some of the bimolecular radical termination reactions contained in the k,[P fg term can actually give rise to the formation of polymer molecules possessing the desired functionality. This is best illustrated by listing the various combination and disproportionation reactions that can occur between two radicals in a matched chain transfer free-radical polymerisation. 

Now (1 -H 2Z -I- Z ) is always <(2 + 3Z + Z ) for Z 0, Therefore to, for type 3 polymerisations will always be smaller than to, for type 4 polymerisations, i.e. the use of a functionalised initiator with type monomers is desirable since it reduces the amount of unwanted polymer formed during a matched chain transfer free radical polymerisation. For type monomers (see Sect. 3.2) the opposite effect is predicted i.e. the use of a functionalised initiator actually increases ft),. 

In Sec. 2 and 3 expressions were derived for 00, and a),. The total weight fraction of polymer molecules produced in a matched chain transfer free radical polymerisation which do not possess the required functionality ( ) is given by ... 

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