Radicals termination, primary

Primary radicals under ideal conditions would contribute to chain initiation only. But in certain systems and under special conditions in certain others, they may also contribute to chain termination (primary radical termination or primary termination), partly or exclusively, giving rise to significant deviations from the ideal kinetics (see Problem 6.29). 

Problem 6.29 Show that in the case of radical polymerization where both bimolecular and primary radical terminations occur simultaneously, the parameter (R / [I] [M] ) is not constant but is dependent on Rj, / [M] ). Hence suggest a method of analysis of the primary radical termination effect from the experimental Rp data. 

If bimolecular and primary radical terminations, (d) and (e), occur simultaneously, then considering steady concentrations of [M-] and [R-] separately, namely. 

Equation (P6.29.5) permits a plot of the left hand side against Rp / [M] ), a negative slope for the linear plot then being indicative of primary radical termination. The magnitude of the slope also gives a measure of the parameter ktp/ kikp). 

Making use of the fact that for vinyl polymerization, ktplkikp) Rpl Mf-) 1, Eq. (P6.25.4) can be simplified to the form  

The end group fonned by combination is similar to that foiTned by head addition to monomer differing only in the orientation of the penultimate monomer unit. 

If the rate of addition to monomer is low, primary radical termination may achieve greater importance. For example, in photoinitiation by the benzoin ether 12 both a fast initiating species (13, high k ) and a slow initiating species (14, low 

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

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The departure of dependence of Rp on the concentration of CHP from 0.5 order might be ascribed to induction decomposition of ROOH type to form ROO- radical, which has very low activity to initiate monomer polymerization [40], but can combine with the propagation chain radical to form the primary radical termination. For the same reason, the order of concentration of TBH was also lower than 0.5 when the TBH-DMT system was used as the initiator in MMA bulk polymerization. But in the BPO-DMT initiation system as shown in Table... 

According to cq. 1, the term/should take into account all side reactions that lead to loss of initiator or initiator-derived radicals. These include cage reaction of the initiator-derived radicals (3.2.8), primary radical termination (3.2.9) and transfer to initiator (3.2.10). The relative importance of these processes depends on monomer concentration, medium viscosity and many other factors. Thus/is not a constant and typically decreases with conversion (see 3.3.1.1.3 and 3.3.2.1.3). 

Primary radical termination may involve combination or disproportionation with the propagating radical. It is often assumed that small radicals give mainly combination even though direct evidence for this is lacking. Both pathways are observed for reaction of eyanoisopropyl radicals with PS (Scheme 3.14) (Section 7.4.3.2). The end group formed by combination is similar to that formed by head addition to monomer differing only in the orientation of the penultimate monomer unit. 

The concentration of monomers in the aqueous phase is usually very low. This means that there is a greater chance that the initiator-derived radicals (I ) will undergo side reactions. Processes such as radical-radical reaction involving the initiator-derived and oligomeric species, primary radical termination, and transfer to initiator can be much more significant than in bulk, solution, or suspension polymerization and initiator efficiencies in emulsion polymerization are often very low. Initiation kinetics in emulsion polymerization are defined in terms of the entry coefficient (p) - a pseudo-first order rate coefficient for particle entry. 

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

Depending on the nature of the substituent R, the radical 76 (Scheme 3.53) may be slow to add to double bonds and primary radical termination can be a severe complication (see 3.2.9).30 40The problems associated with formation of a relatively stable radical are mitigated with certain tx-alkoxy (77) and a-alkanesulfonyl derivatives (79).280 In both cases the substituted benzyl radicals formed by a-scission (78 and 80 respectively) can themselves undergo a facile fragmentation to form a more reactive radical which is less likely to he involved in primary radical termination (Scheme 3.55, Scheme 3.56). 

The S-S linkage of disulfides and the C-S linkage of certain sulfides can undergo photoinduced homolysis. The low reactivity of the sulfur-centered radicals in addition or abstraction processes means that primary radical termination can be a complication. The disulfides may also be extremely susceptible to transfer to initiator (Ci for 88 is ca 0.5, Sections 6.2.2.2 and 9.3.2). However, these features are used to advantage when the disulfides are used as initiators in the synthesis of tel ec he lies295 or in living radical polymerizations. 96 The most common initiators in this context are the dithiuram disulfides (88) which are both thermal and photochemical initiators. The corresponding monosulfides [e.g. (89)J are thermally stable but can be used as photoinitiators. The chemistry of these initiators is discussed in more detail in Section 9.3.2. 

Primary radical termination involving alkyl radicals is described in Sections 2.5 and 7.4.3. Their reactions with monomers are also discussed in Sections 2.3 (fundamental aspects) and 4.5.4 (model propagation radicals). Their chemistry has been reviewed by Fischer and Radom/41 Giese,342,343 Tedder,344 Beckwith,345 Riichardt,76 and Tedder and Walton.346,347... 

The rate constants for benzoyloxy and phenyl radicals adding to monomer are high (> KF M-1 s for S at 60 CC - Table 3.7). In these circumstances primary radical termination should have little importance under normal polymerization conditions. Some kinetic studies indicating substantial primary radical termination during S polymerization may need to be re-evaluated in this light.161 Secondary benzoate end groups in PS with BPO initiator may arise by head addition or transfer to initiator (Section 8.2.1). 

NMR methods can be applied to give quantitative determination of initiator-derived and other end groups and provide a wealth of information on the polymerization process. They provide a chemical probe of the detailed initiation mechanism and a greater understanding of polymer properties. The main advantage of NMR methods over alternative techniques for initiator residue detection is that NMR signals (in particular nC NMR) are extremely sensitive to the structural environment of the initiator residue. This means that functionality formed by tail addition, head addition, transfer to initiator or primary radical termination, and various initiator-derived byproducts can be distinguished. 

Serelis and Solomon108 found that primary radical termination of oligo(MAN) radicals (16) with 15 also gives predominantly combination. The ratio kllt/klc was found to have little, if any, dependence on the oligomer chain length (n<4). As with PMMA, disproportionation involves preferential abstraction of a methyl... 

A substantial number of studies give information on kJkK for polymerizations of S (5.2.2.2.1) and MMA (5.2,2.2.2). There has been less work oil other systems. One of the main problems in assessing kjk lies with assessing the importance of other termination mechanisms (i.e. transfer to initiator, solvent, etc., primary radical termination). 

The nature of the termination reaction in MMA polymerization has been investigated by a number of groups using a wide range of techniques (Tabic 5.5), There is general agreement that there is substantial disproportionation. However, there is considerable discrepancy in the precise values of k tk. In some cases the difference has been attributed to variations in the way molecular weight data are interpreted or to the failure to allow for other modes of termination under the polymerization conditions (chain transfer, primary radical termination).154 In other eases the reasons for the discrepancies are less clear. MALDI-TOF mass... 

Nair et a/." 7 have proposed a modified Mayo equation for use when retardation through primary radical termination with transfer agent-derived radicals is significant. 

A third technique is to examine the products of primary radical termination in polymerizations carried out with high concentrations of initiator.176 177 Values of rtid/Ajc ratios in primary radical termination have been reported for a number of polymerizations carried out with A1BN (model for PM AN ) or AlBMe (model for PMM.V) initiation. 

Barton et al.1X7 have reported that primary radical termination between PBMA and cyanoisopropyl radicals (7) involves largely disproportionation. 

The synthesis of telechelics by what Tobo]sky,9> termed dead-end polymerization is described in several review s.191,191 In dead-end polymerization very high initiator concentrations and (usually) high reaction temperatures are used. Conversion ceases before complete utilization of the monomer because of depletion of the initiator. Target molecular weights are low (1000-5000) and termination may be mainly by primary radical termination.. The first use of this methodology to prepare lelechelic polystyrene was reported by Guth and Heitz.177... 

Disulfide derivatives and hexasubstituted ethanes2,15 may also be used in this context to make cnd-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers.206 Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. The use of similar compounds as photoin iferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

Certain, Y, Y-dialkyl dithioearbamates [e,g. benzyl A)/V-diethyl dithiocarbamate (14)] and xanthates have been used as photoinitiators. Photodissociation of the C-S bond of these compounds yields a reactive alkyl radical (to initiate polymerization) and a less reactive sulfur-centered radical (to undergo primary-radical termination) as shown in Scheme 9.9.30 41 4 ... 

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

Otsu and Tazaki90 have reported on the use of triphenylmethylazobenzene (39) as an initiator. In this case, phenyl radical initiates polymerization and the triphenylmethyl radical reacts mainly by primary radical termination to form a macroinitiator. The early report91 that triphenylmethyl radical does not initiate MMA polymerization may only indicate a very low rate of polymerization. The addition of triphenylmethyl radical to MMA has been demonstrated in radical... 

A further problem with these iniferters is loss of living ends through primary radical termination by disproportionation. The ratio of reported for the cross... 

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