Head addition

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

Vlayl acetate polymerises chiefly ia the usual head-to-tail fashion, but some of the monomers orient head-to-head and tail-to-tail as the chain grows. The fraction of heat-to-head addition iacreases with temperature. For example, a 1.15 mol % head-to-head stmcture and a 1.86 mol % stmcture were determined at 15°C and 110°C, respectively (86). 

Head-to-head units formed in a molecule have not only been considered as initiation sites for the dehydrochlorination but also as termination points for the growing polyene sequences [19,66,68]. Head-to-head units can either be formed through termination by combination [Eq. (19)] or by head-to-head addition during propagation [Eq. (20)]. 

Fitting s Additional velocity heads Additional length pipe diameters f= 0.025... 

We shall consider now the various degrees of order which characterize polymeric molecules. The addition of a monomeric unit to a growing chain may take place in more than one way. In the case of a vinyl or vinylidine monomer, i.e., CH2—CHA or CH2—CAB, head-to-head or head-to-tail addition may occur. In most cases the head-to-tail addition has a vastly greater probability than the head-to-head or tail-to-tail addition, and thus the latter is responsible only for small imperfections in the chain structure. Studies of head-to-tail and head-to-head additions were vigorously pursued in the 30 s and 40 s, and a good account of this work is available, for example, in Flory s recent monograph.15... 

A stereoelectronic requirement in radical addition to carbon-carbon double bonds first became apparent from studies on radical cyelization and the reverse (Fragmentation) reactions.54 56 It provides a rationalization for the preferential formation of the less thermodynamically stable exo-product (i.e. head addition) from the cyelization of to-alkcny] radicals (16 - Scheme 1.5). s 57 G4... 

For olefins with Ji-substitucnts, whether electron-withdrawing or electron-donating, both the HOMO and LUMO have the higher coefficient 021 the carbon atom remote from the substituent. A predominance of tail addition is expected as a consequence. However, for non-conjugated substituents, or those with lone pairs (e.g. the halo-olefins), the HOMO and LUMO are polarized in opposite directions. This may result in head addition being preferred in the case of a nucleophilic radical interacting with such an olefin. Thus, the data for attack of alkyl and fluoroalkyl radicals on the fluoro-olefins (Table 1.2) have been rationalized in terms of FMO theory.16 Where the radical and olefin both have near neutral philicity, the situation is less clear.21... 

The reactions of the radicals (whether primary, secondary, solvent-derived, etc.) with monomer may not be entirely regio- or ehemoseleetive. Reactions, such as head addition, abstraction or aromatic substitution, often compete with tail... 

For reactions with S, specificity is found to decrease in the series cyanoisopropyl mcthyl Fbutoxy>phcnyl>bcnzoyloxy. Cyanoisopropyl (Scheme 3.3),7 f-bntoxy and methyl radicals give exclusively tail addition. Phenyl radicals afford tail addition and ca l% aromatic substitution. Benzoyloxy radicals give tail addition, head addition, and aromatic substitution (Scheme 3.4). ... 

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. 

Alkyl radicals, when considered in relation to heteroatom-centered radicals (e.g. r-butoxy, benzoyloxy), show a high degree of chcmo- and rcgiospecificity in their reactions. A discussion of the factors influencing the rate and rcgiospecificity of addition appears in Section 2.3. Significant amounts of head addition arc observed only when addition to the tail-position is sterically inhibited as it is in a,p-disubstituted monomers. For example, with p-alkylacrylates, cyclohexyl... 

Cyanoisopropyl radicals generally show a high degree of specificity in reactions with unsaturated substrates. They react with most monomers (c.g. S, MMA) exclusively by tail addition (Scheme 3.4). However, Bcvington et al.11 indicated that cyanoisopropyl radicals give ca 10% head addition with VAc at 60 °C and that the proportion of head addition increases with increasing temperature. 

Aryl radicals are produced in the decomposition of alkylazobenzenes and diazonium salts, and by f)-scission of aroyloxy radicals (Scheme 3.73). Aryl radicals have been reported to react by aromatic subsitution (e.g. of Sh) or abstract hydrogen (e.g. from MMA10) in competition with adding to a monomer double bond. However, these processes typically account for <1% of the total. The degree of specificity for tail vs head addition is also very high. Significant head addition has been observed only where tail addition is retarded by sleric factors e.g. methyl crotonate10 and -substituted methyl vinyl ketones 79, 84). 

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). 

Grant et a/.397 examined the reactions of hydroxy radicals with a range of vinyl and a-methylvinyl monomers in organic media. Hydroxy radicals on reaction with AMS give significant yields of products from head addition, abstraction and aromatic substitution (Table 3.8) even though resonance and steric factors combine to favor "normal tail addition. However, it is notable that the extents of abstraction (with AMS and MMA) arc less than obtained with t-butoxy radicals and the amounts of head addition (with MMA and S) are no greater than those seen with benzoyloxy radicals under similar conditions. It is clear that there is no direct correlation between reaclion rale and low specificity. 

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. 

Even allowing for the above-mentioned complication, the number of head-to-head linkages is unlikely to equate exactly with the number of tail-to-tail linkages. The radicals formed by tail addition (T ) and those formed by head addition (H ) arc likely to have different reactivities. 

Consideration of data on the reactions for small radicals (Section 2.3) suggest that the primary alkyl radical (H ) is more likely to give head addition than the normal propagaLing species (T-) for three reasons ... 

However, head addition is usually a very minor pathway and is difficult to determine experimentally. Analysis of the events which follow head addition presents an even more formidable problem. Therefore, there is little experimental data on polymers with which to test the above-mentioned hypothesis. Data for fluoro-olefms indicate that H gives less head addition than T (Section 4.3.1.3). No explanation for the observation was proposed. 

The classic method for establishing the proportion of head addition occurring in VAc polymerization involves a two step process.5 The PVAc is converted to PVA by exhaustive hydrolysis and the number of 1,2-glycol units is determined by periodate cleavage. 

The reaction conditions (solvent, temperature) may also influence the amount of head addition and determine whether the radical formed undergoes propagation or chain transfer. 

Although head addition occurs during PVC polymerization to the extent of ca 1%, it is now thought that PVC contains few, if any, head-to-head linkages (<0.05%).61-6- Propagation from the radical formed by head addition is not competitive with a unimolecular pathway for its disappearance, namely, 1,2-chlorine atom transfer (see Scheme 4.8). 

Rigo e.t al,6j were the first to propose that head addition does occur but is immediately followed by a 1,2-chlorine atom shift. The viability of 1,2-chlorine atom shifts is well established in model studies and theoretical calculations.64 Experimental support for this occurring during VC polymerization has been provided by NMR studies on reduced PVC /lfl Starnes et a/.61 proposed that head addition is followed by one or two 1,2-chlorine atom shifts to give chloromethyl or dichloroethyl branch structures respectively (Scheme 4.8). There also is kinetic data to support this hypothesis. 

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