Head-tail addition

Constitution refers to the binding situation, i.e. the sequence of atoms in a molecule. Systems which have the same sum formula, but different structural formulae are called constitutional isomers. The transformation of one constitutional isomer into another requires breaking and forming chemical bonds with activation energies > 1 eV (96.5 kj/mol). Polymerization defects like head-head-addition (in place of the desired head-tail-addition) introduce constitutional isomers which sometimes have pronounced effects on the material properties. For simplicity however, an ideal, defect-free constitution of the systems inquired will be assumed throughout this review, and constitutional isomerism will not be discussed. 

Let us consider further the consequences of head-tail additions. If an additional molecule of IPP is added head-to-tail to farnesyl pyrophosphate geranylgeranyl pyrophosphate, a diterpene, is obtained. The series of events outlined above can now be repeated at a higher level of complexity geranylgeranyl pyrophosphate can either be converted to other diter-penes or two molecules of geranylgeranyl pyrophosphate can be joined tail-to-tail to give 40 C bodies. In this way tetraterpenes, i.e. carotenoids, are obtained. Further head-to-tail additions of IPP lead, finally, to the polyterpenes rubber, gutta-percha, and balata. 

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... 

The incidence of these defects is best determined by high resolution F nmr (111,112) infrared (113) and laser mass spectrometry (114) are alternative methods. Typical commercial polymers show 3—6 mol % defect content. Polymerization methods have a particularly strong effect on the sequence of these defects. In contrast to suspension polymerized PVDF, emulsion polymerized PVDF forms a higher fraction of head-to-head defects that are not followed by tail-to-tail addition (115,116). Crystallinity and other properties of PVDF or copolymers of VDF are influenced by these defect stmctures (117). 

Propagation occurs by head-to-tail addition of monomer ... 

Size of hucket, in (mm), and hucket spacing, in (mm)f Elevator centers, ft Capacity, tons/h (metric tons/h) Size of lumps handled, in (mm)t Bucket Bucket speed, ft/min (m/min) r/min, head shaft hp required at head shaft Additional hp/ft for intermediate lengths Head Tail Head Tail Belt width, in... 

The head-to-tail addition mode produces the most stable intermediate. For example, styrene polymerization mainly produces the head-to-tail intermediate ... 

Key Terms addition polymer head-to-head, tail-to-tail polymer polymer... 

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... 

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... 

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). ... 

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). 

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. 

The tendency for radicals to give tail addition means that a head-to-head linkage will, most likely, be followed by a tail-to-tail linkage (Scheme 4.5). Thus, head-to-head linkages formed by an "abnormal" addition reaction are chemically distinct from those formed in termination by combination of propagating radicals (Scheme 4.6). 

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. 

Table 4.2 Temperature Dependence of Head V.V Tail Addition for Fluoro-olefin...

The very high levels of head addition and the substituent effects reported in these studies are inconsistent with expectations based on knowledge of the reactions of small radicals (see 2.3) and are at odds with structures formed in the intermolecular step of cyclopolymerization of diallyl monomers (see 4.4.1.1) where overwhelming tail addition is seen. 

When used in conjunction with unsymmetrical dienes with substituents in the 2-position, the term tail addition has been used to refer to addition to the methylene remote from the substituent. Head addition then refers to addition to... 

Propagation in copolymerization could, in principle, be discussed under the same headings as used for the discussion of propagation in Chapter 4. However, remarkably little information is currently available on the tacticity, extents of head vs tail addition, and propensity for rearrangement in copolymerization. 

NMR studies15 1 1 on polymers prepared with, 3C-labeled BPO have shown that the primary benzoyloxy and phenyl end groups formed by tail addition to monomer are thermally stable under conditions where the polymer degrades. They persist to > 50% weight loss at 300°C under nitrogen. Thus, these groups are unlikely to be directly responsible for the poor thermal stability of PS prepared with BPO as initiator. On the other hand, the secondary benzoate end groups, formed by head addition or transfer to initiator, appear extremely labile under these conditions. Their half life at 300°C is <5 min. 

It is also possible that complexation of monomer or propagating species could influence the regiospecificity of addition. However, since the effect is likely to be an enhancement of the usual tendency for head-to-tail addition, perhaps it is not surprising that such effects have not been reported. 

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