Head-to-tail propagation

Semiempirical molecular orbital calculations on this model [309] suggest that, in the case of propylene polymerisation, equatorial 2,4-substitution of the metallacyclopentane ring is the most stable form this would lead to regiose-lective head-to-tail propagation during the polymerisation of propylene and, moreover, to the formation of isotactic polypropylene [51]. Such calculations concern a case, however, that has not been confirmed by experiments a coordination of propylene at Ti(II) species and subsequent reaction according to the above scheme is not as obvious as that of ethylene. 

Heptadiyne was first chosen for polymerization since this monomer could be expected to give the most facile polymerization reaction via an internal head-to-tail propagation which would lead to a six-membered carbocyclic recurring unit. Stille and Frey first reported the study on the polymerization of nonconjugated diynes, especially 1,6-heptadiyne with Ziegler type catalysts. ... 

The extensive structural characterization of iPP produced by ZN catalysts has established unequivocally that the polymer backbone arises from a predominant head-to-tail propagation of monomer units, with occasional head-to-head dyad regioerrors being incorporated. In terms of the Cossee-Arlman mechanism, these structural data can be accounted for by assuming a preferred primary 1,2-regioseIective mode of migratory insertion, with secondary 2,1-regioisomeric... 

This mode of propagation is known as the head-to-tail propagation. This is the predominant pathway. Other modes of propagation such as head-head or tail-tail are less favored. 

There are several sources of differences in stereostructures of macromolecules. In the case of vinyl monomers, the newly formed covalent bond may involve the substituted carbon or methylene group of the monomer. These additions would lead, respectively, to head-to-head or head-to-tail propagation (regiospecificity) (Scheme 41). 

The head-to-tail propagation is favored because when the substituents on the monomer molecule are bulky or polar groups, a steric or coulombic repulsion adds markedly to the head-to-head formation and consequently to the activation energy. 

The free-radical polymerization of acrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-tail growth of the polymeric free radical by attack on the double bond of the monomer. 

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

The polymerization of vinyl chloride monomer, in common with other vinyl monomers, proceeds by a free-radical mechanism involving the usual steps of initiation, propagation, and termination. Poly(vinyl chloride) is formed in a regular head-to-tail manner Eq. (1) [3-6]. 

Propagation then occurs by successive monomer molecules additions to the intermediates. Three addition modes are possible (a) Head to tail (b) Head to head, and (c) tail to tail. 

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. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

After the initial reaction of a radical with the first monomer unit, a series of propagation steps follows, rapidly building up the molecular weight and degree of polymerization. The important part of this mechanism is therefore the (3), (3), etc. noted. This is what makes the polymer With unsymmetrical monomers the head-to-tail addition is preferred because whatever it is in the R group that stabilized the radical once will do so each time a propagation step happens. 

Propagation, or chain growth, takes place in a head-to-tail configuration as a result of resonance stabilization and steric factors by carbocation (M+) addition to another monomer molecule. The head stabilizes the cation best so it is the growing site while the least sterically... 

If some head-to-head configuration is detected in a polymer chain known to propagate by head-to-tail addition, what type of termination has occurred ... 

Considerations involving the heats of reaction of head-to-tail, tail-to-tail, and head-to-head addition of the tertiary carbonium ion to the isobutylene indicated why the polymer chain is propagated chiefly by head-to-tail addition (Evans and Polanyi, 84). 

This type of arrangement (III) is usually referred to as a head-to-tail (H-T) or 1,3-placement of monomer units. An inversion of this mode of addition by the polymer chain propagating alternately via Eqs. 3-9 and 3-8 would lead to a polymer structure with a 1,2-placement of substituents at one or more places in the final polymer chain. 1,2-Placement is usually... 

The polymer has a predominantely head-to-tail structure with propagation occurring almost exclusively by attack at the P-carbon—the less sterically hindered site (Eq. 7-11), that is, an Sn2 attack [Kasperczyk and Jedlinski, 1986 Oguni et al., 1973 Price and Osgan, 1956 Quirk, 2002], Propylene oxide and other substituted epoxides polymerize more slowly than does ethylene oxide because of steric hindrance. 

A similar but direct electron transfer from the metal to the monomer is operative when alkali and alkaline-earth metals (e.g., sodium) are used as the initiator.169,173,177 In this case, however, initiation is slow relative to propagation because of the low metal surface area available, and this method is used only for special purposes. 1,1-Dipehylethylene, for example, forms a dianion that, for steric reasons, is not capable for further head-to-tail addition of the monomer, but it can be used to initiate the polymerization of other monomers.178... 

Propagation in free-radical styrene polymerization proceeds through stable benzylic radicals by head-to-tail addition of the monomer ... 

Under the standard conditions, alternating copolymerization is initiated by the insertion of propene to the Pd-carbon bond of Pd(COOMe) species. Addition of a large amount of an oxidant results in shortening the copolymer chain, that is, inhibiting propagation. Thus, with this protocol, dimers were prepared to estimate the enantiopurity of the chiral centers in the polymer main chain [131). For example, a head-to-tail and anti-dimer with >98% ee was obtained in the reaction catalyzed by a cationic Pd-[MeO-BICHEP] complex with 85% selectivity over other diastereomers (Scheme 7.13). 

Propagation of the polymerization occurs nearly exclusively by head-to-tail reactions, with only a small fraction of head-to-head reactions. The relative ratio of these two reactions is only a function of temperature and has been found to be independent of molecular weight, polymerization solvent, and method of polymerization. The head-to-head addition yields a 1,2-glycol structure in the resulting poly(vinyl alcohol), which in turn influences the degree of crystallinity, strength, solubility, and thermal stability. 

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