Head-to-tail enchainment

As it is well known, an "asymmetric carbon atom is a carbon atom bound to four different substituents. According to this definition, the main chain tertiary carbon atoms of a macromolecule obtained by polymerizing a vinyl monomer are, with few exceptions, asymmetric . In fact even by assuming that 1) the macromolecule is linear 2) all monomeric units have a head-to-tail enchainment and 3) the terminal groups are such as to render both terminal segments of the macromolecule identical, the tertiary carbon atoms (with the possible exception of the central tertiary carbon atom of the macromolecule) are bound to two chain sections of different length and are therefore asymmetric . 

The high-molecular-weight poly (propylene oxide) produced with hexacyanometalate salt complexes shows no crystallinity. Moreover, it was shown by Price et al. (18) and confirmed in our laboratory that these polymers have more than 953 head-to-tail enchainment. The amorphous fractions of partially crystalline polymers made with metal-alkyl and ferrio-chloride-based catalysts were shown by those authors to have considerable head-to-head enchainment. They postulated that this was the cause of the amorphous nature of these fractions. It seems clear, however, that the amorphous nature of the polymers prepared with hexacyanometalate salt complexes must be the result of their low degrees of tacticity. 

A complete analysis of the data obtained on cationic polymers synthesized at —78 to —96° C. indicated that about 30% of the polymer was present in the form of 1,2 head-to-tail enchainment. 

Of great industrial interest are the copolymers of ethene and propene with a molar ratio of 1/0.5, up to 1/2. These EP-polymers show elastic properties and, together with 2-5 wt% of dienes as third monomers, they are used as elastomers (EPDM). Since they have no double bonds in the backbone of the polymer, they are less sensitive to oxidation reactions. As dienes, ethylidenenorbomene, 1,4-hexadiene, and dicyclopentadiene are used. In most technical processes for the production of EP and EPDM rubber in the past, soluble or highly disposed vanadium components are used [69]. Similar elastomers can be obtained with metallocene/MAO catalysts by a much higher activity which are less colored [70-72]. The regiospecificity of the metallocene catalysts toward propene leads exclusively to the formation of head-to-tail enchainments. The ethylidenenor-bornene polymerizes via vinyl polymerization of the cyclic double bond and the tendency to branching is low. The molecular weight distribution of about 2 is narrow [73]. 

The commercial product has a random structure with head-to-tail enchainments. Only small amounts of crystallinity are present. The allyl glycidyl units provide sites for sulfur-cross-linking and also reduce the stereoregularity of the polymer. 

In principle, the monomer can be enchained by head-to-tail linkages or head-to-head, tail-to-tail enchainments (4-4). Poly(vinyl fluoride) actually has about 15% of its monomers in the head-to-head, tail-to-tail mode. This is exceptional, however. Head-to-tail enchainment appears to be the predominant or exclusive constitution of most vinyl polymers because of the influence of resonance and steric effects. 

Vinyl monomers polymerize by attack of an active center (4-5) on the double bond. Equation (4-1) represents head-to-tail enchainment ... 

Because of the complete regioselectivity (only head-to-tail enchainments), the stereospecificity in the polymerization of a-olefins to isotactic polymers indicates that... 

The work of Natta et aL was so misunderstood that some authors actually wrote (4) that it is concluded from that work that random copolymers do not contain regularly head-to-tail enchained Cj units . 

Two kinds of linkages may join vinyl or vinylidene monomers into polymeric chains the head-to-tail bonds or head-to-head or tail-to-tail linkages. The head-to-tail enchainment... 

There are four possible combinations of regiochemistry and stereochemistry within diad units. The olefins can join in a head-to-head or head-to-tail fashion. Most polyolefins formed by early metal catalyts are formed by strict head-to-tail enchainment, but this head-to-tail enchainment can occur by a series of 1,2-insertions in which the a-olefin substituent is located P to the metal in tihe insertion product or 2,1-insertions in which the a-olefin substituent is located a to the metal in the insertion product. In addition, the olefins can join to give rise to a diad unit containing identical (meso, m) or opposite (racemo, r) stereochemical relationships to the last inserted monomer unit. Site control of polymerization to form isotactic polymer gives rise to rr-defects m the polymer from stereoerrors, but cham-end control of polymerization to form isotactic polymer gives rise to r-defects from stereoerrors. 

Except in monomers like ethylene and tetra uoroethylene where the substituents on the two carbons are identical, the two carbons of the double bond in a vinyl type monomer are distinguishable. One of them can be arbitrarily labeled the head and the other the tail of the monomer, as shown below for vinyl chloride (IV). During polymerization, the monomer in principle can be joined by head-to-tail or head-to-head/tail-to-tail (V) additions, as shown by Eqs. (P2.7.1) and (P2.7.2), respectively. However, head-to-tail enchainment is the predominant constitution of most vinyl monomers (see Problem 2.7). 

The properties of polychloroprene are influenced by polymerization conditions as well as by the nature of the additives. In the radical polymerization the monomer is built into the polymer in trans-, 4, cA-1,4, 1,2, and 3,4 structures [Structures (37)-(40)] [298]. In addition to head-to-tail-enchainments, also head-to-head and tail-to-tail enchainments occur, with a probability of 10 to 15% (Table 12) [297, 299]. Polymers with a high trans (>98%) or cis content (>95%) are both possible [300, 301]. The glass transition temperature for the /raw -polychloroprene is — 45 °C that for the cis polymer is — 20 °C at a degree of crystallinity of about 12%. 

Initiation takes place at the central zinc-internal methano-late hond, °° ° represented in red in Scheme 25. Indeed the diethylzinc/alcohol systems may lead to the formation of two polymer fractions one essentially isotactic with high molar masses and a second one ataaic with lower molar masses. Both contain 95% of head-to-tail enchainments, which suggests that the amorphous fraaion is not formed by cationic polymerization. The high molar masses, typically lO gmok, result from a slow initiation and only 10 -10 mol of polymer is obtained per 1 mol of initiator. On the basis of a... 

The covalent nature of the aluminum-X bond suggests that polymerization occurs following a coordination process. This hypothesis is reinforced by the exclusive presence of regular head-to-tail enchainments ° and tactidty which is dependent on porphyrin stmcture. ° Indeed the substituents in ortho positions of peripheral phenyl groups affect the reactivity of the Al-X bond polar groups lead to a very high initiation activity, whereas nonpolar substituents result in an opposite effect. 

More recently, the case of a non-regular polyelectrolyte derived from L-lysine and 1,3-benzene disulfonylchloride (XX) was reported [45]. Although asymmetric centers are in main chains, the polymer was considered to be non-stereoregular due to the possibility of head to head and head to tail enchainments for disymmetric diamine moieties of monomeric units during the polycondensation. 

---