Helical conformation, polyolefins

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

Although many stereoregular polymers have a helical conformation in the solid state (5,96], the conformation is lost in solution in most cases, except in the case of some polyolefins with optically active side groups [12], because the dynamics of the polymer chain are extremely fast in solution. Therefore, isotactic polystyrene [15,16] and polypropylene [17] prepared with an optically active catalyst do not show optical activity due to a helical conformation. However, a helical conformation can be maintained in solution for some polymers having a rigid main chain or bulky side groups that prevent mutation to random conformation, and the conformation may... 

The existence of helical conformations in polyolefin melts was originally suggested by Garth Wilkes et al in the early 1970s [35]. These authors observed... 

In the debate about existence of pre-ordered states in the polymer melt, as advocated recently, polyolefins with chiral side chains may well become a major investigation tool. Indeed, the macromolecular amplification induces a pre-organization, or at least a preferred helical conformation in the polymer melt or solution. As such, these polymers display very precisely the behavior that is assumed by some of the recent crystallization schemes or scenarios. Furthermore, for the P4MH1 systems considered so far at least, the confor-mationally racemic character of the stable crystal structure implies that half of the stems must change their helical hands at some stage in the crystallization process - which may greatly delay the formation of this stable crystal structure, as illustrated by P(S)4MH1. 

A helical structure for vinyl polymers with an excess helicity in solution was realized for isotactic poly(3-methyl-1-pentene) by Pino in I960.12 Although the chiral side groups affect the helical conformation in the polyolefin, the single-handed helix of poly-... 

The isotactic polyolefins prepared using a Ziegler— Natta catalyst form a helical conformation in the solid state (crystalline regions).11 38,42 This helical structure persists in solution, but because of fast conformational dynamics, only short segments of the helix exist among disordered conformations. When an isotactic polyolefin is prepared from an optically active monomer having a chiral side group, the polymer shows the characteristic chiroptical properties which can be ascribed to a helical conformation with an excess helicity.12,43-46 The chiroptical properties arise in this case predominantly from the helical conformation of the backbone. 

Helical conformations were also proposed for the isotactic copolymer derived from (/T)-3,7-d imethyl-1 -octene and styrene.48,49 The copolymer showed intense CD bands based on the styrene units incorporated into the polymer chain. The CD intensity was much larger than that of a model compound of an adduct of the chiral olefin and styrene. The helical structure of polyolefins has also been supported by force field calculations.50 The relationship of these considerations to isotactic vinyl polymers and more recent studies have recently been reviewed.41... 

The above reasoning regarding helical hand in the crystal rests on the assumption that the polymer melt is either made of random coils, or that, if helical stretches exist in the melt, both right- and left-handed helices exist for chiral but racemic polymers such as isotactic (or syndiotactic) polyolefins. For random coils, the conformation of the incoming chain would simply have to adapt to the crystalline substrate structure. When helical stretches do exist, the sorting-out process described above would have to be fully operative. 

A deeper understanding of the crystallization process is desirable and would be achieved if the helical chain conformation in the melt could be determined more exactly, if it were possible to know which helical hand the chains adopt in the melt, and compare it with that of the crystal that is formed. This issue is now addressed by considering (a) the evidence that the polymer melt is indeed organized to some extent - but this does not require a spinodal decomposition, and (b) the chiral polyolefins mentioned in the Introduction , for which the helical hand in the melt is known, and for which the helical hand in the crystal is also known. 

It is well known that isotactic polyolefins often exist as equimolar mixtures of right- and left-handed helices in the crystalline state [19,35]. Upon dissolution they typically undergo rapid conformational changes due to a lack of rotational barriers [36]. In 1974, Drenth demonstrated that polymers bearing bulky side groups exist as stable helices in solution by resolving poly(fert-butyl isocyanide) into optically active fractions [37]. 

It has been revealed by X-ray studies106,107) that in polyethylene the carbon-carbon bond distance is 1.54 A and the valence angle 108°, suggesting that the carbon atom in the polymer chain is tetrahedral. As all of the polyolefins including polyethylene possess helical coiled structure, the tetrahedral stereochemistry for the carbon atom in the polymer chain would persist in all of them. With bulky pendant groups linked to the polymer chain, this tetrahedral structure is most likely to be under considerable strain. For example, polyethylene, through a carbon-carbon bond scission in the polymer chain, forms the radical (XV) in which the unpaired electron and two C-H bonds on CH are in trans or gauche conformation with the two C 2-H bonds and the C T-Cn bond. The radical end... 

Synthetic polymers with conformational chirality have become a research field of widespread interest in recent years, and a wide range of polymers with conformational chirality have been synthesized from various types of monomers including vinyl monomers [9, 61-63, 128-136]. The existing examples of optically active vinyl polymers with conformational chirality include isotactic, helical polyolefins bearing asymmetric side chains [133-135] and isotactic, hehcal polymethacrylates bearing bulky, achiral side chains [61-63,136]. These polymers have stereocenters in the main and/or side chains. Optically active poly(PDBS) is the first vinyl polymer with conformational chirality bearing no stereocenters in the main and side chains whose chiroptical properties arise only from a chiral conformation. 

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