Chain conformation isotactic 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. 

The configurational structure (stereoregularity) of 1-butene and of the higher polyolefins up to 1-nonene has been studied by NMR spectroscopy in solution [38, 39], interpreted with the aid of chemical shift calculations, consideration of the y effect and of the rotational isomeric state model of Flory. The evaluation of the results favors the bicatalytic sites model of polymerization [40] over simple Markovian statistics. In contrast to polypropylene, side-chain conformation also has to be considered. Comparison with alkane model compounds indicates that in meso-units of poly-1-butene, trans conformation of backbone is less favored than in isotactic polypropylene because of contiguous ethyl group interactions. Introduction of racemic units in both... 

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 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. 

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... 

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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