Poly 4-methyl pentene conformation

The other three polymers have additional rotation angles in the side chains, x and/or x. For poly(3-methyl-1-butene), the minimum was found in the three-dimensional plot. For poly(U-methyl-l-pentene) and poly(methyl methacrylate), the stable conformation of the side chain was first calculated with the fixed main chain conformation corresponding to the (7/2) and (5/1) helices, respectively. The potential energy was calculated against the main chain rotation angles, x and t2, by fixing x and x of the side chain at the values thus obtained. ... 

The case of conformational isomorphism of lateral groups occurs, for instance, in the crystal structure of isotactic poly(S-3-methyl-pentene), characterized by the presence of fourfold helices of only one sense (left-handed), with lateral groups which may adopt statistically two conformations of minimum energy [99,103]. 

Similarly, isotactic poly(4-methyl pentene) has been crystallized with several different unit cells with different ordered conformations in some cases.(264)... 

Figure 2.10 Maps of conformational energy of various isotactic polymers as function of backbone torsion angles 0i and 02 (a) Isotactic polystyrene, (b) polypropylene, (c) poly(l-butene), and (d) poly(4-methyl-l-pentene). Succession of torsion angles. .. 0i020i02 [s(M/N) symmetry] has been assumed. Isoenergetic curves are reported every 10 (a,c,d) or 5 (b) kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero.

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. 

Recently, a similar analysis of the conformational energy has been performed also for various new syndiotactic polymers.27,47 The conformational energy maps of syndiotactic polypropylene (sPP),48 polystyrene (sPS),49 poly butene (sPB),25 and poly(4-methyl-l-pentene) (sP4MP)26 are reported in Figure 2.12. A line repetition group s(M/N)2 for the polymer chain, and, hence, a succession of the torsion angles. .. 0i, 0i, 02, 02,..., has been... 

Figure 2.14 Maps of conformational energy as function of backbone torsion angles 9i and 02 of a chain of isotactic poly((S)-3-methyl-l-pentene) for (a,b) left-handed helix and (c) right-handed helix.29 For each pair of Oi and 02, reported energy corresponds to minimum obtained by varying torsion angles of lateral group 03 and 04. Curves are reported at intervals of 0.5 kcal/mol of monomeric unit. Values of energies corresponding to minima are also indicated. (Reprinted with permission from Ref. 29. Copyright 1976 by Elsevier Science.)...

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

In spite of the similarity of the structure of the monomer units the two corresponding isotactic polymers crystallize in two different chain conformations tiie helix of poly-3-methyl-l-butene contains four monomer units per turn (4/1) with a chain repeat of 6.85 A the helix of poly-4-methyl-l-pentene contains 3.5 units per turn (7/2) and has a repeat of 13.85 A. The copolymers tend to crystallize. Their chain conformation and cross sectional area in the crystal lattice are analogous to those of the homopolymer corresponding to the predominant comonomer. For 4-methyl-l-pentene contents higher than 50% some evidence exists that the system simultaneously contains both chain conformations. 

The conformational analysis, according to Brewster, of isotactic poly-a-olefins demonstrates that, in the case of poly-(S)-3-methyl-1-pentene and poly-(S)-4-methyl-l-hexene, only 3 conformations are allowed for each monomeric unit two of them correspond to a left-handed helix conformation of the principal chain and one to a right-handed helix conformation (Table 25). 

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-... 

FIGURE 2. Allowed conformations for isotactic poly(U-methyl-1-pentene)... 

The concept of the optically active aromatic chromophore as conformational probe in stereoregular copolymers can be further extended by employing optically active aromatic monomers. Thus the coisotactic copolymers of (R)-U-phenyl-l-hexene with U-methyl-l-pentene [29] (Figure 7) can be used for correlating the sign of CD of the aromatic Il-electrons system with the macromol-ecular conformation. Both the above copolymer and poly[(R)-U-phenyl-l-hexenej show negative CD between 270 and 250 nm ( Lj -band)... 

Higher values of the ratio r/R, close to V2 -1 (similar to the situation stabilizing NaCl), are obtained in the cases of many vinyl polymers with chains in complex helical conformation s M N) with a fractional ratio MIN, ranging between 3 and 4, such as form I of isotactic poly(4-methyl-l-pentene) (7/2 helix) [38], form II of isotactic poly(l-butene) (11/3 helix) [46], isotactic poly(m-methylstyrene) (11/3 helix) [86], and syndiotactic poly(4-methyl-l-pentene) (12/7 helix) [72]. In these cases, a tetragonal packing of enantiomorphous chains. 

This angular dependence of the interaction parameter can be extended by using the rotational isomeric state approximation to calculate the conformer populations. This approach was used to interpret the solid-state spectrum of poly(3-methyl-l-pentene), which in the amorphous state can have either a four-fold right-handed helix or a nearly equi-energetic four-fold left-handed helix. Three of the six ehemical shifts of a right-handed unit differ from the corresponding resonances of a left-handed unit, and these stereochemical differences are observed [35]. 

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