Poly-butadienes conformation

The solution properties of dendrigraft polybutadienes are, as in the previous cases discussed, consistent with a hard sphere morphology. The intrinsic viscosity of arborescent-poly(butadienes) levels off for the G1 and G2 polymers. Additionally, the ratio of the radius of gyration in solution (Rg) to the hydrodynamic radius (Rb) of the molecules decreases from RJRb = 1.4 to 0.8 from G1 to G2. For linear polymer chains with a coiled conformation in solution, a ratio RJRb = 1.48-1.50 is expected. For rigid spheres, in comparison, a limiting value RJRb = 0.775 is predicted. 

Contrary to the usual procedure which considers only the local conformations of minimum energy, the mean-square end-to-end distance of crs-1,4-poly butadiene is recalculated taking into account the whole continuum of conformational states elastic strain on the C—C-C bond angles is also allowed, but only the values which minimize the conformational energy are retained, for every set of rotation angles. 

Figure 5-9. Conformations, lattice constants (fl, and c), density p, and crystal forms of the four isomeric poly(butadienes) (after G. Natta and P. Corradini).

Spherical micelles have indeed been observed in various systems. Experimental values of L of Doly(styrene)-poly(butadiene) diblock copolymer micelles in n-heptane seem to conform to scaling law (Eq. (42).3 ... 

As an example of chemical structure and torsional angles, a sketch of a 1,4-poly butadiene chain in the planar all-trans conformation is shown in Figure 6. Rotational angles about bonds of the chain skeleton are measured relative to the planar conformation shown, for which (j)=0°. X-Ray diffraction studies on cis- and trans-polybutadiene indicate that CH2-CH2 bonds such as bond (i-l-2) connecting C,+i and Cj+2 in Figure 6 assume the trans conformation in the crystalline state. The rotational states accessible to these bonds in the amorphous state or in solution are concluded... 

Solvent polarity is also important in directing the reaction bath and the composition and orientation of the products. For example, the polymerization of butadiene with lithium in tetrahydrofuran (a polar solvent) gives a high 1,2 addition polymer. Polymerization of either butadiene or isoprene using lithium compounds in nonpolar solvent such as n-pentane produces a high cis-1,4 addition product. However, a higher cis-l,4-poly-isoprene isomer was obtained than when butadiene was used. This occurs because butadiene exists mainly in a transoid conformation at room temperature (a higher cisoid conformation is anticipated for isoprene) ... 

Figure 2.16 reports the conformational energy maps as a function of the torsion angles 0i and 02 of the two single bonds adjacent to the double bonds for 03 = T = 180° for cis-1,4-poly (1,3-butadicnc) (cisPBD),69 tranx-l,4-poly(l,3-butadiene) (transPBD),70 ds-l,4-poly (isoprene) (cisPI),68 trans-1,4-poly(isoprene) (transPI),71 ds-l,4-poly(2,3-dimethyl-l,3-butadiene) (cisPMBD),68 and lrans-, 4-poly(2,3-dimethy 1-1,3-butadicnc) (transPMBD).68 These polymers are representative examples of polydienes with A = A = H... 

Figure 2.16 Maps of conformational energy of various cis and trans polydienes as function of torsion angles 9i and 0268 (a) cis-l,4-poly( 1,3-butadiene), (b) /ra .v-l,4-poly( J, 3-buladiene), (c) cis-l, 4-poly(isoprene), (d) trans-1,4-poiy(isoprenej, (e) cis-1,4-poly(2,3-dimethyl-1,3-butadiene), and (/) trans- 1,4-poly(2,3 -dimethyl-1,3-butadiene). Isoenergetic curves are reported every 2 kJ/mol of monomeric units with respect to absolute minimum of each map assumed as zero. (Reproduced with permission from Ref. 68. Copyright 1986 by the Societa Chimica Italiana.)...

Crystalline polymers characterized by disordered conformations of the chains are, for instance, polytetrafluoroethylene (PTFE), /ra .s-1,4-poly (1,3-butadiene), and cis-1,4-poly(isoprcnc). 

Another example of a conformationally disordered mesomorphic form is the high-temperature phase of trans-1,4-poly(1,3-butadiene).111112 As discussed in Section 2.6, in this phase the disorder corresponds to a statistical succession... 

There is an even more striking change in the conformational preference of poly-(isoprenyl)lithium on increasing solvent polarity than observed in the poly(butadienyl)-lithium model. There is no detectable trans-carbanion in THF, and even changing solvent from benzene to ether causes a decrease in the trans content from 65 to 25 %. In THF, the exclusively cis conformation is noted 198) for the sodium and potassium ion pair of the isoprene model the butadiene model carbanion is 22% trans with sodium but less than 10% trans with potassium (all at —20 °C). 

Molecular orbital calculations on fluorinated butadienes and hexatrienes were used to model the effects of fluorination on the properties of poly(acetylene). Like poly(acetylene), "head-to-head" poly(fluoro- acetylene), (-CH=CF-CF=CH-), is predicted to adopt a planar, all trans structure, but poly(difluoro-acetylene) favors a non-planar skewed chain conformation. "Head-to-tail" poly(fluoroacetylene), (-CH=CF-CH=CF-) is predicted to favor a nearly planar cis structure stabilized by intramolecular CF-HC hydrogen binding. Calculations on 2-fluoroethanol and on both 2-fluoroacetaldehyde enol and its alkali metal (Li, Na, K) enolates reveal moderately strong intramolecular CF—HO hydrogen bonds(1.9 and 3.2 kcal/mol, respectively) and even stronger intramolecular coordination of CF to alkali metal cations (9-12 kcal/mol). 

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