Poly butene conformation

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... 

Isotactic polymers with two chain atoms per monomeric unit thus tend to occur in more or less ideal TG conformations. In addition, the low energy difference for slight deviations from the ideal torsion angle can lead to various helix types. Rapid crystallization of it-poly (butene-1) produces a 4i helix, for example, which, as a high-energy form, changes into a 3i helix on annealing (see also Chapter 10). 

The phenomenon by which various monomer units can replace each other in the lattice is termed isomorphism. Isomorphism is possible in copolymers if the corresponding unipolymers show analogous crystal modifications, similar lattice constants, and the same helix type. For example, according to Table 5-5, the y form of it-poly(propylene) and modification 1 of it-poly (butene-1) possess triclinic crystal form, similar lattice constants for the c dimension, and the same helix type. The copolymers of propylene and butene-1 therefore show isomorphism. Isomorphism occurs particularly readily in helix-forming macromolecules, since the helix conformations lead to channels in the crystal lattice, which can easily accommodate different substituents. 

Polymorphism is observed relatively frequently in long-chain macromolecules, for which approximately isoenergetic structures exist. The stable crystal form of poly(ethylene), for example, possesses an orthorhombic lattice, but on elongation, triclinic and monoclinic modifications are observed. Three modifications are known in it-poly(propylene) a (monoclinic), P (pseudohexagonal), and y (triclinic). Since the molecules are in a 31 helix conformation in all the modifications, differences in the packing of the chain must be responsible for this polymorphism. The three modifications appear at varying crystallization temperatures. In it-poly(butene-l), however, the various modifications correspond to different kinds of helix, so that variations in conformation must be important (see also Table 5-5). 

Raman spectroscopy will not supplant X-ray diffraction for the determination of the conformation of a polymer in the solid state, but Raman spectroscopy can be useful for those systems that are unstable and cannot be satisfactorily oriented to give a proper X-ray pattern. Poly(butene-l) is an example where Raman spectroscopy has been of value for the determination of the conformation of the polymer in the solid state. 

Poly(butene-l) exists in at least three crystalline forms. Form I has a hexagonal unit cell with 3i helices. Form II is prepared by casting a film from different solvents, but it will transform slowly and irreversibly into form I at room temperature, so X-ray diffraction patterns of form II are difficult to obtain. Form III of poly(butene-l) transforms to form II upon heating, and then spontaneously transforms to form I. The Raman spectra of forms II, III, and I are substantially different because of differences in the conformations [71]. Detailed analysis (including the dreaded use of normal coordinate analysis) can establish the conformation of poly(butene-l) from the Raman spectra. 

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. 

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 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. ... 

For isotactic poly (1-butene) (i-PlB) [45], it is well known that the solid structures are three kinds of (-TG-) helical conformations as shown in Fig. 

The considerations discussed in detail in connection with the chemical, configurational, and conformational structure of polypropylene apply similarly to poly-1-butene and the higher poly-1-olefins. Also with poly-1-butene, the preferred backbone conformation is tgtg for the isotactic chain [37]. The various crystalline modifications correspond to greater or lesser deviations from these ideal chain conformations, coupled with variations in chain packing in the crystal lattice. 

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... 

Fig. 1 - Internal energy of isotactic poly- a-butene as a function of the two internal rotation angles in the backbone 0 and 02 For each pair, both the bond angles and the internal rotation angles of the lateral group assume the values that minimize the internal energy. The curves are reported at intervals of 0.5 Real (mol of CRU) . The open curves are the loci of points which correspond to the unit twist of the three conformations experimentally observed in the crystalline state.

The calculations for poly-a-butene are reported, as an example, in fig. 1. The E(0, 02) map represented is that one relevant to the possible conformations of left-handed helices. Di ferently from the case of PP (above cited lecture) the energetic minimum is split into two. Correspondigly, chain polymorphism is experimentally observed for PB(2). The different crystalline modifications have s(3/l)l, s(ll/3)l, s(4/l)l chain conformations the loci of points corresponding to such symmetries are also represented in the figure. 

Several polymers form helices that are simple in the sense that AC is a fraction of 360°, like 180°, 120° or 90°. These are called m/l-helices, m giving the number of monomeric units arranged along one 360°-turn. Examples are polypropylene, polystyrene or poly-1-butene, which all form 3/1-helices. The all-trans-conformation of polyethylene and the all-gauche-conformation of poly(oxymethylene) correspond to 2/1-helices. The next general helical form is given by the m/n-helices. The name is meant to indicate that m monomeric units are equally distributed over n turns. 

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