Macromolecules zyxwvu 1995,28, 5507-5511 5507 Crystal Structure of Form I11 of Syndiotactic Poly(pmethy1styrene) Claudio De Rosa,* Vittorio Petraccone, Francesco Dal Poggetto, Gaetano Guerra, Beniamino Pirozzi, Maria Laura Di Lorenzo, and Paolo Corradini Dipartimento di Chimica, Universitd di Napoli "Federico II", Via Mezzocannone zy 4, 80134 Napoli, Italy Received December 28, 1994; Revised Manuscript Received zyxw April 13, 1995@ ABSTRACT: The crystal structure of form zyxwvu I11 of syndiotactic poly(p-methylstyrene) is presented. Chains in nearly truns-planar conformation are packed in an orthorhombic unit cell with axes a zyxw = 13.36 A, b = 23.21 A, and c = 5.12 A. The crystalline density is 0.988 g/cm3 with four chains in the unit cell (8 monomeric units); space group Pnam. Structure factors and packing energy calculations are shown. A refinement of the structure with the Rietveld method is also presented. Introduction The discovery of homogeneous catalytic systems which consist of soluble titanium or zirconium compounds and methylaluminoxane for the polymerization of styrene to fully syndiotactic polymer1,2 has also allowed re- searchers to obtain fully syndiotactic substituted poly- styrenes. 3-8 The very complex polymorphic behaviorg-ll and the crystalline structures of various modifications of syn- diotactic polystyrene (S-PS)~~-'~ have been reported. Recent structural studies have shown a very complex polymorphic behavior also for syndiotactic poly(p- methylstyrene) (S-PPMS).~~-~~ s-PPMS, which is generally crystalline as polymer- ized, does not crystallize by cooling from the melt or by annealing the amorphous phase. However, it crystal- lizes readily when cast or precipitated from solution as well as by solvent-induced crystallization from the amorphous phase.lg For s-PPMS, four different crystalline forms and a mesomorphic form as well as several clathrate struc- tures (which include molecules of solvent) have been fo~nd.~~,~~ The different crystalline forms and the clathrate structures are formed depending on the sol- vent, the crystallization technique (e.g., casting, pre- cipitation, or solvent induction from the amorphous state) or other parameters (e.g., temperature of casting and nature of the nonsolvent in the precipitati~n).~~,~~ The chain conformation involved in the crystalline forms of S-PPMS~~-~~ are those found for s-PS zyxwvut (trans planar and s(2/1)2 helical). Following the nomenclature proposed in refs 19 and 22, form I, form 11, and the clathrate structures includ- ing molecules of solvent (for instance, tetrahydrofuran or o-dichlorobenzene) present chains in the s(2/1)2 helical conformation with a repeating distance of nearly 7.8 &20-22 while form 111, form V, and the mesomorphic modification (form IV) present chains in a trans- lanar For the various crystalline forms only these few structural data have been suggested so far. This paper presents a complete structural characterization of form 111. A model for the crystal structure of form I11 is suggested on the basis of structure factors and packing conformation with a repeating distance of -5.1 zyxwvu x .20-22 @ Abstract published in Advance ACS Abstracts, June 1, 1995. 0024-929719512228-5507$09.00/0 energy calculations. A refinement of the model, per- formed with the Rietveld method, is also presented. Experimental Section s-PPMS was synthesized as described in refs 8 and 19. The syndiotacticity of the polymer was evaluated by 13C-NMRs the fraction of rrrr pentads is higher than 95%. Form I11 of s-PPMS was obtained by annealing form I at 210 "C for 12 h.19 As described in ref 19, form I recrystallizes into form I11 above its melting temperature (-180 "C). Form I was obtained by casting at 50 "C from 10 wt % solution in toluene. An amorphous sample of s-PPMS was obtained by melting the as-polymerized sample and cooling to room temperature. Partially oriented fibers of s-PPMS in form I11 can be obtained by drawing films of both crystalline forms I and 11; this produces first a transition to the mesomorphic modifica- tion (form IV). Successive annealing of the fiber in the mesomorphic modification, holding the ends fmed, at 210 "C produces transition to form III.22 X-ray powder diffraction spectra were recorded at room temperature with a Philips diffractometer, using Ni-filtered Cu Ka radiation and a step-scan procedure. The range of 28 diffraction angle examined was 2.5-50", the count time for each step was equal to 60 slstep, and the step width was 0.05" The packing energy has been evaluated as half of the sum of the interaction energies between the atoms of one mono- meric unit and all the surrounding atoms of the neighboring macromolecules.23The calculations have been performed with the parameters for the nonbonded energy reported by taking the methyl groups as a single rigid unit.25 The conformation of the chain has been kept constant and the interactions have been calculated within spheres of twice the sum of the van der Waals radii for each pair of atoms. Calculated squared structure factors (Fez) have been ob- tained as F? = ElF12Mt, where M, is the multiplicity factor and the summation is taken over all the reflections included in the 28 range of the corresponding reflection peak obsellred in the Geiger spectrum. It was assumed that the atomic scattering factors are as in ref 26. The refinement procedure was done through the use of the program DEBVIN, first developed by ImmirziZ7 and hence revised and implemented by B r u ~ k n e r . ~ ~ - 3 ~ Method of Refinement The refinement was performed with the full profile X-ray powder diffraction refinement procedure (Rietveld method), using the same approach employed in the case of the crystal structure refinement of other polymer^.^-^^ (28). 0 1995 American Chemical Society