IntroductionSyndiotactic polystyrene (sPS) can be synthesized by means of a number of homogeneous catalytic systems formed by titanium complexes and a co-catalyst like methylaluminoxane (MAO). [1,2] According to published studies, [3][4][5][6] the mechanism of insertion of styrene monomer into the metal-carbon bond is secondary, that is, the carbon atom of the polymer chain bearing the phenyl ring is directly bonded to the titanium atom in each propagation step. The regioregularity of insertion is extremely high; as a consequence,head-to-headunitsareabsentfromthepolymerchain.The syndiospecific control arises from the steric repulsive interaction between the phenyl group of the last inserted unit of the growing chain and the phenyl group on the incoming monomer (1,3-asymmetric induction), as represented in Scheme 1.The stereospecificity is determined by the nature of the substituents on the titanium catalytic system (for instance, it is well known [7] that [(Cp*)TiCl 3 ] affords a perfectly syndiotactic sPS, whereas [(Cp)TiCl 3 ] behaves differently) and by polymerization conditions, mainly the reaction temperature. [7,8] A perfectly syndiotactic polystyrene is fully constituted by racemic (r) dyads (Scheme 2a), whereas the occurrence of a meso (m) dyad determines a steric defect in the chain (Scheme 2b).Full Paper: Several syndiotactic polystyrene (sPS) samples have been synthesized by using different catalytic systems. Their stereochemistry has been determined by 13 C NMR spectra in both the aliphatic CH 2 and aromatic C 1 resonance regions. The observed peaks have been unambiguously assigned to specific hexads and heptads, respectively, and their intensities have been used to draw the percent of defects (meso dyads) in the polymer chains. On the hypothesis that chain defects are at the origin of chain folding and thus determine the thickness of crystalline lamellae, we performed differential scanning calorimetry (DSC) analysis on the same samples, and their thermal parameters were measured. A model was developed to determine the amount of steric defects from the DSC melting-peak profiles, and the results obtained were compared with the NMR results. A satisfactory agreement was found (correlation factor 0.96) in the explored range of defect concentrations (up to 2.5% of meso dyads). The possible influence of the extraction procedure of the amorphous fraction was found to be negligible. Thus, information on stereochemistry can be obtained from DSC experiments starting from as-prepared (not extracted) samples.Stereospecificity is commonly measured by means of nuclear magnetic resonance spectroscopy (NMR). [4,[7][8][9][10][11][12][13] Generally, syndiotactic polystyrene is not obtained as the exclusive reaction product. Indeed, the cocatalyst may initiate the nonstereospecific polymerization of styrene affording low-molecular weight atactic polystyrene (aPS). [14] Moreover, at a temperature above 90-100 8C, the thermal initiated self-polymerization of styrene produces highmolecular-weight aPS. The atactic fraction ...