Quantitative Correlation between Steric Defects and Thermal Behavior in Highly Syndiotactic Polystyrene: A Study Based on DSC and ¹³C NMR Spectroscopy

Abstract

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 ...

One-pot synthesis of isotactic-capped syndiotactic polystyrene with a bimetallic homogeneous catalytic system

“…It is known that the higher the steric defect content in sPS, the lower the crystalline lamellae thickness and the melting temperature, and these parameters have been quantitatively correlated. 13,16 Figure 2 shows the %m-T m relationship for several high-M w sPS samples obtained by using different catalysts (Cp*TiCl 3 , …”

“…Molecular weight calibration was performed using monodisperse aPS standards. 13 C-NMR spectra were run on an AM-300 Bruker instrument (Bruker Corp., Madison, WI, USA) at 110 1C. The samples were prepared by dissolving 150 mg of polymer in 2.5 ml of o-dichlorobenzene.…”

“…The degree of stereoregularity of the polymers was evaluated from the intensities of the aromatic quaternary carbon peak according to reported methods. 13 Differential scanning calorimetry spectra were recorded with a Perkin-Elmer DSC-Series 7 system (Perkin Elmer Inc., Waltham, MA, USA), using 5-10 mg of polymer for each run. Samples were heated (20 1C min À1 ) up to 300 1C.…”

“…For all samples the T m values reported are those corresponding to the first heating scan, although in a more rigorous treatment (namely, measuring the T m in the second heating scan), a slow cooling and re-melting should be applied. 13 …”

“…For instance, it is known that (pentamethylcyclopentadienyl)titanium derivatives activated by methylaluminoxane (MAO) afford a highly sPS, with %m (defined as meso diads content, calculated by 13 C-nuclear magnetic resonance (NMR)) below the instrumental detection levels, and T m ¼271 1C, contaminated by small amounts of aPS by-product. 3 On the other hand, nickel derivatives/MAO systems afford an isotactic-rich polystyrene (%m¼70-80), lacking any melting temperature; 4 also in this case aPS is formed as a by-product.…”