Porous Organic Polymers-Supported Zeigler-Natta Catalysts for Preparing Highly Isotactic Polypropylene with Broad Molecular Weight Distribution

Abstract

Porous organic polymers (POPs) have attracted much attention in numerous areas, including catalysis, adsorption and separation. Herein, POP supported Ziegler-Natta catalysts were designed for preparation of isotactic polypropylene (iPP). The POPs-based Ziegler-Natta catalysts exhibited the characteristic of broad molecular weight distribution (MWD > 11) with or without adding an extra internal electron donor. The added internal electron donor 3-methyl-5-tert-butyl-1,2-phenylene dibenzoate (ID-2) used in cat-2 showed good propylene polymerization activity of 15.3 × 10⁶ g·PP/mol·Ti·h, high stereoregularity with 98.2% of isotacticity index and broad molecular weight distribution (MWD) of 12.3. Compared to the MgCl₂-supported Ziegler-Natta catalysts (cat-4) with the same ID-2, cat-2 showed higher chain stereoregularity for propylene polymerization. As seen in the TREF results, the elution peak of PP-2 (124.0 °C, 91.7%) is 1.5 °C higher than the isotactic fraction from PP-4 (122.5 °C, 87.2%), and even 1.2 °C higher than PP-5 prepared from ID-3 with the characteristics of high stereoregularity. Moreover, the pentad methyl sequence mmmm of PP-2 (93.0%) from cat-2 is 0.5% higher than that of PP-4 from cat-4. XPS analysis revealed that the minute difference in binding energy of Ti, Mg, C and O atoms exist between the inorganic MgCl₂ and the organic polymer based Z-N catalysts. The plausible interaction mechanism of active sites of Mg and Ti with the functional groups in the POP support and the added ID was proposed, which could be explained by their high stereoregularity and the broad molecular weight distribution of the POP-based Z-N catalysts.

Keywords: Ziegler–Natta catalyst; broad molecular weight distribution; isotactic polypropylene; porous organic polymer (POP).

Figure 1

(a) Isotherm curves of POP3120T and POP3100; (b) pore size distribution curves based on nonlocal density functional theory (NLDFT) dV(d) vs. d (V: pore volume, d: pore diameter (Left); cumulative PV (Right).

Figure 2

(a) FTIR results from the POP3120T and the prepared POP-based and MgCl₂-based Z–N catalyst; (b) PXRD curves from POP3120T and the prepared catalysts containing MgCl₂ nanocrystals. The peak around 21.7° can be assigned to the polyethylene crystal from the plastic protection film. No PE film used in POP3120T.

Figure 3

(a–d) XPS characterization results from the POP-based and MgCl₂-based Z–N catalysts.

Figure 4

Plausible interaction mechanism of Mg/Ti active sites for the prepared POP-based Ziegler–Natta catalyst.

Figure 5

Molecular weight distribution curves of the prepared PPs from five different catalysts.

Figure 6

DSC results of the PPs obtained from the POP-based and MgCl₂-based Z–N catalysts.

Figure 7

TREF results of the obtained PPs from cat-2/3/4.

Figure 8

The ¹³CNMR spectra of three PP samples from cat-1, cat-2 and cat-4.

Figure 9

SEM photos of PP-1 from cat-1 (a,b); PP-2 from cat-2 (c,d); PP-4 from cat-4 (e,f).