Tacticity Changes during Controlled Degradation of Polypropylene

Abstract

The controlled reactive degradation of polypropylene (CPP) in a twin-screw extruder has been investigated from the perspective of tacticity. CPP is initiated by the decomposition of a peroxide producing radicals that abstract hydrogen from the chain backbone creating tertiary radicals. Since C atoms with tertiary radicals have no longer sp3 configurations, they temporarily lose their stereospecificity. The C atoms resume stereospecificity upon termination, whereby methyl groups may assume a different orientation than before, causing a tacticity change. Degradation experiments in a twin-screw extruder of isotactic PP samples have been performed and tacticity changes are observed by C-13 NMR measurements revealing a decrease of isotactic mmmm pentads and increases of pentads with racemic sequence pairs. Statistical analysis of NMR data shows that the change of tacticity is nonrandom, i.e., units on the chain backbone are more likely to change orientation if they are close together. We attribute the nonrandomness to isomerization reactions, where through cyclic intermediates, a radical from one tertiary C carbon is transferred to anotherradical chain walking. To validate this chain-walking hypothesis, a kinetic Monte Carlo simulation algorithm has been developed that predicts the change of the pentad distribution from the reactions taking place: initiation of tertiary radicals, transfer to polymer, chain walking, and termination by disproportionation. PP chains simulated consist of around 109 units with orientations of methyl groups inferred from the pentad distribution measured by NMR. Simulated pentad changes are compared to those measured from NMR, which reveal a relatively high racemic content. kMC simulations show that this can only be explained by assuming chain walking. Also, this isomerization via ring-shaped intermediates influences the patterns of changes in pentad distribution. These findings demonstrate that molecular weight changes are accompanied by tacticity changes. The kMC models confirmed by NMR data show that these tacticity changes are clustered locally along PP chains and that tacticity changes happen within the length scale of individual pentads. Chain-walking isomerization as a main cause of the tacticity changes is supported by predictions from the kMC model.

Multi-Analytical Approach to Characterize the Degradation of Different Types of Microplastics: Identification and Quantification of Released Organic Compounds

“…As seen in Figure 4 a, the 1 H NMR spectrum of artificially aged PP (from t 0 + 1 w to t 0 + 4 w) contains only signals due to isotactic PP. While all degradation processes typically take place preferentially in the amorphous phase, the observed change in tacticity is in agreement with the results reported in a recent paper [ 34 ], suggesting a preferential degradation of syndiotactic PP. Another important feature concerns the appearance of several structured signals in the region between 3 ppm and 6 ppm in the spectra of PP upon aging (see Figure 4 b), which are totally absent in the pristine PP (t 0 ).…”

Modelling the Photodegradation of Marine Microplastics by Means of Infrared Spectrometry and Chemometric Techniques

“…Concerning PP isotacticity (Figure S1h) the results showed a decrease with irradiation time. This result can be interpreted as the consequence of the formation under oxidation conditions of tertiary radicals in carbon atoms, which temporarily lose their sp 3 configurations and, therefore, their stereospecificity [27].…”

Scaling Acceleration Algorithm for Hybrid Kinetic Monte Carlo Simulation of Linear Radical Polymerization

“…Fierens et al proposed a new design strategy to achieve sequence-controlled polymers and select appropriate mediating agents using KMC simulation. Iedema et al developed a KMC algorithm to track the pentad distribution from polypropylene degradation reactions, revealing the main reason for the tacticity changes. Luo et al , performed KMC simulations to investigate the network formation, such as the cross-linked network and the interpenetrating polymer network (IPN), and evaluate its characteristic properties.…”