Oxidative dehydrogenation of polypropylene

A significant barrier to conventional chemical recycling of polyolefins is

their high ceiling temperature which makes backbone modification energy

intensive. In the current work we investigate the oxidative dehydrogenation

of isotactic polypropylene (i-PP) as a potential upcycling route for this

polymer. The main goal of this method is the formation of terminal

vinyl groups (TVD) which can act as nucleophilic centers for subsequent

repolymerization. We perform thermal oxidation of i-PP in air at 150, 200,

and 240 ◦C and 1 atm for up to 24 hours. We observe major loss of VOCs

from the samples’ surface, which we hypothesize acts like a sacrificial layer.

This causes slow diffusion of oxygen to the bulk which is thus only partially

oxidized. Using H-NMR, HSQC and FTIR-ATR, we observe an increase

in the degree of dehydrogenation of the backbone with both time and

temperature. We observe a maximum overall yield of ∼ 2.5% of total vinyl

double bonds (TVD) at 240 ◦C at 24 hours. However, after accounting

for the loss of weight and structural integrity using GPC and TGA, we

note that the optimal yield is obtained only after 4 hours, with a weight

loss of 35%. Overall, we demonstrate that a precise control of process

parameters (in particular, surface/volume ratio, temperature and oxygen

pressure) allows to achieve a relatively high degree of dehydrogenation

while preventing full polymer decomposition