Tuning Reprocessing Temperature of Aliphatic Polyurethane Networks by Alkoxyamine Selection

Recent studies have shown that the largest employed thermoset family, polyurethanes (PUs), has great potential to be reprocessed due to the dynamic behavior of carbamate linkage. However, it requires high temperatures, especially in the case of aliphatic PUs, which causes side reactions besides the desired exchange reaction. To facilitate the reprocessing of aliphatic PUs, in this work, we have explored the dynamic potential of alkoxyamine bonds in PU networks to facilitate the reprocessing under mild conditions considering their fast recombination ability. Taking advantage of the structural effect of the nitroxide and alkyl radicals on the dissociation energy, two different alkoxyamine-based diols have been designed and synthesized to generate PU networks. Our study shows that replacing 50 mol % of a nondynamic diol chain extender with these dynamic blocks boosts the relaxation times of the networks, enabling reprocessing at temperatures as low as 80 °C.

The PPG polyol, named Desmophen® 4042 BT, exhibits the following characteristics as specified in the product datasheet: Hydroxyl number of 44.5 ± 1.5 mg KOH/g.
To determine the Mn, we conducted GPC analysis: Adjustments are necessary for the Mn obtained via GPC, considering the calibration was conducted using Polystyrene.The Mn of PPG, a Poly(Propylene oxide), is determined using the following equation: 1     =   −   + (  + 1)    + 1 Where: is the unknown Mn (the one of the PPG); are the Mark-Houwink parameters for Polystyrene (in THF); are the Mark-Houwink parameters for Poly(Propylene oxide) (in THF); is the Mn of the Polystyrene (found via GPC) Substituting the values, the equation yields: = 11 • 10 −3 − 12.9• 10 −3 + (0.725 + 1)4482 0.75 + 1 Thus,   results to be 3630 g/mol.

Gel content
The gel content (insoluble content) was determined according to previously reported procedures. 2,3The method measures the fraction of a polymer sample that is insoluble in a certain solvent under reflux conditions.For determination of the insoluble content, the dry sample was extracted in boiling THF for 24 h using Soxhlet apparatus.After extraction, the samples were dried in a vacuum oven at 65 °C for 24 h and weighed.The gel content was calculated according to the following equation: Where   is the dried insoluble fraction or residual mass and  0 is the initial mass.The gel content did not change significantly between the pristine (non-reprocessed sample) and after the first reprocessing.

Density Functional Theory (DFT) calculations
All geometry optimizations were carried out within density functional theory (DFT) using the M062X functional (Y.Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215-241) combined with the 6-31+G(d,p) basis set.(W.J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 56, 2257-2261) To confirm that the optimized structures were minima or transition states on the potential energy surfaces, frequency calculations were carried out at the same level of

Figure
Figure S7.a) Influence of the alkoxyamine concentration in the relaxation time at 100 ºC.b) Arrhenius plot of characteristic relaxation times of cross-linked PU with different amounts of PV2 alkoxyamine and their respective activation energies (Ea) (black, 0%; blue, 10 %; red, 25 %; green 50 % and yellow, 100 %).

Figure S8 .
Figure S8.Reprocessing attempt using 5 equivalents of tributyl tin hydride, a radical inhibitor (left) vs reprocessing without additives for PU network containing 50 % of PV2.

Figure S11 .
Figure S11.GPC analysis of the PPG polyol in THF.

Figure S12 .
Figure S12.TGA analysis of the PU crosslinked polymer after treating it in at 80 °C for 24h.a) TGA analysis of the PU pristine.b) TGA analysis of the PU containing 50% of PV2.

Figure S13 .
Figure S13.Dynamic mechanical analysis (DMA) analysis of the PU containing the 10% of PV2, before reprocessing (in black) and after reprocessing (in green).The reprocessing conditions were the following: 45 minutes; 90 °C; 400 bar.

Table 1 .
Calculated values of gel content uf PU-PV1 and PU-PV2 samples before and after reprocessing.