Fast Bulk Depolymerization of Polymethacrylates by ATRP

Fast bulk depolymerization of poly(n-butyl methacrylate) and poly(methyl methacrylate), prepared by atom transfer radical polymerization (ATRP), is reported in the temperature range between 150 and 230 °C. Depolymerization of Cl-terminated polymethacrylates was catalyzed by a CuCl2/TPMA complex (0.022 or 0.22 equiv vs P-Cl) and was studied using TGA, also under isothermal conditions. Relatively rapid 5–20 min depolymerization was observed at 230 and 180 °C. The preparative scale reactions were carried out using a short-path distillation setup with up to 84% depolymerization within 15 min at 230 °C.


Instrumentation
The apparent molecular weights (Mn) and dispersity (Ð) were measured relative to poly(methyl methacrylate) (PMMA) standards by gel permeation chromatography (GPC) conducted with a Waters 515 pump and Waters 2414 differential refractometer using PSS columns (SDV 10 5 , 10 3 , 500 Å) with THF as eluent at 35 °C and at a flow rate of 1 mL·min −1 . 1 H NMR spectra were collected using a Bruker Advance 500 MHz NMR spectrometer with CDCl3 as a solvent at room temperature. The Differential Scanning Calorimetry analysis was performed with a Perkin Elmer DSC 4000 using a rate of heating 10 °C/min. Thermogravimetric Analysis (TGA) was performed with a TA TGA 550 using high-resolution mode.

Deconvolutions
The deconvolution of macroinitiators chain extensions GPC traces was performed with ORIGIN software. The GPC trace was converted from weight distribution to number distribution in order to highlight the presence of small peaks corresponding to dead initial polymers.  Figure S1-2).

Synthesis of PBMA-Cl (DP=25)
PBMA with shorter length was synthesized using the same procedure but stopping the reaction earlier at 40% conversion to obtain a polymer with DP=26, Mn= 3900, Đ=1.17

Chain Extension Experiments
The chain extension reactions were performed with both PBMA-Cl and PMMA-Cl using identical polymerization conditions except using the obtained polymers as initiators. The targeted DPs were 25 times those of the macroinitiators. The GPC analyses demonstrated that the chain-end fidelity of the precursor polymers was almost quantitative, as no initial polymers were observed after chain extensions.

Bulk Depolymerization
In a 25 ml round bottom flask was added the macroinitiator, the catalyst, and the amine in acetone. Acetone was removed using a rotary evaporator. Then, the flask was weighed prior to depolymerization. The depolymerization was started by placing the flask in an oil bath at controlled temperatures under a vacuum. The receiving flask was placed in liquid nitrogen in the short distillation path to collect the generated monomers.

Calculation of contributions of depolymerization, termination by disproportionation and lactonization for PMMA
The 1 H NMR analysis of the residual PMMA provides information on the contributions of the involved depolymerization and termination processes. Termination in radical polymerization of methacrylates is dominated by disproportionation over recombination. The level of disproportionation increases with temperature and at 200 o C exceeds 93.5%. [3][4][5] Therefore, we neglected the contribution of termination by recombination in the analysis below. The residual PMMA had DP=25, meaning that the total concentration of terminal groups (P t ) is 4%, which are lactone, vinyl or hydrogen end groups: P L , P = , P H . Integration of vinyl end groups ((1.00 + 1.12)/2 = 1.06) vs methoxy groups (573.6/3) indicates 0.55 % of unsaturated chain end. The disproportionation yields the same amount of unsaturated and saturated chain ends P = = P H = 0.55% This indicates P L = P t -(2 x P = )= 2.9% By normalizing P T to 100%, one can obtain values of P = = P H = 13.75% and P L = 72.5% The percentage of the remaining monomer was calculated by comparing the integrated ratios of one of the vinyl hydrogen peaks of the monomer with those of pendant methoxy protons. The average integrated ratio of vinyl protons of the monomer {(7.3 + 8.2) / 2 = 7.8} over the integral area of pendant methoxy protons (573.6/3=191.2): (7.8 / 191.2) × 100% = 4.1 % When the depolymerization conversion is considered by adding the remaining monomer (4.1%) to the depolymerization yield (69%), the disproportionation level can be calculated as follows: residual polymer = 100% -(69% + 4%) = 27% Therefore, this enables the calculation of the contribution of the processes involved: Depolymerization: 69% + 4% = 73% Termination by disproportionation: 27% x (13.75% + 13.75%) = 7.4 % Lactonization: 27% x 72.5% = 19.6%