Short-Chain Branching Structures in Ethylene Copolymers Prepared by High-Pressure Free-Radical Polymerization:  An NMR Analysis

Abstract

It is well-known that short-chain branching (SCB) reactions (intramolecular H-abstraction) play an important role in determining the properties of ethylene homopolymers produced under high pressure by free-radical polymerization. There is little information, however, regarding SCB mechanisms that occur during the synthesis of ethylene copolymers under similar reaction conditions. This work describes SCB structures for a wide range of ethylene copolymers of varying composition (ethylene with n-butyl acrylate (nBA), methyl acrylate (MA), vinyl acetate (VAc), n-butyl methacrylate (nBMA), acrylic acid (AA), and methacrylic acid (MAA)), as determined by proton, 13 C, and 2D NMR techniques. Close examination of the resonances reveals that for many (if not all) of these copolymers, a significant fraction of the SCBs contain comonomer as a result of CH2-radical to CH2 backbiting around a comonomer unit. In addition, SCBs are formed not only by hydrogen abstraction from CH2 polyethylene units but also by abstraction of hydrogen from the comonomer methine units. This latter mechanism does not occur during production of E/VAc, E/nBMA, or E/MAA but is important for E/AA and acrylate (E/MA and E/nBA) copolymers; for these systems 10-20% of the comonomer groups in the polymer are alkylated. Implications of these findings to the polymerization kinetics are discussed.

Reassessment of Free-Radical Polymerization Mechanism of Allyl Acetate Based on End-Group Determination of Resulting Oligomers by MALDI-TOF-MS Spectrometry

“…In this connection, it is notably recalled that the free-radical polymerizations of ethylene, [32][33][34][35][36][37] vinyl chloride, 38,39 vinyl acetate, [40][41][42][43] and acrylate esters [44][45][46][47][48][49] are governed by the polymerization mechanism involving mid-chain radical formed through backbiting hydrogen abstraction of growing polymer radical. …”

Ab initio based kinetic Monte Carlo analysis to unravel the propagation kinetics in vinyl acetate pulsed laser polymerization

“…9,[15][16][17][18][19] Tail radicals and MCRs could not be detected via EPR in a study of Kattner and Buback,9 while in NMR studies contradictory findings regarding the importance of backbiting have been reported. [15][16][17][18][19] It should however be noted that for VAc radical polymerization, the analysis of NMR and EPR spectra is significantly complicated due to the occurrence of head-to-head propagation, possibly leading to low concentrations of tail and mid-chain radicals that could be close to the detection limits. An interesting alternative approach is ab initio based kinetic modeling, 20,21 especially for a complicated reaction mechanism involving many side reactions, as in the case of VAc radical polymerization.…”

Computer‐Aided Optimization of Conditions for Fast and Controlled ICAR ATRP of n‐Butyl Acrylate

“…[15,[19][20][21][22][23] In contrast to RP of ethylene and vinyl acetate in which the occurrence of chain transfer to polymer reactions is a long-standing fact that has been well documented since its discovery more than a half century ago, [24][25][26][27][28] the importance of branching in acrylate RP has only emerged in the early nineties by the detection of quaternary carbons via 13 C NMR spectroscopy. [29][30][31] Interestingly, Ahmad et al [32] have recently reported that the branching level of poly(nbutyl acrylate) can be reduced significantly by performing a CRP instead of an FRP. One of the most frequently used CRP techniques is (normal) atom transfer radical polymerization (ATRP), [33,34] the principle of which is given in Scheme 2a.…”