Abstract
The occurrence of intramolecular transfer to polymer in the radical polymerization of acrylic monomers has been extensively documented in the literature. Whilst it has been largely assumed that intramolecular transfer to polymer leads to short chain branches, there has been some speculation over whether the mid-chain radical can migrate. Herein, by the matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry (MS) of poly(n-butyl acrylate) synthesized by solution polymerization under a range of conditions, it is shown that this mid-chain radical migration does occur in the radical polymerization of acrylates conducted at high temperatures, as is evident from the shape of the molecular weight distribution. Using a mathematical model, an initial approximation of the rate at which migration occurs is made and the distribution of branching lengths formed in this scenario is explored. It is shown that the polymerizations carried out under a low monomer concentration and at high temperatures are particularly prone to radical migration reactions, which may affect the rheological properties of the polymer.
Highlights
The polymerization kinetics and final polymer microstructure of acrylic polymers produced by radical polymerization are strongly affected by transfer to polymer events that lead to the formation of a so-called mid-chain radical (MCR) [1,2,3,4]
The formation of branched structures and chain scission can have a huge influence on the rheological properties of acrylic polymers and understanding the formation and subsequent fate of mid-chain radicals is critical in the synthesis of tailored acrylic polymers
Through the use of mass spectrometry, Vandenbergh and Junkers observed the occurrence of radical migration in products of dormant polymer chains made by controlled radical polymerization activated at high temperature in the absence of monomer [29,30]
Summary
The polymerization kinetics and final polymer microstructure of acrylic polymers produced by radical polymerization are strongly affected by transfer to polymer events that lead to the formation of a so-called mid-chain radical (MCR) [1,2,3,4]. Through the use of mass spectrometry, Vandenbergh and Junkers observed the occurrence of radical migration in products of dormant polymer chains made by controlled radical polymerization activated at high temperature in the absence of monomer [29,30] They observed that the distribution of macromonomers produced by this reaction followed a distinct pattern which could be directly linked to the occurrence of the n:n + 4 transfer. Reactions were conducted in solution with varying monomer concentrations and temperatures to observe how reaction conditions impact on the migration process These experimental results are backed up by a mathematical model which predicts the distribution of individual species, including effects of radical migration along the backbone, allowing for an estimate of the rate of radical migration.
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