Chain transfer to polymer in free-radical solution polymerization of n-butyl acrylate studied by NMR spectroscopy

The effects of the initial monomer concentration, [M](0), and percent conversion on the extent of chain transfer to polymer in free-radical solution polymerization of n-butyl acrylate has been studied. The polymerizations were carried out in cyclohexane at 70 degrees C using 0.1% (w/w) 2,2'-azobis(2-cyanopropane) as initiator and the mole percent branched repeat units (mole percent branches) in the poly(n-butyl acrylate) was determined from unique resonances of branch-point carbons in the C-13 NMR spectra. At [M](0) > 10%(w/w) the mole percent branches is independent of [M](0) and increases from 0.8 to similar to 2.2% as conversion increases from 35 to similar to 95%. However, for more dilute solutions, with [M](0), less than or equal to 10% (w/w) the mole percent branches increases as [M](0) decreases and is higher than at equivalent conversions for the more concentrated solution polymerizations; e.g., at similar to 25% conversion the mole percent branches increases from 2.7% for [M](0) = 10% (w/w) to 5.9% for [M](0) = 3%(w/w). These observations are explained in terms of the ratio of the concentrations of polymer repeat units and monomer in the vicinity of the propagating chain end. In more concentrated solutions, intermolecular chain transfer to polymer dominates because, at all except the lowest percent conversions, the overall polymer repeat unit concentration is sufficient for overlap of individual polymer coils. However, in the dilute solutions the overall polymer repeat unit concentration is too low for overlap of individual polymer coils and intramolecular chain transfer to polymer dominates. Under these conditions, the local polymer repeat unit concentration within the isolated propagating chains is defined by the chain statistics and so is approximately constant, whereas the monomer is distributed uniformly throughout the solution. Thus, for dilute solutions, as [M], decreases, the probability of chain transfer to polymer land hence the mole percent branches) increases.