No abstract
The effects of the initial monomer concentration, [M]o, 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 °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 13C NMR spectra. At [M]o > 10% (w/w) the mole percent branches is independent of [M]o and increases from 0.8 to ∼2.2% as conversion increases from 35 to ∼95%. However, for more dilute solutions, with [M]o ≤ 10% (w/w), the mole percent branches increases as [M]o decreases and is higher than at equivalent conversions for the more concentrated solution polymerizations; e.g., at ∼25% conversion the mole percent branches increases from 2.7% for [M]o = 10% (w/w) to 5.9% for [M]o = 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]o decreases, the probability of chain transfer to polymer (and hence the mole percent branches) increases.
A comprehensive overview of the fundamentals of emulsion polymerization and related processes is presented with the object of providing theoretical and practical understanding to researchers considering use of these methods for synthesis of polymer colloids across a wide range of applications. Hence, the overview has been written for a general scientific audience with no prior knowledge assumed. Succinct introductions are given to key topics of background science to assist the reader. Importance is placed on ensuring mechanistic understanding of these complex polymerizations and how the processes can be used to create polymer colloids that have particles with well-defined properties and morphology. Mathematical equations and associated theory are given where they enhance understanding and learning and where they are particularly useful for practical application. Practical guidance also is given for new researchers so that they can begin using the various processes effectively and in ways that avoid common mistakes.
No abstract
Chain transfer to polymer (CTP) in conventional free-radical polymerizations (FRPs) and controlled radical polymerizations (ATRP, RAFT and NMP) of n-butyl acrylate (BA) has been investigated using (13) C NMR measurements of branching in the poly(n-butyl acrylate) produced. The mol-% branches are reduced significantly in the controlled radical polymerizations as compared to conventional FRPs. Several possible explanations for this observation are discussed critically and all except one refuted. The observations are explained in terms of differences in the concentration of highly reactive short-chain radicals which can be expected to undergo both intra- and inter-molecular CTP at much higher rates than long-chain radicals. In conventional FRP, the distribution of radical concentrations is broad and there always is present a significant proportion of short-chain radicals, whereas in controlled radical polymerizations, the distribution is narrow with only a small proportion of short-chain radicals which diminishes as the living chains grow. Hence, irrespective of the type of control, controlled radical polymerizations give rise to lower levels of branching, when performed under otherwise similar conditions to conventional FRP. Similar observations are expected for other acrylates and monomers that undergo chain transfer to polymer during radical polymerization.
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