Hyperthermal Intact Molecular Ions Play Key Role in Retention of ATRP Surface Initiation Capability of Plasma Polymer Films from Ethyl α-Bromoisobutyrate

Abstract

We report a systematic study of the plasma polymerization of ethyl α-bromoisobutyrate (EBIB) to produce thin film coatings capable of serving as ATRP initiation surfaces, for which they must contain α-bromoisobutyryl functional groups. In the deposition of polymeric coatings by plasma polymerization there generally occurs considerable fragmentation of precursor ("monomer") molecules in the plasma; and the retention of larger structural elements is challenging, particularly when they are inherently chemically labile. Empirical principles such as low plasma power and low pressure are usually utilized. However, we show that the α-bromoisobutyryl structural moiety is labile in a plasma gas phase and in low pressure plasma conditions, below the collisional threshold, there is little retention. At higher pressure, in contrast, fragmentation of this structural motif appears to be reduced substantially, and coatings useful for ATRP initiation were obtained. Mass spectrometry analysis of the composition of the plasma phase revealed that the desired structural moiety can be retained through the plasma, if the plasma conditions are steered toward ions of the precursor molecule. Whereas at low pressure the plasma polymer assembles mainly from various neutral (radical) fragments, at higher pressure the deposition occurs from hyperthermal ions, among which the protonated intact molecular ion is the most abundant. At higher pressure, a substantial population of ions has low kinetic energy, leading to "soft landing" and thus less fragmentation. This study demonstrates that relatively complex structural motifs in precursor molecules can be retained in plasma polymerization if the chemical and physical processes occurring in the plasma phase are elucidated and controlled such that desirable larger structural elements play a key role in the film deposition.

An Acid Test: Facile SI‐ARGET‐ATRP of Methacrylic Acid

“…We have been exploring substrate independent ATRP as a way of decorating medical device surfaces with bioactive compounds such as those that combat infections . Plasma polymerization is a versatile and scalable technology for preparing substrates for SI‐ATRP grafting; it can be used on medically‐relevant plastics and polymers in two or even one processing step . Building on this platform, we further seek to expand the versatility of a polymer conjugation scaffold by exploring polymers with different functional groups and have focused on carboxylic acid moieties in this study.…”

“…[27] Plasma polymerization is a versatile and scalable technology for preparing substrates for SI-ATRP grafting; it can be used on medically-relevant plastics and polymers in two or even one processing step. [31,34] Building on this platform, we further seek to expand the versatility of a polymer conjugation scaffold by exploring polymers with different functional groups and have focused on carboxylic acid moieties in this study. Another motivation for our work was to explore ARGET as a way to eliminate the need for low-oxygen atmospheres in the processing of potential biomedical device coatings.…”

Synthesis of highly functionalised plasma polymer films from protonated precursor ions via the plasma α–γ transition

“…Following this, plasma deposition of ethyl bromoisobutyrate (EBIB) was studied with the aim of retaining the a-bromo isobutyryl group which can be used to initiate atom transfer radical polymerisation (ATRP) (see Table 1). 36 The fragility of the a-bromo isobutyryl group in the plasma was observed by neutral mass spectrometry with only very small peaks featuring this group measured under all conditions tested; many of the fragments observed in the plasma had lost the Br atom which is critical to the utility of the deposits. Protonated EBIB ions were observed however, showing that if plasma conditions could be tuned to promote ionic deposition, surfaces capable of initiating ATRP may be possible.…”

“…This is consistent with previous measurements using this plasma chamber for ethanol and ethyl bromoisobutyrate. 34,36 The ion flux measured for EIB was approximately 10 times lower than those for MIB and ETMA at equivalent power inputs. The reason for this is unclear, but we speculate that subtle changes in chemical structure result in changes in collisional cross-section and/or ionisation potential.…”

“…This is consistent with previous observations for unsaturated esters and carboxylic acids including propionic acid 35 and ethyl bromoisobutyrate. 36…”