Atom transfer radical polymerization from multifunctional substrates

by Carlmark, Anna

Atom transfer radical polymerization (ATRP) has proven to be a powerful technique to obtain polymers with narrow polydispersities and controlled molecular weight. It also offers control over chain-ends. The technique is the most studied and utilized of the controlled/”living” radical polymerization techniques since a large number of monomers can be polymerized under simple conditions. ATRP can be used to obtain polymer grafts from multifunctional substrates. The substrates can be either soluble (i. e. based on dendritic molecules) or insoluble (such as gold or silicon surfaces). The large number of growing chains from the multifunctional substrates increases the probability of inter-and intramolecular reactions. In order to control these kinds of polymerizing systems, and suppress side-reactions such as termination, the concentration of propagating radicals must be kept low. To elaborate such a system a soluble multifunctional substrate, based on 3-ethyl-3-(hydroxymethyl)oxetane, was synthesized. It was used as a macroinitiator for the atom transfer radical polymerisation of methyl acrylate (MA) mediated by Cu(I)Br and tris(2-(dimethylamino)ethyl)amine (Me6-TREN) in ethyl acetate at room temperature. This yielded a co-polymer with a dendritic-linear architecture. Since most solid substrates are sensitive to the temperatures at which most ATRP polymerisations are performed, lowering the polymerization temperatures are preferred. ATRP at ambient temperature is always more desirable since it also suppresses the formation of thermally formed polymer. The macroinitiator contained approximately 25 initiating sites, which well mimicked the conditions on a solid substrate. The polymers had low polydispersity and conversions as high as 65% were reached without loss of control. The solid substrate of choice was cellulose fibers that prior to this study not had been grafted through ATRP. As cellulose fibers a filter paper, Whatman 1, was used due to its high cellulose content. The hydroxyl groups on the surface was first reacted with 2-bromoisobutyryl bromide followed by grafting of MA. Essentially the same reaction conditions were used that had been elaborated from the soluble substrate. The grafting yielded fibers that were very hydrophobic (contact angles > 100°). By altering the sacrificial initiator-to-monomer ratio the amount of polymer that was attached to the surface could be tailor. PMA with degrees of polymerization (DP’s) of 100, 200 and 300 were aimed. In order to control that the polymerizations from the surface was indeed “living” a second layer of a hydrophilic monomer, 2-hydroxymethyl methacrylate (HEMA), was grafted onto the surface. This dramatically changed the hydrophobic behavior of the fibers.