Facile synthesis of macromonomers and their utilization for building complex polymer architectures
Final Report Abstract
A high temperature one-pot – one-step polymerization method, used to synthesize macromonomers, was demonstrated to be a versatile tool for conducting (follow-up) chemistry based on macromonomers. The straightforward preparation of the vinyl terminated macromolecular building blocks allowed for their facile incorporation into complex macromolecular architecture. Polymeric structures were obtained by utilizing a modular ligation technique for the formation of dendritic structures as well as a (co)polymerization method for the generation of (block) copolymers. Initially, a fundamental macromonomer library based on acrylate and acrylate-type monomers was established. The reaction conditions for the high temperature polymerization process were investigated in order to obtain high purity macromonomers. It was found that macromonomers with a major quantity of unsaturated chain termini were obtained using n-butyl acrylate as a monomer. In addition, it was demonstrated that the addition of radical initiators to the macromonomer formation reaction reduces the inhibition phase observed during initiator-free synthesis, without interfering in the actual polymerization. Generally, 82 % to 95 % (as estimated by ESI-MS) of the macromonomer generated in a single reaction possess the geminal double bond, depending on the type of monomer. The molecular weight of the macromonomers that can be achieved is located between 800 g∙mol-1 and 2000 g∙mol-1, with polydispersities close to 1.6. The initial generated macromonomer library was expanded by synthesizing dendronized macromonomeric structures. Dendronized acrylates based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) were synthesized from dendrimers which subsequently were functionalized via click chemistry (i.e. copper catalyzed azide-alkyne cycloaddition (CuAAC)). Utilizing this synthetic procedure, dendronized acrylates from 1st to 3rd generation were generated that contained a 6 or 9 carbon chain spacer between the acrylate functional group and the dendron core. The dendronized macromonomers were obtained by subjecting the dendronized acrylates to the high temperature polymerization process. The achieved number-average molecular weight (Mn) of the dendronized macromonomers was between 1700 and 4400 g•mol-1. The obtained vinyl terminated oligomers with their geminal substitution were found to have a purity of as high as 83 %, where the 1st generation dendronized acrylates provided the highest purity. Moderate deprotection of the acetonide groups occurred spontaneously during the macromonomer formation process and was most serve for the 3rd generation dendrimers. In addition to creating dendritic structures, copolymers were synthesized either via free radical copolymerization or by ring-opening polymerization (ROP). An n-butyl acrylate macromonomer (BAMM) was the starting point of the copolymer synthesis. The successful copolymerization of BAMM with benzyl acrylate (BzA) as a co-monomer to give statistical copolymers featuring pendant side chains was described. The obtained pBAMM-co-pBzA were found to have a Mn between 8000 g•mol-1 and 77000 g•mol-1 with a polydispersity of 1.30 to 2.12. The composition of the copolymers was calculated from the 1H NMR integrals associated with resonances for the BzA aromatic protons and the CH2 protons of the BAMM. These results were subjected to a (‘terminal model’) Mayo-Lewis analysis resulting in estimated reactivity ratios (at ≈ 40 % conversion) of rBzA = 2.46 and rBAMM = 1.79, indicating a copolymer composition of FBA < 0.65 for the copolymers derived from fBzA > 0.9 up to a copolymer composition of FBA > 0.9 for copolymers with a comonomer feed of fBzA < 0.6. As in recent years a strong interest was developed in employing thiol-ene chemistry for macromolecular construction, it seemed apt to subject n-butyl acrylate macromonomers to radical thiol-ene chemistry with the aim of generating star polymer structures as detailed in the original grant application. To this date, no other research group had attempted to conduct polymer-polymer conjugation reactions (i.e. the joining of two or more independent polymer strands) via radical thiol-ene chemistry. All existing studies focused on the conjugation of small molecules to polymer backbones or surfaces. Surprisingly, it was found that radical thiol-ene chemistry is not a very suitable tool for efficient polymer-polymer conjugation. While in difunctional and tetrafunctional thiol-ene systems some conjugation is observed via ESI-MS, the process is far from quantitative. While initially thought that the observed inefficiency of the radical thiol-ene conjugation reaction may be due to the structure of our employed macromonomers, it became rapidly clear that the failure of radical thiol-ene chemistry for polymer-polymer ligation is of a general nature. In parallel to our experimental investigations a through theoretical assessment of the macromonomer formation process was carried out, via an extensive PREDICI® simulation of the high temperature macromonomer formation process. A complex kinetic model was developed which accounted for the key reactions constituting the macromonomer formation process. On the basis of the kinetic model, the important rate coefficients governing acrylate polymerization (e.g. -scission and termination rate coefficients of midchain radicals, backbiting and intramolecular chain transfer rate coefficients) as well as the reaction conditions (e.g. initial monomer concentration, reaction temperature, radical flux) were systematically varied and their influence on the high temperature macromonomer formation process is critically evaluated. In summary, the work carried out in grant proposal BA3751/2-1 has demonstrated that a facile and efficient methodology exists for the formation of macromonomeric species from acrylate based entities. The process can be theoretically well-described. Importantly, these macromonomeric moieties can be successfully employed in free radical polymerization with conventional acrylate species. Finally, it has been established that the terminal double bonds in the macromonomers are unsuitable as conjugations points for polymer-polymer ligation via radical thiolene click chemistry, although small molecule additions functions well via the radical thiolene process. The origin of these limitations (inability of form polymer-polymer conjugates) is based in the non-orthogonal nature of radical thiolene chemistry, rather than in the nature of the vinylic double bonds of the generated macromonomers.
Publications
- "Direct Macromonomer Synthesis via High Temperature Acrylate Polymerization". Controlled Radical Polymerization Meeting, Houffalize, Belgium, September 17th to 18th 2009, Book of Abstracts
Zorn, A.-M.; Junkers, T.; Barner-Kowollik, C.
- "Optimum Reaction Conditions for the Synthesis of Macromonomers via High-Temperature Polymerization of Acrylates". Macromol. Theory Simul. 2009, 18, 421‐433
Junkers, T.; Barner-Kowollik, C.
- "Synthesis of a Macromonomer Library from High‐Temperature Acrylate Polymerization". Macromol. Rapid Commun. 2009, 30, 2028‐2035
Zorn, A.-M.; Junkers, T.; Barner-Kowollik, C.
- "Direct Radical Appraisal: Complex Reactions in Acrylate Polymerizations". Bunsenkolloqium, Göttingen, Germany, April 8th 2010
Junkers, T.; Koo, S.P.S.; Zorn, A.-M.; Barner-Kowollik, C.
- "Efficient Polymer Material Design via Modular Building Blocks". Dresden Polymer Discussion 18th to 21st of April 2010, Meissen
C. Barner-Kowollik
- "Limitations of Thiol‐ene Reactions for Polymer‐Polymer Conjugation". J. Polym. Sci. – Polym. Chem. 2010, 48, 1699‐1713
Koo, S.P.S.; Stamenović, M. M.; Prasath, R.A.; Inglis, A.J.; Du Prez, F.E.; Barner‐Kowollik, C.; Van Camp, W.; Junkers, T.
- "Mass Spectrometry as a Mechanistic Tool". PacifiChem 2010, 16th to 20th of December 2010, Honolulu
C. Barner-Kowollik
- "A Detailed Investigation of the Free Radical Copolymerization Behavior of n-Butyl Acrylate Macromonomers". Macromolecules 2011, 44, 6691–6700
Zorn, A.-M.; Junkers, T.; Barner‐Kowollik, C.
- "High Temperature Synthesis of Vinyl Terminated Polymers Based on Dendronized Acrylates: A Detailed Product Analysis Study". Polym. Chem. 2011, 2, 1163‐1173
Zorn, A.-M.; Malkoch, M.; Carlmark, A.; Barner‐Kowollik, C.
- "The Free Radical Copolymerization Behavior of n-Butyl Acrylate Macromonomers". Bayreuth Polymer Symposium, Bayreuth, Germany, September 11th to 13th 2011, Bock of Abstracts
Zorn, A.-M.; Junkers, T.; Barner-Kowollik, C.
