Synthesis of tailored polymer structures by the combination of cationic and anionic polymerizations
Jesper Feldthusen (01/1998-01/1998)
A new method for the synthesis of tailored polyisobutylene-based (PIB-based) block copolymers by the combination of controlled/living cationic and controlled/living anionic polymerizations has been developed. In addition, parallel to this subject new synthetic routes for preparation of telechelic PIBs and amphiphilic networks have been investigated.
The PIB precursors used for subsequent controlled/living anionic polymerization of various methacrylates and ethylene oxide were prepared by controlled/living cationic polymerization of isobutylene followed by quenching with 1,1-diphenylethylene (DPE) under selected conditions. Depending on the quenching procedure two different endgroups, either a diphenylmethoxy- or a diphenylvinyl endgroup were obtained.
Since the diphenylmethoxy endgroup can be obtained in quantitative yield by quenching with methanol under carefully selected conditions, quenching experiments involving the diphenyl-cation with different alcohols were performed, e.g. with 2-hydroxyethyl methacrylate (HEMA) leading to in situ formation of a macromonomer. However, since the competing reaction between the Lewis acid (TiCl4) and the alcohol is more pronounced in case of these alcohols only partial functionality was reached (with HEMA » 65 % was detected). The side product is the diphenylvinyl endgroup formed as consequence of the reduced Lewis acidity (TiCl4 + xROH Ti(OR)xCl4-x). Further experiments with these alcohols using different Lewis acids might lead to improvements.
Preliminary model studies involving the transformation from cationic to anionic polymerization with low molecular weight compounds having the same active sites as the corresponding PIB precursors were performed. The compounds used in these experiments were; diphenylmethane (DPM), triphenylmethane (TPM), 3,3,5,5-tetramethyl-1,1-diphenylhex-1-ene (TMP-DPV), and 1-methoxy-3,3,5,5-tetramethyl-1,1-diphenyl (TMP-DPOMe).
With the first two compounds (DPM and TPM) < 85 % conversion was reached under different conditions. Lithiation of TMP-DPV was performed with n/s-BuLi. However, in contrast to DPE, the addition of BuLi is not quantitative due to sterical hindrance. Direct metalation of TMP-DPV with K/Na alloy or Cs led to quantitative conversion within 15 min verified by 1H NMR and UV spectroscopy. Interestingly, coupling was not detected which is the case in a similar reaction with DPE. Under the same conditions metalation of TMP-DPOMe was completed also in 15-20 min. In both experiments the same product was formed after quenching with methanol, 3,3,5,5-tetramethylhexane. By addition of LiCl, the corresponding lithium initiator was achieved. That means that by this procedure (macro)initiators with various counterions; K+, Cs+, and Li+ are available in quantitative yield in less than 20 min (metalation with Na-mirror is rather slow). Anionic polymerizations of methacrylates with the synthesized initiator, 3,3,5,5-tetra-methylhexyllithium from TMP-DPV and/or TMP-DPOMe indicate quantitative initiating efficiency.
In order to analyze effects of potential Clt-terminated PIB (due to incomplete DPE-capping) metalation experiments were carried out with a sample containing 85 % DPV and 15 % Clt endgroups and one with 100 % Clt-terminated PIB. With the former sample coupling was observed due to nucleophilic substitution of the Clt endgroup with PIB-DPE-. Experiments with the 100 % Clt-terminated PIB led to interesting observations. Metalation with K/Na alloy in n-hexane contain-ing N,N,N,N-tetramethylethyldiamine resulted in quantitative formation of isopropenyl capped PIB faster and more selectively than existing methods (instead of the expected generation of a PIB mac-rocarbanion which is partially formed in pure n-hexane together with the isopropenyl endgroup).
Metalation of the resulting DPE-capped PIB precursor with K/Na alloy or Cs led also to quantitative formation of a PIB macrocarbanion within less than 60 min independent of the molecular weight. The independence of the metalation and anionic polymerization on the ratio of the diphenylvinyl and diphenylmethoxy endgroups is a important finding. This means that it is not necessary to consider the chain end composition of the DPE-capped PIB in the course of its preparation by controlled/living cationic polymerization.
Well-defined OH-terminated PIBs with quantitative functionality were synthesized by adding ethylene oxide to the lithium salt of the PIB macroinitiator. Using the corresponding potassium or cesium salt of the PIB macroinitiator tailored amphiphilic i.e. water soluble PIB-b-PEO block copolymers were prepared.
Different tailored PIB-based AB, ABA, and (AB)3 block copolymers of various methacrylates were synthesized in order to obtain materials which can subsequently be characterized. Near to quantitative blocking efficiencies were normally reached proved by SEC analysis (and 1H NMR).
Thermoplastic elastomers (TPEs) based on linear poly(methyl methacrylate)-b-poly-isobutylene-b-poly(methyl methacrylate) PMMA-b-PIB-b-PMMA and star-shaped (PIB-b-PMMA)3 block copolymers with tensile strength exceeding 20 MPa have been prepared. The synthesized samples have similar properties compared to corresponding polystyrene-based block copolymers (PSt-b-PIB-b-PSt). In addition, the morphology was examined by small angle X-ray scattering. Lamellar and cylindrical morphologies were observed.
Amphiphilic polyisobutylene-b-poly(methacrylic acid) (PIB-b-PMAA) block copolymers have been synthesized and characterized. The characterization of these block copolymers in aqueous media with dynamic and static light scattering, fluorescence correlation spectroscopy, ultracentrifu-gation, and transmission electron microscopy verified the formation of well-defined micelles with an aggregation number and a hydrodynamic radius which depend primarily on the chain length of the PIB block segment. Compared to other polymer systems, an extremely low CMC was detected for all the PIB-b-PMAA samples, about 2-3 orders of magnitude lower than comparable PMMA-b-PMAA block copolymers.
Preliminary studies regarding synthesis of amphiphilic networks (APNs) based on block copolymer units have been carried out using different methods: A series of APNs were prepared by adding ethylene glycol dimethacrylate to living block copolymer units. A second curing process involving crosslinking reactions between alcohol groups at the polymer chain ends and isocyanates or acid chlorides was examined, too. Optimization of the curing procedures and systematic characterization of the resulting APNs have to be performed in a future work.