Synthesis of oligo (vinyl ether)s in ab initio cationic polymerisation.

Terminally functionalised oligo(vinyl ether)s were produced in ab initio
cationic polymerisations. Various polymerisations and chain end functionalisation
systems were investigated. MALDI-TOF mass spectrometry was applied to analyse
the obtained oligomers and thus the polymerisation and chain end functionalisation
process.
Oligo(isobutyl vinyl ether), also oligo(ethyl vinyl ether) and oligo(methyl
vinyl ether) were synthesised in cationic polymerisation. Silyl enol ethers were added
to the polymerisation as end-capping agents before initiations and they compete with
monomer to cap the carbocationic chain end. The methodology relies on a comparable
end capping rate to chain propagation rate so that oligomers can still be produced in
the presence of reactive end-capping agents whilst other side reactions are suppressed.
Polymerisation temperatures investigated range from -78°C to the room
temperature (21 °C), 4 out of 6 silyl enol ethers were applied and are proved to be
reactive in the end-capping. Initiation systems investigated include iBVEHCl/Yb(OTf)3
and iBVE-HCl/SnC14 and both produced oligomers with high chain
end functionalities. Generally silyl enol ether functionalised oligo(vinyl ether)s have
lower molecular weights and broader molecular weight distributions than their
identical control polymerisations without end-capping. The iBVE-HCl/SnCI4
initiation system produced functionalised oligomers with narrower molecular weight
distribution than iBVE-HCl/Yb(OTf)3 initiation system.
Different silyl enol ether reactivities in end-capping were observed. It was also
observed that ab initio chain end functionalisation by reactive silyl enol ethers largely
suppressed the majority of side reactions during polymerisation. This suppression was
attributed to the higher rates of end-capping than side reaction rates.
When (1-tert-butyl-vinyloxy)trimethyl-silane was applied as end-capping
agent, the polymerisation system produced oligomers with narrower molecular weight
distributions than the control polymerisations without end-capping while the chain
end functionalities were also obtained. This indicates the possibility of setting up a
controlled ab initio chain end functionalisation cationic polymerisation system in
which the oligomer's molecular weight, polydispersity and chain end functionality
can be regulated at the same time.
MALDI-TOF MS, NMR and SEC are mainly applied in the oligomer
characterisation. Side reactions in this polymerisation system were examined from
these analyses. Under less critical polymerisation conditions 7 different chain ends
from side reactions are observed in MALDI-TOF mass spectra. Based on the MS and
NMR observation the various chain end structures are identified. Side reactions are
also postulated which mainly include ß-proton elimination, water capping of the
carbocationic chain end and combinations of these.
Sample preparation for MALDI-TOF MS analysis of oligo(vinyl ether)s is
investigated. Direct laser desorption of oligo(isobutyl vinyl ether) was observed for
the samples of molecular weight of up to 2k Daltons. Complementary information of
oligo(isobutyl vinyl ether)s obtained from ESI MS indicates a serious mass
discrimination in MALDI-TOF MS technique and thus it is regarded that MALDITOF
MS can not provide reliable molecular weight distributions for polymers with
broad molecular weight distributions.
Combination of SEC and MALDI-TOF MS to calibrate SEC columns and
quantitative application of MALDI-TOF MS to analyse the oligomers' chain end
functionalities were also explored.