Reactivity of allyl and vinyl pentosides in photo-initiated donor-acceptor copolymerization - Carbohydrate Chemistry: Chemical and Biological Approaches - page 282

by size exclusion chromatography (SEC) using THF as the eluent and
polystyrene standards for calibration. The number-average molecular
weights of the copolymers synthesize d i n solution and isolated at low
conversion were very low (at p = 70% M n ¼ 2500 g mol 1 for copolymers
from vinylethers, DEF-EVE, and less than 1000 g mol 1 at p = 26%
for allyloxy derivatives, DEF-EAE), whereas the polymerization in
solvent-free conditions en abl ed to obtain longer polymer chains with
hig her molecular weight (M n ð vinyloxy copolymer Þ 4110000 g mol 1 vs.
M n ð allyloxy copolymer Þ 1200 g mol 1 ) which shows the detrimental
influence of dilution on the polymerization (see Table 2). Indeed, the
higher average initiation rate in the samples irradiated in the form of
film cannot account for the observed differences in MW for the resulting
polymers.
As the kinetic chain length is inversely proportio nal to the instant
initiation rate (see equation 1 below), the low values of M n obtained could
be explained by the high eciency of the initiation process.

R p
R in þ P j R t;j

l ¼

ð 1 Þ

With l : kinetic chain length
R p : Instant polymerization rate
R in : Instant initiation rate
R t,j : Rate of each transfer reaction
Presuming the occurrence of a bimolecular termination mechanism
by combination, it can be assumed that when the PI content ([ PI])
is subject to a fourfold decrease, the resulting average molecular weight M n
should be doubled, if transfer reactions are negligible. A series of
experiments was conducted with various PI contents, yet maintaining low
absorbance conditions. The limited decrease in M n as a function of the
photo-initiator content (Fig. 4) clearly suggests a large contribution of
transfer reactions.

Table 2 Influence of the polymerization conditions on M n of various copolymers
(equimolar amounts of donor and acceptor monomers).
In solvent-free conditions c In solution b

M n a
(g mol 1 ) PDI a

M n a
(g mol 1 ) PDI a

Conversion
p D N (%)

Conversion
p D N (%) Copolymer Copolymer

DEF-AHE 49 1200 1.8 DEF-EAE 26 900 1.2

DEF-AIG 40 1540 1.9 DEF-EVE 70 2500 1.5

DEF-HVE 100 11900 1.7
DEF-VIG 100 16600 2.0
a SEC analyzes carried out at 35 1C with a PL-GPC50 Plus system (Polymer Laboratories):
solvent: THF; flow rate: 1mL min 1 ; PS standards.
b C monomer = 90 wt-% in ethyl acetate; C PI = 0.25 wt-% of Darocur 1173;

Irradiation:

l = 350 nm, P = 13-14 mW cm 2 , duration = 30 min.
c Photosensitive formulations sandwiched between two NaCl plates for oxygen shielding,
C PI = 5 wt-% of Darocur 1173; Irradiation: l = 365 nm, P = 8mW cm 2 , duration = 30 min.