Spiropyran transformations in polymer matrix
Russ.Chem.Bull., Int.Ed., Vol. 56, No. 2, February, 2007
201
a special architecture of macromolecules of the copolyꢀ
mer preventing fast bleaching of 1´. Perhaps, either the
high sulfur content or an increase in the degree of cyclizaꢀ
tion of the branched structure, or both these factors siꢀ
multaneously are significant in this case. Macromolecules
of copolymer 4a of composition 100 : 21 : 21 have the
shortest primary polymer chains (10 units), contain many
terminal chains, and are characterized by the high conꢀ
tent of the —SC H groups (DT residues inserted into
At/A0
1.0
0.5
2
1
0
21
1
3
the polymer chain due to chain transfer).
2
0
40
t/min
Therefore, the composition of copolymer 4a equal to
1
00 : 12 : 12 is most optimal for the t1/2 value, and polyꢀ
Fig. 3. Kinetics of transformation 1´ → 1 in the matrices of
branched copolymers 4c of composition 100 : 12 : 12 (1), 4d of
composition 75 : 25 : 12 : 12 (2), and 4e of composition
mer chains of this copolymer are characterized by
the maximum molecular mobility compared to other
poly(methyl methacrylates) obtained using EGDM as a
branching agent.
1
00 : 12 : 12 (3) (T = 20 °C).
The MMA—EGDM copolymer (4b) of composition
domly distributed in the main polymer chain were asꢀ
sumed to favor worsening of the molecular packing and,
hence, an increase in the free volume of the polymer. It is
noteworthy that in this polymer matrix the frequency of
the absorption band maximum of form 1´ upon UV irraꢀ
diation is shifted to higher wavelengths (bathochromic
shift). This indicates a decrease in the level of dipoleꢀ
dipole interaction of form 1´ with the polymer matrix.
As a result, the rate of transformation 1´ → 1 should
increase and the t1/2 value should decrease compared to
the corresponding values for copolymer 4a of composiꢀ
tion 100 : 12 : 12. However, an opposite effect is observed:
the introduction of the LMA units into the polymer chain
decreases the rate of 1´ bleaching and increases the
II
–3
1
00 : 12 prepared in the presence of Co Pn (1•10
–
1
mol L ) was also studied as a polymer matrix. Copolyꢀ
mer 4b differs from those studied above by the absence in
the polymer chains of the —SC H groups affecting the
1
0
21
mobility of the reaction medium and by the presence of
an additional amount of the C=C terminal bonds formed
due to catalysis of chain transfer.12 As found for this case,
the bleaching rate of form 1´ decreases (see Fig. 2, curve 4)
and the t1/2 value increases, correspondingly, over that for
polymer 4a of the same composition obtained in the presꢀ
ence of DT. Copolymer 4b has T (see Table 1) by almost
g
2
1
0 °C higher than that of copolymer 4a of composition
00 : 12 : 12. Thus, the decrease in the bleaching rate of
merocyanine 1´ correlates with the decrease in the moꢀ
lecular mobility of the polymer matrix of 4b.
t1/2 values in spite of a decrease in T (see Table 1). The
g
most probable reason for the retarded photochromic transꢀ
formations of compound 1 in the matrix of branched
copolymer 4d is the formation of ordered structures due
to the enhancement of hydrophobic interactions between
the hydrocarbon units of the —C H substituents and
It was of considerable interest to study photochromic
transformations in the matrices of branched copolymers 4
containing the bulky BMA and LMA units. The replaceꢀ
ment of MMA by methacrylic monomers with bulky alkyl
substituents can substantially change the architecture and
properties of copolymers 4, first of all, the free volume of
the polymer matrix and mobility of the polymer chains
and, hence, can effect the rate of photochromic transforꢀ
mations of compound 1. For this purpose, we studied
the kinetics of bleaching 1´ → 1 in the matrices of
the branched copolymers BMA—EGDM—DT (4c) and
MMA—LMA—EGDM—DT (4d) of compositions
12
25
the free ends (—SC H ) inserted in the polymer due to
1
0
21
the chainꢀtransfer reaction. The formation of ordered
structures is characteristic of the polymers containing more
4
than 10 carbon atoms in the side chain.
1
3
It is known that the architecture and properties of
the branched copolymers are determined by the monoꢀ
mer : branching agent : chainꢀtransfer agent ratio and also
by the structure of the branching agent. We studied the
MMA—TEGDM—DT copolymer (4e) of composition
100 : 12 : 12 as a polymer matrix. The TEGDM branching
agent has the same reactivity as EGDM but differs subꢀ
stantially in length of the oligomeric unit between the
methacrylic fragments. Elongation of the latter (when
TEGDM is used instead of EGDM) can create additional
steric hindrance for the interaction of these fragments.
Thus, the elongation of the branching agents should inꢀ
crease the molecular mobility of the polymer matrix. The
bleaching kinetics of merocyanine 1´ due to transforꢀ
mation into the initial compound 1 in branched copolyꢀ
1
00 : 12 : 12 and 75 : 25 : 12 : 12, respectively (Fig. 3),
whose physicochemical properties are described in Table 1.
Transformation 1´ → 1 in the matrix of branched coꢀ
polymer 4c occurs with a high rate and is characterized by
the t1/2 value close to that observed for copolymer 4a with
the same composition. Thus, the increase in the alkyl
substituent size from one to four carbon atoms exerts no
effect on the rate of photochromic transformations of
compound 1 in the matrix of polymer 4c.
Different results were obtained for copolymer 4d with
the composition 75 : 25 : 12 : 12. The LMA units ranꢀ