the concentration of macroradicals, as well as the probability of termination, decreases with time. The growing radicals
therefore predominantly react with X
• , which is present at much higher concentration, rather than with themselves (see
Fig. 4.41B).
In such fast-initiated systems, hX n i is expected to evolve as follows:
X n
h i ¼
M
½ 0
f Ini-X
½
0
∙ α
ð4:26Þ
where [Ini-X] 0 is the initial concentration of the initiator, f [Ini-X] 0 represents the fraction of initiated chains at the early
stage of the reaction, and α ¼ ([M] 0 – [M])/[M] 0 is the extent of conversion of the monomer (see § 4.6.3, Annex 4.3,
Eq. 4.23). The molecular mass of the polymer is therefore expected to evolve linearly with the degree of conversion,
which is the typical signature of a “living” synthesis system (Bertin and Boutevin 1996), whereas linearity can be
observed only at low conversion when polymerization is performed by RP (see Annex 4.3, Fig. A4.3.1, and Braun 2009;
Misha and Kumar 2012).
Synthesis systems based on DT (namely RAFT) follow typical RP kinetics with slow initiation and fast
termination. The concentration of transfer agent is much larger than that of radical initiators, and its transfer constant
is typically high (rate of transfer larger than rate of propagation, i.e. C T much higher than 1). Thus, the transfer agent plays
the role of the dormant species. The monomer is consumed by a very small concentration of radicals which can terminate
but also, with a higher probability, degeneratively exchange with the dormant species (the highly transfer-effective
reagent TA-Y can be reversibly bound to the growing chains and indifferently exchanged between them). This reversible
and rapid distribution of the transfer agent between active and dormant species allows to readjust the drift of the
incorporated monomers’ amount in each growing chain at the early stage of the polymerization (see Fig. 4.41C), resulting
in the following linear relation:
X n
h i ¼
M
½ 0
TA À Y
½
0
∙ α
ð4:27Þ
where the initial concentration of transfer agent is [TA-Y] 0 .
Living polymerization is achieved by using specific reagents (some examples of structures of which are given in
Fig. 4.42), which reversibly bind to the growing chain. Monomers are incorporated only either after homolytic rupture of
this reversible link (meaning that, the covalent bond being symmetrically broken, one of the two electrons involved in the
covalent link is captured by each of the two vicinal atoms, generating two radicals) or during a degenerative exchange
process (reversible chain transfer). Whatever the mechanism, polymerization performed by CRP evolves more homogeneously than when performed by RP, leading to a significantly lower dispersity.
The choice of the method is dictated by the experimental conditions (temperature, solvent, type of monomer) and
the structure to be obtained (see Annex 4.5). For instance, a limitation of NMP is that only a few successful examples
have been reported when the method was applied to acrylate derivatives (Knoop and Studer 2003). Alternative
approaches (mainly ATRP and RAFT) are often preferred for CRP of acrylic monomers (Braunecker and Matyjasziewski
2007; Grubbs 2011).
Because RAFT does not involve the use of toxic metal salts and it is compatible with a wide range of monomers, it
was initially often preferred to ATRP (Boyer et al. 2016). However, constant adaptations and modernizations of the
method have resulted in drastic limitation of the concentrations of metal complexes (Pintauer and Matyjaszewski 2008;
Boyer et al. 2016). Emerging trends are even aiming at developing metal-free ATRP. One successful typical example has
been recently reported, where ATRP was photoinduced (Treat et al. 2014). After 20 years of development and
improvements, numerous ATRP-based methods have found applications for a wide range of monomers without the
Table 4.6 Some key references to the principles of controlled radical polymerization (CRP).
CRP
method Reviews
Articles
NMP
Braunecker and Matyjasziewski (2007) and
Grubbs (2011)
Georges et al. (1993), Veregin et al. (1993),
and Bertin and Boutevin (1996)
ATRP
Braunecker and Matyjasziewski (2007).
Pintauer and Matyjaszewski (2008),
Matyjaszewski and Tsarevsky (2014), and
Boyer et al. (2016)
Wang and Matyjaszewski (1995) and
Buback et al. (2016)
RAFT
Barner-Kowollik et al. (2006), Braunecker
and Matyjasziewski (2007), and Moad et al.
(2012)
Chiefari et al. (1998), Feldermann et al.
(2004), Meiser et al. (2011), Meiser and
Buback (2012), and Buback et al. (2016)
4.6 Annexes
221
therefore predominantly react with X
• , which is present at much higher concentration, rather than with themselves (see
Fig. 4.41B).
In such fast-initiated systems, hX n i is expected to evolve as follows:
X n
h i ¼
M
½ 0
f Ini-X
½
0
∙ α
ð4:26Þ
where [Ini-X] 0 is the initial concentration of the initiator, f [Ini-X] 0 represents the fraction of initiated chains at the early
stage of the reaction, and α ¼ ([M] 0 – [M])/[M] 0 is the extent of conversion of the monomer (see § 4.6.3, Annex 4.3,
Eq. 4.23). The molecular mass of the polymer is therefore expected to evolve linearly with the degree of conversion,
which is the typical signature of a “living” synthesis system (Bertin and Boutevin 1996), whereas linearity can be
observed only at low conversion when polymerization is performed by RP (see Annex 4.3, Fig. A4.3.1, and Braun 2009;
Misha and Kumar 2012).
Synthesis systems based on DT (namely RAFT) follow typical RP kinetics with slow initiation and fast
termination. The concentration of transfer agent is much larger than that of radical initiators, and its transfer constant
is typically high (rate of transfer larger than rate of propagation, i.e. C T much higher than 1). Thus, the transfer agent plays
the role of the dormant species. The monomer is consumed by a very small concentration of radicals which can terminate
but also, with a higher probability, degeneratively exchange with the dormant species (the highly transfer-effective
reagent TA-Y can be reversibly bound to the growing chains and indifferently exchanged between them). This reversible
and rapid distribution of the transfer agent between active and dormant species allows to readjust the drift of the
incorporated monomers’ amount in each growing chain at the early stage of the polymerization (see Fig. 4.41C), resulting
in the following linear relation:
X n
h i ¼
M
½ 0
TA À Y
½
0
∙ α
ð4:27Þ
where the initial concentration of transfer agent is [TA-Y] 0 .
Living polymerization is achieved by using specific reagents (some examples of structures of which are given in
Fig. 4.42), which reversibly bind to the growing chain. Monomers are incorporated only either after homolytic rupture of
this reversible link (meaning that, the covalent bond being symmetrically broken, one of the two electrons involved in the
covalent link is captured by each of the two vicinal atoms, generating two radicals) or during a degenerative exchange
process (reversible chain transfer). Whatever the mechanism, polymerization performed by CRP evolves more homogeneously than when performed by RP, leading to a significantly lower dispersity.
The choice of the method is dictated by the experimental conditions (temperature, solvent, type of monomer) and
the structure to be obtained (see Annex 4.5). For instance, a limitation of NMP is that only a few successful examples
have been reported when the method was applied to acrylate derivatives (Knoop and Studer 2003). Alternative
approaches (mainly ATRP and RAFT) are often preferred for CRP of acrylic monomers (Braunecker and Matyjasziewski
2007; Grubbs 2011).
Because RAFT does not involve the use of toxic metal salts and it is compatible with a wide range of monomers, it
was initially often preferred to ATRP (Boyer et al. 2016). However, constant adaptations and modernizations of the
method have resulted in drastic limitation of the concentrations of metal complexes (Pintauer and Matyjaszewski 2008;
Boyer et al. 2016). Emerging trends are even aiming at developing metal-free ATRP. One successful typical example has
been recently reported, where ATRP was photoinduced (Treat et al. 2014). After 20 years of development and
improvements, numerous ATRP-based methods have found applications for a wide range of monomers without the
Table 4.6 Some key references to the principles of controlled radical polymerization (CRP).
CRP
method Reviews
Articles
NMP
Braunecker and Matyjasziewski (2007) and
Grubbs (2011)
Georges et al. (1993), Veregin et al. (1993),
and Bertin and Boutevin (1996)
ATRP
Braunecker and Matyjasziewski (2007).
Pintauer and Matyjaszewski (2008),
Matyjaszewski and Tsarevsky (2014), and
Boyer et al. (2016)
Wang and Matyjaszewski (1995) and
Buback et al. (2016)
RAFT
Barner-Kowollik et al. (2006), Braunecker
and Matyjasziewski (2007), and Moad et al.
(2012)
Chiefari et al. (1998), Feldermann et al.
(2004), Meiser et al. (2011), Meiser and
Buback (2012), and Buback et al. (2016)
4.6 Annexes
221