Page 264 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: GNH/GRI P2: GTV Final pages
Encyclopedia of Physical Science and Technology EN012B-596 July 27, 2001 18:18

Polymers, Synthesis 773

TABLE XVI Heats and Entropies of Polymerization for Cyclic termination steps help to control irreversible chain ter-
Ethers mination and result in polymerization behavior with lin-
ear molecular weight increase with time similar to living
−∆H −∆S
Monomer Ring size (kcal/mole) (cal/k mole) anionic polymerizations. Based on the earlier work of
Rizzardo et al. in nitroxide-mediated stable free-radical
Ethylene oxide 3 22.6 —
polymerization of methyl acrylate, Georges et al. first re-
Oxacyclobutane 4 16.1 —
ported the preparation of polystyrene with low polydis-
3,3-Bis(chloromethyl)- 4 20.2 19.9 persity using bulk free-radical polymerization of styrene
oxacyclobutane
initiated by a conventional free-radical initiator, ben-
1,3-Dioxolane 5 6.2 —
zoyl peroxide (BPO), in the presence of the stable ni-
Tetrahydrofuran 5 5.3 11.5
troxide free radical, 2,2,6,6-tetramethyl-1-piperidinyloxy
Tetrahydropyran 6 0.4 —
◦
(TEMPO) at 125 C. The SFRP process involves a de-
m-Dioxane 6 0.0 —
sirable reversible equilibrium between nitroxide-capped
1,3-Dioxepane 7 3.6 —
polymer chains and uncapped polymer radicals. The un-
capped polymer radicals are then able to chain extend via
proceeds by nucleophilic attachment of monomer at styrene monomer addition. The success of this method
the carbon next to the oxonium ion. Other counterions, arises from the unique feature that the nitroxide radicals
−
such as (PF 6 ) and (SbCl 6 ) , are also utilized. In some will react with carbon radicals at near-diffusion-controlled
−
systems, there are essentially “living” or nonterminated rates, but will not react with other oxygen-centered radi-
oxonium ions, which are somewhat analogous to the cals or initiate additional polymer chains.
“living” carbanionic polymerizations discussed earlier. One drawback of the initial SFRP technique is that the
Many ring-opening polymerizations display ring–chain polymerization requires long reaction times to achieve
equilibrium, and hence one may have to separate cyclic high conversion. The addition of camphorsulfonic acid
monomer from high-molecular weight linear chains at dramatically increases the rate of styrene polymerization
the end of the polymerization. and high yields could be achieved with reaction times less
than 6 hr. Addition of acylating agents such as acetic
anhydride to styrene SFRP dramatically reduces reac-
D. Controlled and Living Radical
tion time. The development of unimolecular initiators for
Polymerization
SFRP is a viable method to control molecular weight.
Controlled polymerization routes permit the synthesis of The classic initiating system is bimolecular and consists
well-defined macromolecules with controlled chemical of benzoyl peroxide as the initiating radical together with
composition, predictable molecular weight, and narrow TEMPO as the mediating radical. Disadvantages of the
molecular weight distribution. The ability to control poly- bimolecular initiating system include lack of control over
mer architecture is essential in advanced technological structural features such as molecular weight, chain ends,
applications where well-defined macromolecular archi- and architecture. A unimolecular initiator can be synthe-
tectures are required. Control of chain-growth polymer sized from benzoyl peroxide, TEMPO, and styrene. Using
architecture has been traditionally achieved using living this unimolecular initiator as well as derivatives, the syn-
anionic, cationic, or group-transfer polymerization proce- thesis of narrow-polydispersity materials with controlled
dures. Synthetic methodologies for controlled polymer- molecular weights, chain ends, and chain architectures is
ization have been expanded with recent developments in feasible.
stable free-radical polymerization (SFRP), atom transfer A very successful approach to the controlled nitro-
radical polymerization (ATRP), and the radical addition xide-mediated polymerization of acrylates uses β-phos-
and fragmentation technique (RAFT). phonate-substituted nitroxide with 2,2 -azobisisobutyro-

The basic feature of controlled polymerization is the nitrile (AIBN) as the initiating radical source. Two major
absence of transfer and termination processes in chain structural features of the β-phosphonate-substituted ni-
growth reactions as discussed earlier. Szwarc first de- troxide distinguish it from previously studied nitroxides.
fined such systems as “living polymerizations” in 1956 First, it is acyclic; second, it contains a α-hydrogen to
based upon his work on anionic polymerizations. Sev- the nitroxide functionality. Both of these features are
eral decades later, the idea of living polymerizations was expected to decrease the stability of nitroxide and increase
extended to free-radical systems. The use of initiator- decomposition. It is anticipated that the limitation of
transfer-agent-terminators, or iniferters, to reduce irre- polymerization of acrylates and other monomer families
versible chain termination in free-radical polymerization is the control of excess free nitroxide that accumulates
processes is a viable approach. The reversible radical during polymerization. Therefore, the decreased stability