Common Chain-Growth Polymers - Principles of Polymer Chemistry - page 370

Solvents influence the rate of free-radical homopolymerization acrylic acid and its copolymerization
with other monomers. Hydrogen bonding solvents slow down the reaction rates [ 219 ]. Due to electron
withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not
by cationic mechanisms. Lithium alkyls are very effective initiators of

-methyl methacrylate
polymerization yielding stereospecific polymers [ 213 ]. Isotactic poly(methyl methacrylate) forms in
hydrocarbon solvents [ 214 ]. Block copolymers of isotactic and syndiotactic poly(methyl methacry-
late) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene
glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity [ 215 ] in the
following order:

a

tetrahydrofuran tetrahydropyran dioxane diethyl ether

> > >

Furthermore, polymerizations in solvating media, like ethylene glycol dimethyl ether, tetrahydro-
furan, or pyridine, using biphenylsodium or biphenyllithium yield virtually monodisperse syndiotactic
poly(methyl methacrylate) [ 216 ].
The nature of the counterion in anionic polymerizations of methyl methacrylate in liquid ammonia
with alkali metal amide or alkali earth metal amide catalysts is an important variable [ 217 ]. Lithium
and calcium amides yield high molecular weight polymers, though the reactions tend to be slow.
Sodium amide, on the other hand, yields rapid polymerizations but low molecular weight polymers.
Polymers formed with sodium amide, however, have a narrower molecular weight distribution than
those obtained with lithium and calcium amides. Calcium amide also yields high molecular weight
polymers from ethyl acrylate and methyl methacrylate monomers in aromatic and aliphatic solvents
at temperatures from

8 to 110 C. When, however, tetrahydrofuran or acetonitrile is used as solvents
much lower molecular weight products form [ 218 ].
Products from anionic polymerizations of methyl methacrylate catalyzed by Grignard reagents
(RMgX) vary with the nature of the R and X groups, the reaction temperature, and the nature of the
solvent [ 219 - 221 ]. Secondary alkyl Grignard reagents give the highest yields and the fastest rates of
the reactions. Isotacticity of the products increases with the temperature. When anion-radicals from
alkali metal ketyls of benzophenone initiate polymerizations of methyl methacrylate, amorphous
polymers form at temperatures from 78 to +65 C[ 222 ].

Sodium dispersions in hexane yield syndiotactic poly(methyl methacrylate) [ 223 ]. A 60-65%
conversion is obtained over a 24-h period at a reaction temperature of 20-25 C. Lithium dispersions
[ 224 ], butyllithium [ 203 ], and Grignard reagents [ 225 , 226 ] yield crystalline isotactic poly( -butyl

acrylate). The reactions take place in bulk and in hydrocarbon solvents. Isotactic poly(isopropyl
acrylate) forms with Grignard reagents [ 226 , 227 ].
Coordination polymerizations of methyl methacrylate with diethyliron-bipyridyl complex in
nonpolar solvents like benzene or toluene yield stereoblock polymers. In polar solvents, however,
like dimethylformamide or acetonitrile, the products are rich in isotactic placement [ 229 ].
There are many reports in the literature on polymerizations of acrylic and methacrylic esters with
Ziegler-Natta catalysts [ 230 - 233 ]. The molecular weights of the products, the microstructures, and
rates of the polymerizations depend upon the metal alkyl and the transition metal salt used. The ratios
of the catalyst components to each other are also important [ 234 , 235 ].
In 1992 Yasuda et al. [ 236 , 237 ] reported that organolanthanide complexes of the type Cp* 2 Sm-R
(where Cp* is pentamethyl cyclopentadienyl, and R is either an alkyl, alkylaluminum or a hydride)
initiate highly syndiotactic, living polymerizations of methacrylates. It was also reported that
lanthanide complexes such as Cp* 2 Yb(THF) 1-3 , Cp* 2 Sm(THF) 2 , and (indenyl) 2 Yb(THF) 2 can also
initiate polymerizations of methylmethacrylate [ 238 ]. Although very low initiator efficiencies were

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