Chemistry Reference
In-Depth Information
Fig. 7.2
TEM micrograph of polyethylene
latex particles prepared by catalytic poly-
merization (
M
w
2.1
10
5
g mol
-1
,
M
w
/
M
n
2.2; crystallinity 54%; catalyst precursor
9b
) [82].
due to significant co-monomer concentration drifts during the polymerization. This
translates into
T
m
, as measured by DSC, spanning over as much as 60
C.
In the aforementioned ethylene polymerizations yielding stable latexes, low-mo-
lecular-weight polymer was obtained. To exploit the property profile of polymer la-
texes, the synthesis of dispersions of polyolefins with higher molecular weights is
necessary. Mecking et al. have employed miniemulsions of the somewhat water-
sensitive catalyst precursor
9b
for ethylene polymerization. Hereby, stable latexes
of high molecular weight semicrystalline polyethylene (
M
w
4
10
5
g mol
-1
,
M
w
/
M
n
2 to 4) could be obtained [82].
The polyethylene latexes obtained in the different emulsion polymerization pro-
cedures using the various aforementioned nickel(II) complexes display average
particle diameters of 100 to 600 nm. A number of anionic surfactants or neutral
stabilizers are suitable, i.e. compatible with the catalysts and capable of stabilizing
the latex. Solids contents of up to 30% have been reported to date. A typical TEM
image is shown in Fig. 7.2. By comparison to smooth, spherical latex particles of
amorphous polystyrene as a well studied hydrocarbon polymer prepared by free-
radical emulsion polymerization, the ruggedness of the particles shown can be ra-
tionalized by their high degree of crystallinity.
Very recently, an aqueous olefin polymerization using an early transition metal
catalyst has also been reported [84]. A toluene solution of styrene is prepolymer-
ized briefly by a catalyst prepared by combination of [(C
5
Me
5
)Ti(OMe)
3
] with a bo-
rate and an aluminum-alkyl as activators. The reaction mixture is then emulsified
in water, where further polymerization occurs to form syndiotactic polystyrene
stereoselectively. It is assumed that the catalyst is contained in emulsified droplets
and is thus protected from water, with the formation of crystalline polymer en-
hancing this effect. Cationic or neutral surfactants were found to be suitable,
whereas anionic surfactants deactivated the catalyst. The crystalline polystyrene
formed was reported to precipitate from the reaction mixture as relatively large
particles (500
m).
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