The copolymerization of styrene and ethylene has been performed
using a titanocene-based
catalytic system of cyclopentadienyltitanium triphenoxide
(CpTi(OPh)3) and methylaluminoxane (MAO).
The catalyst system exhibited a high catalytic activity of
104−105 g (mof Ti·h)-1 and
was found to selectively
(more than 90 wt %) give an elastoplastic and amorphous
styrene−ethylene (S−E) copolymer with a
well-defined random/alternating microstructure and a single glass
transition (T
g) as thoroughly
characterized by solvent fractionation, GPC, 13C NMR, DSC, and WAXD.
Under the reaction conditions employed,
up to 54.5 mol % of styrene could be introduced into the copolymer
chains. The composition,
microstructure, molecular weight of the copolymers, and catalytic
activity of the copolymerization are
strongly dependent upon the comonomer feed ratio, polymerization
temperature (T
p),
CpTi(OPh)3/MAO
mole ratio, and trimethylaluminum (TMA) content or structure in MAO.
For 300 ⩽ Al/Ti < 1000, the
copolymerization product was essentially the random S−E copolymer.
For 1000 < Al/Ti ⩽ 2000, the
copolymerization product was significantly SPS homopolymer. The
results showed that 24.5−28.2 mol
% TMA content in MAO (oligomerization degree (n) ≈
18−20) was optimum for the copolymerization,
but 30.2−35.7 mol % TMA in MAO (oligomerization degree
(n) ≈ 24−28) for styrene syndiospecific
homopolymerization, even in the presence of ethylene feed monomer.
The external addition of TMA or
triisobutylaluminum (TIBA) inhibited the copolymerization but promoted
the styrene homopolymerization.
ESR spectroscopic analysis combined with copolymerization results
suggests the presence of a Ti(IV)
active center which is responsible for the formation of polyethylene, a
Ti(III) species which is active in
the syndiospecific polymerization of styrene, and, moreover, the
presence of a third intermediate which
contributes to promoting the copolymerization of styrene with ethylene
to produce S−E copolymer.
Sulfonation of highly stereoregular syndiotactic polystyrene has been accomplished in 1,1,2-trichloroethane and chloroform (60/40 v/v) mixed solvent. FTIR spectroscopy was used to con®rm that the sulfonated syndiotactic polystyrene was the product of the sulfonation reactions. Sodium, potassium, zinc (II), manganese (II) and cobalt (II) salts of the sulfonated polymers exhibited behaviour indicative of strong interactions. FTIR spectroscopy and DSC data showed that the roles of the cation±anionic site interactions in alkali form and transition metal form ionomers are somewhat different. The DSC data also showed that the alkali metal cations had more pronounced effect on T g than did the transition metal cations. In addition, the crystallization behaviour of the ionomers with a low degree of sulfonation also exhibited considerable differences in comparison with the neat syndiotactic polystyrene. The melting points (T m ) and the degree of crystallization (X c ) were signi®cantly lowered by the presence of the sulfonic acid groups or the sulfonate metal groups. Moreover, the ionomers were more thermally stable and more hygroscopic than the unmodi®ed polymer.
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