reaction in the rac-Me 2 Si[2-Me-4-Ph(Ind)] 2 ZrCl 2 /MAO-catalyzed polymerization
of propylene is evidenced by its comparison with two control reactions that were
carried out under similar reaction conditions – without the chain transfer agent
(control 1) and with p-MS only (control 2). A small amount of p-MS effectively
stops the polymerization of propylene. The introduction of hydrogen restores the
catalyst activity, as shown in run IX-I-1, which exhibits about 85% of the catalytic
activity of control 1 (without chain transfer agents). Hydrogen is clearly needed to
complete the chain transfer cycle during polymerization, as shown in Scheme 8. In
general, both p-MS/hydrogen and styrene/hydrogen chain transfer systems showed
very similar results: the higher the concentration of the p-MS (or styrene), the lower
the molecular weight of the resulting polymer. The polymer with very low molecular weight (just a few thousand) has been obtained and the molecular weight
distribution is generally narrow, which is consistent with single site polymerization
processes. The catalyst activity was also proportionally depressed with the concentration of p-MS (or styrene), which reflects the competitive coordination at
metallocene active sites between monomer and chain transfer agents.
Figure 13 shows the plot of the polymer molecular weight (M n ) versus the mole
ratios of [propylene]/[p-MS] and [propylene]/[St]. There is a linear proportionality
between the polymer molecular weight and molar ratio of [propylene]/[p-MS] or
[propylene]/[St]. It is clear that the chain transfer reaction to styrenic molecule
(with rate constant k tr ) is the dominant termination process, with very high chain
transfer constant k tr /k p ~ 1/6.36 and 1/7.5 for p-MS and styrene, respectively. The
300
250
200
mV
150
100
20
22
(a)
(b)
(c)
(d) (e)
24
26
Minutes
28
30
32
Fig. 12 GPC curves of (a) PP (control 1) and (b–e) several PP-t-p-MS polymers prepared by racMe 2 Si[2-Me-4-Ph(Ind)] 2 ZrCl 2 -mediated propylene polymerization with various amounts
(increasing from b to e) of p-MS/hydrogen chain transfer agent
258
T.C.M. Chung
of propylene is evidenced by its comparison with two control reactions that were
carried out under similar reaction conditions – without the chain transfer agent
(control 1) and with p-MS only (control 2). A small amount of p-MS effectively
stops the polymerization of propylene. The introduction of hydrogen restores the
catalyst activity, as shown in run IX-I-1, which exhibits about 85% of the catalytic
activity of control 1 (without chain transfer agents). Hydrogen is clearly needed to
complete the chain transfer cycle during polymerization, as shown in Scheme 8. In
general, both p-MS/hydrogen and styrene/hydrogen chain transfer systems showed
very similar results: the higher the concentration of the p-MS (or styrene), the lower
the molecular weight of the resulting polymer. The polymer with very low molecular weight (just a few thousand) has been obtained and the molecular weight
distribution is generally narrow, which is consistent with single site polymerization
processes. The catalyst activity was also proportionally depressed with the concentration of p-MS (or styrene), which reflects the competitive coordination at
metallocene active sites between monomer and chain transfer agents.
Figure 13 shows the plot of the polymer molecular weight (M n ) versus the mole
ratios of [propylene]/[p-MS] and [propylene]/[St]. There is a linear proportionality
between the polymer molecular weight and molar ratio of [propylene]/[p-MS] or
[propylene]/[St]. It is clear that the chain transfer reaction to styrenic molecule
(with rate constant k tr ) is the dominant termination process, with very high chain
transfer constant k tr /k p ~ 1/6.36 and 1/7.5 for p-MS and styrene, respectively. The
300
250
200
mV
150
100
20
22
(a)
(b)
(c)
(d) (e)
24
26
Minutes
28
30
32
Fig. 12 GPC curves of (a) PP (control 1) and (b–e) several PP-t-p-MS polymers prepared by racMe 2 Si[2-Me-4-Ph(Ind)] 2 ZrCl 2 -mediated propylene polymerization with various amounts
(increasing from b to e) of p-MS/hydrogen chain transfer agent
258
T.C.M. Chung