Alexandru Dragos Asandei , University of Connecticut
The Cp2TiCl mediated radical ring opening of epoxides, SET reduction of aldehydes, halide abstraction and redox reaction with peroxides (Scheme 1) were demonstrated as novel initiating methodologies for the living radical polymerization (LRP) of styrene. Cp2TiCl2 was shown to be the only know transition metal catalyst that can provide radical initiation from four different classes of initiators. The advantages of epoxides and aldehydes rely on their commercial availability with a wide variety of chain ends and the in situ generation of alcohol chain ends which can be further employed in the synthesis of block copolymers, whereas the halide activation provided by Ti is superior to that afforded by Cu in ATRP since Ti can afford initiation even from inactivated substrates such as regular linear alkyl halides (1-bromodecane).
Scheme 1: Mechanism of the Cp2TiCl-catalyzed styrene LRP initiated from epoxides, aldehydes, halides and peroxides.
All 4 Cp2TiCl-activated initiator types were thoroughly comparatively evaluated (Tetrahedron, 2008, 64, 11831) by investigating the effect of reagent stoichiometry (monomer to initiator, Ti/initiator, Zn/Ti) and temperature on initiator efficiency (IE) and polydispersity in the Cp2TiCl-catalyzed styrene LRP. While living polymerization features (linear dependence of molecular weight on conversion and low polydispersity) were obtained in most cases over a wide range of conditions, the comparison of the effect of the reaction variables revealed a set of initiator specific similarities and differences. For brevity only the effect of temperature is presented in Figure 1.
While the polymerizations are sensitive to the initiator structure, larger initiator efficiencies and narrower polydispersity (Mw/Mn ~ 1.2) and are obtained when using excess Cp2TiCl over the initiator and of Zn over Cp2TiCl2 and with decreasing temperature. Therefore, optimum conditions which minimize PDI and maximize IE are [St]/[I]/[Cp2TiCl2]/[Zn] = (50-200)/1/(2-3)/(4-6) at 70-90 °C. However, they are also initiator dependent. Peroxides are good initiators, but do not provide functional chain ends. Halides are more sensitive to the variation in the reaction parameters and their optimum conditions are in a narrower interval. Finally, both epoxides and aldehydes remain synthetically the most useful. Epoxides may provide faster initiation and are more readily accessible on polymer backbones for the Ti-catalyzed synthesis of block and graft copolymers. However, aldehydes not only also allow access to the same PSt-OH functional chain ends but also seem to be the least affected by the reaction conditions and thus the most robust initiator in the series.
Figure 1. Effect of temperature in the Cp2TiCl-catalyzed styrene LRPs initiated from BDE, BA, BEB and BPO: (a) Dependence of Mn and Mw/Mn on conversion; (b) Dependence of IE and PDI on temperature.
The knowledge accumulated on styrene was further applied towards the isoprene LRP since to date there is no example of a transition metal catalyzed isoprene LRP. We have thus provided the first example of such polymerizations using again the Cp2TiCl2/Zn system in conjunction with epoxides, aldehydes and halides and have shown that the polymerization is living, and that block copolymers with styrene can also be synthesized. (ACS Symposium Series, Controlled/Living Radical Polymerization. 2009, 1024, 149-166).
Figure 2. 500 MHz 1H-NMR spectra of isoprene/styrene copolymers synthesized by Cp2TiCl-catalyzed LRP.
The first examples of the use of aldehydes in the living ring opening polymerization of e-CL were presented (J. Polym. Sci.: Part A: Polym. Chem. 2008, 46, 2869-2877, Scheme 2) via the use of the SET reduction of carbonyl groups to generate in situ Ti alkoxides.
Scheme 2. Living ring opening polymerization of caprolactone catalyzed by titanium alkoxides derived from SET reduction of aldehydes.
Figure 2. Temperature effect in the Cp2TiCl catalyzed caprolactone LROP (a) Dependence of Mn and PDI on conversion; (b) First order kinetics.
The living character of the polymerization was demonstrated (Fig. 2) by the linear dependence of Mn on conversion, low PDI values and linear kinetics, while the aldehyde initiation was confirmed (NMR) by the presence of the initiator fragment of the PCL chain end. The effect of the nature of the aldehyde functionality (R-Ph-CHO, R = H, Cl, PhCH2O, NMe2, CH3O, NO2, and CHO), reagent ratios ([CL]/[aldehyde] = 50/1 to 400/1, [aldehyde]/[Cp2TiCl2] = 1/1 to 1/4, [Cp2TiCl2]/[Zn] = 1/0.5 to 1/2) and temperature (T = 75 °C to 120 °C) was investigated over a wide range of values to reveal a living polymerization in all cases with an optimum observed at 90 °C with [CL]/[aldehyde]/[Cp2TiCl2]/[Zn]= 100/1/1/2.
Thus, together with epoxides, aldehydes were introduced as a new class of initiators for the Cp2TiCl-catalyzed living ROP of cyclic esters. This convenient and inexpensive novel methodology precludes the need for prior synthesis of air and moisture sensitive Ti complexes and provides convenient access to PCL with variety of chain ends derived from widely available and structurally diverse aldehyde precursors.
Later efforts were directed towards the application of the current methodologies in complex polymer architecture as well as towards the LRP of dienes. Thus, while no example of metal catalyzed (e.g. ATRP) LRP of diene is available, we were able to show that living polymerizations are successfully accomplished for butadiene, 2-metyl butadiene (isoprene) as well as 2,3-dimethyl butadiene from epoxides, aldydes and halides. An example of the chain ends and of the kinetics of this living polymerization are outlined below:
Figure 3. 500 MHZ-1H NMR of the polymerizations initiated from epoxides.
Figure 4. Temperature effect in the Cp2TiCl catalyzed isoprene LRP.
Figure 5. Grafting of isoprene from polyglycidyl methacrylate: