Not that you can not design IMC for unstable systems, however it is not as easy as designing IMC for stable plants. Since the regular IMC design for unstable plant gives very poor performance, other alternatives for designing IMC filter for unstable plants can be found in the literature. Search for Dr. Wee sit Lee and Dr. M Campi papers. However, you can use pole-placement to stabilise the unstable system and then design your adaptive IMC based on the stable plant.

Have a look at this paper:

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=325405&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D325405

For continuous time linear systems, a complete solution is available.  See our papers. 

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(1) The IMC procedure must be modified for unstable processes. The paper [RL91] has used the procedure developed by the classical book [MZ89] Morari and Zafiriou (1989) to find IMC-based PID controllers for unstable processes.

The book [MZ89] recommended a low-pass filter function which has the form

f(s)=(\gamma s + 1)/(\lamda s + 1)^n

to make unstable processes "be stabilized before applying adaptive IMC".

Quote Isra Abbasi's comment "Hence system has to be stabilized by any robust technique before applying adaptive IMC."

"if there is any specific technique that should be used in this case to stabilize?"

A quick answer: a low-pass filter function is a simple and robust technique used to stablize unstable dynamic system before applying adaptive IMC.

Quote Amin Noshadi's comment "designing IMC filter for unstable plants can be found in the literature. Search for Dr. Wee sit Lee and Dr. M Campi papers."

The paper [CLA93] recommended by Amin Noshadi used a two-degree of freedom filter to stablize unstable dynamic system.

[CLA93] M. Camp, W.S. Lee, B.D.O. Anderson, "Filters for internal model control design for unstable plants," Proceedings of the 32nd IEEE Conference on Decision and Control (IEEE CDC), 1993, pp.1343-1348.

[RL91] G.E. Rotstein and D.R. Lewin, “Simple PI and PID Tuning for Open-Loop Unstable Systems,” Industrial & Engineering Chemistry Research, 30 (8), 1991, pp. 1864-1869.

http://pubs.acs.org/doi/abs/10.1021/ie00056a028

[MZ89] M. Morari and E. Zafiriou, Robust Process Control, Prentice-Hall, Englewood Cliffs, NJ, 1989.

http://www.amazon.ca/Robust-Process-Control-Manfred-Morari/dp/0137821530

The following paper has cited the classical book [MZ89] in the IMC-PID controller design for stable processes using Nyquist Stability Criterion.

W.K. Ho, T.H. Lee, H.P. Han, and Y. Hong, "Self-Tuning IMC-PID Control with Interval Gain and Phase Margin Assignment," IEEE Transactions on Control Systems Technology, 9(3), May 2001, pp. 535-541. Available in the following RG Link.

https://www.researchgate.net/publication/3332273_Self-tuning_IMC-PID_control_with_interval_gain_and_phase_marginsassignment

(2) The low-pass filter function is very useful in the control engineering practice.

For example, the following paper also used the low-pass filter function to make the improper optimal controller be implemented in the real-world network traffic control system.

J(s)=(\gamma s + 1)/(\beta s + 1)^3

Y. Hong, O.W.W. Yang, "Robust Explicit Congestion Controller Design For High Bandwidth-Delay Product Network: A H∞ Approach," Proceedings of IEEE International Conference on Communications (IEEE ICC), Cape Town, South Africa, May 2010. Available in the following RG Link.

https://www.researchgate.net/publication/221167543_Robust_Explicit_Congestion_Controller_Design_for_High_Bandwidth-Delay_Product_Network_A_H_Approach

(3) The papers [ZP09] and [ZPF08] proposed robust adaptive controller design for continuous time linear systems in case of unmodeled system dynamics and noisy system output measurement. Such robust approaches can be integrated with IMC for the dynamic sytem under imperfect state measurement. 

Quote Zigang Pan's comment "For continuous time linear systems, a complete solution is available."

[ZP09] S. Zeng and Z. Pan, “Adaptive controller design and disturbance attenuation for SISO linear systems with noisy output measurements and partly measured disturbances,” International Journal of Control, vol. 82, February 2009, pp. 310–334.

[ZPF08] Q. Zhao, Z. Pan, and E. Fernandez, “Reduced-order robust adaptive control design of uncertain SISO linear systems,” International Journal of Adaptive Control and Signal Processing, vol. 22, September 2008, pp. 663–704.

(4) Recent discussions on control system design approaches can be found in the following RG link.

"What are trends in control theory and its applications in physical systems (from a research point of view)?"

https://www.researchgate.net/post/What_are_trends_in_control_theory_and_its_applications_in_physical_systems_from_a_research_point_of_view2

Article Self-tuning IMC-PID control with interval gain and phase mar...

Conference Paper Robust Explicit Congestion Controller Design for High Bandwi...