1.  Technical Field
The invention relates to a polymerizable compound having two groups, a polymer obtained from the polymerizable compound, a liquid crystal composition including the polymerizable compound or the polymer, and a liquid crystal display device containing the composition.  It also relates to a liquid crystal display device in which the liquid crystal composition is sealed between the substrates of a device, a polymerizable compound is polymerized while a voltage is applied to the device, and the orientation of liquid crystal molecules is adjusted by the effect of the resulting polymer.  The invention also relates mainly to a liquid crystal composition suitable for an AM (active matrix) device and so forth, and to the AM device containing the composition. 
2.  Technical Background
A liquid crystal display device utilizes optical anisotropy, dielectric anisotropy and so forth possessed by liquid crystal molecules in a liquid crystal composition.  A classification based on an operating mode for the liquid crystal molecules includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode and a vertical alignment (VA) mode. 
A liquid crystal display device having a mode in which a polymer is used in combination with a liquid crystal composition.  This is, for example, a polymer sustained alignment (PSA) mode or a polymer stabilized (PS) mode.  In a liquid crystal display device having this mode, a liquid crystal composition to which a polymerizable compound is added is poured into a display device.  A polymer is formed in the liquid crystal composition by the irradiation with ultraviolet light under the conditions of applied voltage between electrodes and by the polymerization of the polymerizable compound.  According to this method, a liquid crystal display device is obtained in which the response time is decreased and the image burn-in is improved. 
This method can be applied to a variety of operating modes of a liquid crystal display device, and modes such as PS-TN, PS-IPS, PS-FFS, PSA-VA and PSA-OCB are known.  A polymerizable compound used for these kinds of modes seems to have a high ability to orient liquid crystal molecules, however, it is not said that the solubility in a liquid crystal composition is high.  An improvement of the solubility in a liquid crystal composition has been tried until now, and there is a tendency that as the solubility increases, the polymerization reactivity is decreased.  Thus, the development of a polymerizable compound having a suitable balance between the solubility and the polymerization reactivity has been expected.