Silicone type cinnamic acid derivative, preparation method thereof, UV-ray absorber, and external skin treatment agent

A method for preparing a silicone type cinnamic acid derivative which is a siloxane having at least one unit represented by the formula (I):

• ##STR41## and having other units which can exist in said siloxane being represented by the formula:
• O.sub.(4-m)/2 SiR.sub.m.sup.3 wherein R.sup.1 represents an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a trimethylsiloxy group, R.sup.2 is a divalent hydrocarbon group having at least two carbon atoms, which may include a heteroatom O, X is an alkoxy group, n is an integer of 0 to 2, a is an integer of 2 or 3, R.sup.3 is an alkyl group having 1 to 4 carbon atoms, a phenyl group, or a trimethylsiloxy group, and m is an integer of 0 to 3, comprising the step of reacting a cinnamic acid ester having the formula:
• ##STR42## wherein Y represents a monovalent hydrocarbon group having at least two carbon atoms and an olefinic unsaturated bond, which may include a heteroatom O, X is an alkoxy group, and a is 2 or 3 with a siloxane having an ##STR43## unit and R.sub.m.sup.3 SiO.sub.(4-m)/2 unit in the presence of a platinum vinyl siloxane catalyst in an organic solvent.