An organic polymer containing at least one reactive silyl group per molecule is known to have a characteristic that it is cross-linked by siloxane bond formation involving hydrolysis or other reactions of the reactive silyl group due to factors such as moisture even at room temperature, whereby a rubbery cured product is obtained. 
Among such reactive silyl group-containing organic polymers, those whose backbone skeleton is a polyoxyalkylene polymer, a saturated hydrocarbon polymer, or a poly(meth)acrylic acid ester copolymer are disclosed in, for example, Patent Literature 1, have already been produced industrially and are widely used in applications such as sealing materials, adhesives, coating materials and paints. 
The market share of these organic polymers in building sealing material applications in particular has been increasing for at least 20 years.  Detached housing that uses siding boards has become predominant in recent years, and sealing materials are also used in the joints between siding boards.  However, since siding boards are porous boards, it has been found that the plasticizer tends to be slowly absorbed into the boards during long-term exposure, thereby reducing the elongation of the cured product.  In order to allow the sealing material to have watertightness and airtightness even after long-term exposure, the initial cured product can be formed to have low modulus and high elongation so that a satisfactory performance can be achieved even when some plasticizer has escaped.  In addition, the co-use with a low specific gravity, hollow resin has been employed recently in order to reduce the cost of the curable composition on a volume basis.  However, it is becoming clear that the use of the hollow resin brings about the problems of reduced workability due to an increase in the viscosity of the curable composition and reduced elongation of the cured product, and therefore an improved technique is desired. 
Some methods exist to allow the cured product of a curable composition containing a reactive silyl group-containing organic polymer to exhibit the physical properties of low modulus and high elongation.  One of these methods is to increase the amount of plasticizer, but this is unsuitable because the weather resistance of the cured product is then reduced.  Another method is that the number of silyl groups in each molecule of the reactive silyl group-containing organic polymer is reduced; however, this method is unfavorable because it causes the stickiness of the surface of the cured product (referred to below as the residual tack) to get worse.  Still another method exists in which the molecular weight of the organic polymer is increased while keeping the number of silyl groups in each molecule constant; however, this raises the viscosity of the organic polymer and the associated increase in the viscosity of the curable composition causes the problem of poor workability. 
Meanwhile, techniques that use a combination of at least two organic polymers containing different amounts of reactive silyl groups are already known from Patent Literatures 2 to 9 and the like; however, these involve only simple mixing at a certain weight ratio and have not considered the nature of each component in the mixture.  In addition, these use an organic polymer having a branched structure and/or use a trialkoxysilyl group as the reactive silyl group, and there is thus room for improvement with regard to the achievement of low modulus and high elongation.