you can try with cell
clc
clear
syms t sigma
r = -4;
w = 2;
a = 1;
b = 1;
W = 1;
x_2 = 1
A_sin = 1;
A_sigma = [0,1;-w^2,-a*((A_sin*sin(W*sigma)^2)/w^4)-b*x_2+r];
B=0;
A{1} = int((A_sigma),sigma, 0, t);
for n = 2:5
A{n} = int((A_sigma*A{n-1}),sigma, 0, t);
B = A{n}+B;
end
phi = [1 0; 0 1]+A{1}+B;