Is the word poltrona the same in Brazilian and European Portuguese?

Yes

The word "poltrona" is identical in meaning, spelling, and grammar in both Brazilian and Continental Portuguese; it refers specifically to an upholstered armchair. The only difference lies in pronunciation. In Brazilian Portuguese, the vowels are typically more open and clearly articulated, and the rhythm of speech is more syllable-timed. In Continental Portuguese, unstressed vowels (such as the final "a") are often reduced or almost silent, and the rhythm is more stress-timed, giving the word a shorter, more clipped sound.

Brazilian Portuguese Examples:

1. Eu estou lendo um livro na poltrona. (I am reading a book in the armchair.)
2. O cachorro está dormindo na poltrona. (The dog is sleeping in the armchair.)
3. Essa poltrona é muito confortável. (This armchair is very comfortable.)
4. Eu comprei uma poltrona nova para a sala. (I bought a new armchair for the living room.)
5. Você pode sentar naquela poltrona? (Can you sit in that armchair?)

Continental Portuguese Examples:

1. Eu estou a ler um livro na poltrona. (I am reading a book in the armchair.)
2. O cão está a dormir na poltrona. (The dog is sleeping in the armchair.)
3. Essa poltrona é muito confortável. (This armchair is very comfortable.)
4. Eu comprei uma poltrona nova para a sala. (I bought a new armchair for the living room.)
5. Podes sentar-te naquela poltrona? (Can you sit in that armchair?)