Polarizations in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>B</mml:mi><mml:mo>→</mml:mo><mml:mi>V</mml:mi><mml:mi>V</mml:mi></mml:math>decays

We demonstrate that the polarization fractions of most tree-dominated $B\ensuremath{\rightarrow}VV$ decays can be simply understood by means of kinematics in the heavy-quark or large-energy limit. For example, the longitudinal polarization fractions ${R}_{L}$ of the ${B}^{0}\ensuremath{\rightarrow}({D}_{s}^{*+},{D}^{*+},{\ensuremath{\rho}}^{+}){D}^{*\ensuremath{-}}$ and ${B}^{+}\ensuremath{\rightarrow}({D}_{s}^{*+},{D}^{*+},{\ensuremath{\rho}}^{+}){\ensuremath{\rho}}^{0}$ modes increase as the masses of the mesons ${D}_{s}^{*+},{D}^{*+},{\ensuremath{\rho}}^{+}$ emitted from the weak vertex decrease. The subleading finite-mass or finite-energy corrections modify these simple estimates only slightly. Our predictions for the $B\ensuremath{\rightarrow}{D}_{(s)}^{*}{D}^{*}$ polarization fractions derived in the perturbative QCD framework, especially ${R}_{L}\ensuremath{\sim}1$ for ${B}^{0}\ensuremath{\rightarrow}{\overline{D}}^{*0}{D}^{*0}$ governed by nonfactorizable $W$-exchange amplitudes, can be confronted with future data. For penguin-dominated modes, such as $B\ensuremath{\rightarrow}\ensuremath{\rho}(\ensuremath{\omega}){K}^{*}$, the polarization fractions can be understood by the annihilation effect from the $(S\ensuremath{-}P)(S+P)$ operators, plus the interference with a small tree amplitude. At last, we comment on the various mechanisms proposed in the literature to explain the abnormal $B\ensuremath{\rightarrow}\ensuremath{\phi}{K}^{*}$ polarization data, none of which are satisfactory.