*2.1. Materials*

PBN was synthesized by the reaction of BCl<sup>3</sup> and NH2CH3, with a softening point of 80 ± 2 ◦C [27]. PCS (main units: (HSi(CH3)CH2), (CH2Si(CH3)2CH2), (Si(CH3)2Si(CH3)2)) with a number-average molecular weight of about 1150 and a softening point of 210 ± 2 ◦C was purchased from Zhongxing New Material Technology Co. Ltd., Ningbo, China. High purity N<sup>2</sup> (>99.99%, Huanyujinghui Co. Ltd., Beijing, China) and NH<sup>3</sup> (>99.99%, Nanfei Co. Ltd., Beijing, China) gases were used. All reactions were carried out under a dry nitrogen atmosphere.
