# Effects of Organophosphorus Flame Retardants on the Dissipation Factor of Flame-Retardant Polymers
## 1 Introduction  
Modern wireless technology features high peak data speeds and ultra-low latency. Since data rates and signal loss are largely decided by the dielectric properties of materials, several polymers with low dielectric constant (Dk) and dissipation factor (Df) have been developed and used as a base material for printed circuit boards (PCBs). For example, thermosetting polyphenylene oxide (PPO), cyanate esters, polyimide, polyolefins and polytetrafluoroethylene have been employed in high-frequency microelectronics with low signal loss [1, 2, 3, 4, 5, 6, 7, 8].  
However, some of the above polymeric materials do not have sufficient flame retardancy. To pass the mandatory flammability tests such as a UL-94 standard [9], a significant amount of flame retardants has to be added. Consequently, flame retardants exert serious influence on the PCB dielectric properties. Additionally, the current trend towardnon-halogenated flame retardants has channeled the electronics industry to use organophosphorus flame retardants instead of brominated flame retardants. As a result of this impetus, some promising organophosphorus flame retardants with stable P-C bonds have been developed, for example, derivatives of 6H-dibenzo[c,e][1,2]oxaphosphinine 6-oxide and diphenylphosphine oxide [10; 11; 12; 13; 14].  
These organophosphorus flame retardants perform well in terms of flame retardancy; however, they are quite expensive, which poses a great challenge for the industry in popularizing halogen-free products. In order to develop low-cost yet high-performance organophosphorus flame retardants for modern communication applications, a relationship between the structures of organophosphorus flame retardants and the dielectric properties of flame-retardant polymers needs to be firstly established. Currently, this relationship has not been clear or at times is misunderstood. In particular, some patents claim that the presence of a hydroxyl group, one of the most polar groups, in structures of organophosphorus flame retardants is disadvantageous for high-frequency PCB design because of its easy polarization, which foreseeably causes an adverse effect on dissipation factor [15; 16; 17]. Consequently, organophosphorus compounds with an OH group in their structures have been largely ignored as flame retardant options in high-frequency and high-speed PCBs so far.  
Nevertheless, considering beneficial effects brought by a hydroxyl group on the melting points and crystallinity of small molecules, which can counter the easy polarization of the hydroxyl group, the influence of an OH group in organophosphorus flame retardants on dielectric losses simply cannot be inferred. This is rather different from the dielectric constant, which has been satisfactorily predicated by various models [18]. In addition, organophosphorus compounds with a P-CH\({}_{2}\)OH moiety can be readily accessed via an economic Pudovik reaction [19]. Thus, organophosphorus compounds with an OH group deserve to be explored and the effect of hydroxyl groups needs to be elucidated.  
In this work, we study the effect of a hydroxyl group on the dielectric losses of flame retardants and flame-retardant polymers. By comparing Df values of 6-methyldibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO-Me) with 6-(hydroxymethyl) dibenzo[c,e][1,2]oxaphosphinine 6-oxide (DOPO-HM) and their blends with polymers ranging from non-polar to polar, we have demonstrated that an OH group can have a positive impact on the dielectric loss of flame retardants and that it is the interaction between flame retardants and polymeric materials that is a key to deciding the Df values of flame-retardant polymers. Our study will help open up a new avenue in the development of high-performance, yet affordable, flame retardants and assist product designers to formulate and choose right flame retardants for their PCBs.