# Effects of Organophosphorus Flame Retardants on the Dissipation Factor of Flame-Retardant Polymers
## 4 Conclusions  
To develop high-performance organophosphorus flame retardants for modern communication applications and understand the structural effect, in particular the effect of a hydroxyl group, on the dielectric losses of flame retardants and flame-retardant polymers, the performance difference between DOPO-HM and DOPO-Me has been independently investigated in non-polar and polar polymers. DOPO-HM differs from DOPO-Me only by the addition of a hydroxyl group.  
It has been demonstrated that DOPO-HM possesses lower Dk and Df values than DOPO-Me at 7-20 GHz because the former is able to form hydrogen bonds that slow down dipole reorientation of the polar groups. Both flame retardants increase the dielectric loss for non-polar polystyrene. However, DOPO-HM causes a slower Df increase than DOPO-Me due to higher crystallizability and poorer plasticization of DOPO-HM.  
In polar PMMA, factors affecting the distribution of conformations of PMMA contribute to changes in Df values. At low loadings and a low processing temperature, both  
Figure 6: Df of PPO/FR: (**a**) PPO-DOPO-Me; (**b**) PPO-DOPO-HM.  
Figure 7: Predicted Df vs. actual values of PPO/FR at 20 GHz.  
DOPO-Me and DOPO-HM cause an initial Df drop of PMMA. However, the reasons are different. For DOPO-Me, it acts as a plasticizer to reduce the \(T_{g}\) of PMMA and facilitate the return of a tt conformation to a tg or gg conformation and hence both the T\({}_{1}\) values of carbonyl carbon and the Df values of PMMA/DOPO-Me reduce initially. On the other hand, DOPO-HM initially works as an anti-plasticizer for PMMA-170; as a result, T\({}_{1}\) increases, owing to fewer dipole-dipole interactions, while the Df value reduces, presumably because of hydrogen bonds between DOPO-HM and PMMA. Nonetheless, like DOPO-Me, DOPO-HM can also act as a plasticizer for PMMA-190, which was processed at a higher temperature. This is because there is a higher concentration of tt conformations in PMMA-190. In any case, PMMA/DOPO-HM only has moderately higher Df values compared to PMMA/DOPO-Me at the same high loadings at the measured frequencies.  
Lastly, in a crosslinked PPO system, DOPO-HM performs better than DOPO-Me, as they do in PS. By utilizing the crystallinity of each component and based on the weight addition, the theoretical Df values of PPO/FR have been calculated. A generally fair-to-good agreement with measurements is obtained, especially at high loadings of flame retardants. The above results prove that the interaction between flame retardants and polymeric materials is really the key to deciding the effect of a hydroxyl group on the Df values of flame-retardant polymers. Consequently, organophosphorus structures with a hydroxyl group deserved to be explored in the development of high-performance, non-halogenated flame retardants for high-speed, high-frequency PCBs.  
Conceptualization, Q.Y.; methodology, Q.Y.; software, P.J.; validation, Q.Y.; investigation, P.J., W.C., J.S. and Y.Z.; resources, Q.Y.; data curation, P.J.; writing--original draft preparation, Q.Y.; writing--review and editing, P.J., W.C., J.S. and Y.Z.; visualization, Q.Y.; supervision, Q.Y.; project administration, Q.Y. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.