Light spectrum and photoinitiators: A NMR study of unreacted monomers

The efficiency of polymerization process depends strongly on the compatibility between the light emitted and the photoinitiator absorption. Poor conversion leads to higher concentration of non-reacted and pendant monomers. The presence of unreacted monomers in dental composite after photo-activation could be detected by NMR¹H analysis. Objectives: The aim of this study was to evaluate the influence of different photoinitiator systems on the percentage of unreacted monomers of an experimental based resin photocured by LED and Quartz-Tungsten-Halogen lights. Methods: Specimens (5mm of diameter X 2mm high) of BisGMA-TEGDMA-UDMA-bisEMA blend contained as initiator 0.3% of camphorquinone with 0.5% of amines (N,N-dimethyl-p-aminobenzoic acid ethyl ester; N,N-cyanoethyl methylaniline; 2-dimethylamino ethyl methacrylate), or 0.5% of other photosensitizers (2,3-butanedione and Lucirin-TPO), which were light-cured for 20s by a LED curing unit (UltraBlue, DMC) and by a QTH unit (Optilux 401, Demetron). The unreacted monomers of each specimen (n=3) were extracted after a one-day storage time in deuteraded chloroform, and analyzed in a spectrometer VARIAN MERCURY 300. Results: ¹H spectra of all specimens showed residual (Table), by the detection of 5.6 and 6.1 ppm signals of CH₂= groups.

% unreacted monomers–mol% (sd)

CQ/

EDB

CQ/

CEMA

CQ/

DMAEMA

CQ/

TPO

CQ/

Optilux

2.48^a

(0.04)

4.03^c

(0.06)

5.09^d,e

(0.09)

3.71^b

(0.08)

5.04^d

(0.11)

UltraBlue

4.06^c

(0.06)

4.95^d

(0.11)

5.17^e,f

(0.12)

5.78^g

(0.13)

5.37^f

(0.11)

* similar letters indicate no significant difference Conclusions: CQ/EDB photocured by Optilux showed lower % of unreacted monomers than all combinations (p<0.001), and also within LED groups (p<0.001). Optilux and LED produced equal amounts of unreacted monomers in CQ/DMAEMA (p=0.98). In the presence of Lucirin TPO, photoactivation by LED produced the highest values of unreacted monomers.